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doi:10.2204/iodp.proc.304305.103.2006

Igneous petrology

This section describes the igneous rock types recovered from the two principal holes drilled at Site U1309: Holes U1309B and U1309D. These holes are 20 m apart, and the upper 132 m of Hole U1309D, cored during the first phase of drilling at Site U1309 (see “Operations”), was compared to Hole U1309B in an attempt to correlate rock types. A total of 770 igneous units were defined in Hole U1309D, including 221 in the upper 401.3 m (Expedition 304) and 549 from 401.3 to 1415.5 mbsf (Expedition 305). Each unit is distinguished on the basis of primary modal mineralogy, igneous contacts, and variations in grain size.

Hole U1309B (Expedition 304)

Hole U1309B (Fig. F35; Table T2) is dominated by two intrusive series separated by a narrow (~2–4 m) interval of relatively undeformed serpentinized harzburgite. The upper intrusive sequence includes diabase and gabbro that show an increase in fracture intensity and alteration downhole. There is an intrusive contact between the lowermost gabbro and the serpentinite. The lower intrusive sequence includes diabase and gabbro that are less altered in appearance and significantly less fractured than those of the upper series. Both the basalt and the gabbro are interpreted to be younger than, and most likely intrusive into, the serpentinized harzburgite, although the key contacts were not recovered.

Upper intrusive series

Upper basaltic sequence

• Depth: ~2–21 mbsf
• Units: 1–13
• Interval: Sections 304-U1309B-1R-3, 25 cm, through 3R-1, 67 cm

The upper basaltic sequence comprises a series of fine- to medium-grained basalt that shows an increase in grain size and ophitic textures downhole. There are a number of intrusive contacts in the core; nowhere in the sequence is there evidence of eruption at the seafloor. With the possible exception of the uppermost basalt unit (interval 304-U1309B-1R-3, 25–150 cm), the entire sequence appears to be intrusive in origin. The total recovered thickness is 4.3 m with a possible maximum thickness of ~19 m.

As seen in hand specimen, the core appears increasingly altered downhole, primarily because of the development of greenish, patchy discoloration. The dominant alteration effects seen in thin section are the almost complete alteration of generally sparse olivine to random aggregates of acicular amphibole and of clinopyroxene to acicular green or green-brown actinolite, generally in optical continuity. Plagioclase appears relatively fresh throughout (see “Metamorphic petrology” for further description).

The coarser grained units have variably developed subophitic textures, and, in places, the distinction between medium-grained basalt and diabase is somewhat arbitrary. Phenocrysts are rarely abundant, although local concentrations of either olivine or plagioclase are present within Units 6–8. There are no vesicles in Hole U1309B core. In some places, sparse trains of altered plagioclase phenocrysts define weak, near-horizontal laminations.

There is clear evidence of intrusive relationships in two places. Unit 10 displays an aphanitic chilled margin at its horizontal contact with Unit 9. Matrix grain size increases downhole away from the contact, but the size and abundance of sparse plagioclase phenocrysts increase upward, suggesting flow concentration near the margin or, perhaps, that plagioclase has floated in a magma body (Fig. F36).

Aphanitic basalt and breccia

• Depth: ~21–31 mbsf
• Units: 14–19
• Interval: Sections 304-U1309B-3R-1, 67 cm, through 5R-1, 42 cm

This distinctive sequence, dominated by aphanitic to fine-grained basalts, has a recovered thickness of ~2 m (maximum = ~10 m). Hand samples have a characteristic dense aphanitic surface texture, reflecting the fine-grained metamorphic recrystallization of the matrix, that is easily recognized on cut surfaces and is distinct from all other Site U1309 rock types. These basalts are characterized by a fine-grained, originally glassy matrix that is now dominated by fine actinolitic amphibole. Within this matrix, fine acicular plagioclase microphenocrysts are preserved (<5%). Cataclastic breccias and fracture networks are common on macro- to microscales throughout the interval, forming a significant fraction of the sequence. At least some of the thicker zones of cataclasite are associated with intrusive contacts. The dominant clast type is the host aphanitic basalt, accompanied by fresh, broken plagioclase phenocrysts and by fragments of more crystalline basalt with subophitic textures. These coarser basalts resemble similar lavas in the overlying upper basalt sequence but differ in grain size and texture from the deeper diabases. The breccia matrix is a complex mixture of highly altered relict clasts and finer material (e.g., Sample 304-U1309B-3R-1, 65–68 cm). The dominant mineral is actinolitic amphibole in a variety of forms, accompanied by lesser chlorite, albite, and opaque minerals (see “Metamorphic petrology”).

Where intrusive contacts are clearly visible, they most commonly display grain-size-graded chilled margins against other similar basalts. The only measurable contact is located within Unit 19. It is steeply dipping (Fig. F37) (interval 304-U1309B-4R-1 [Piece 5A, 36–42 cm]). Elsewhere, more irregular contacts defining small intrusive basalt dikelets have been identified within some of the breccias. A single piece of gabbro with no recovered contact is located at ~21 mbsf (interval 304-U1309B-3R-1, 105–114 cm).

We interpret this sequence as a suite of mutually intrusive fine-grained basalts that have been brecciated by intrusion and hydrothermal alteration and, subsequently, pervasively recrystallized to a very fine grain size. Its relatively small thickness suggests that it represents a suite of narrow dikes or sills intrusive into a deformed or deforming gabbroic protolith.

Upper diabase

• Depth: ~31–35 mbsf
• Units: 20–22
• Interval: Sections 304-U1309B-1R-3, 14 cm, through 6R-2, 67 cm

The units of this sequence appear to constitute a single <5 m thick medium-grained diabase intrusion (recovered thickness = ~3 m). The upper and lower contacts were not recovered, but a single piece of cataclasite at the lower boundary (interval 304-U1309B-6R-2 [Piece 6, 67–74 cm]) suggests a sheared contact with the underlying gabbro. This interval is characterized by a well-developed medium-grained ophitic texture and by a gradation to finer grain size margins approaching both the upper and lower boundaries. The dominant minerals are plagioclase and clinopyroxene, with only minor olivine. An opaque phase, presumed to be magnetite, varies in abundance from a few percent to as much as 5% over certain 10–20 cm intervals. Magnetite abundance is strongly reflected in higher MS values, particularly close to the fine-grained margins (see “Physical properties”). Plagioclase phenocrysts are very sparse through this interval, with rare examples up to 1 cm in length. Occasional sparse trains of a few such crystals locally define a weak subhorizontal magmatic fabric. Similar sparse trains of rounded relict olivine are also present in several places, with a good example in interval 304-U1309B-5R-1 (Piece 12, 72–80 cm) (Fig. F38).

The average mineral composition of this basalt is 50% plagioclase, 40% clinopyroxene, 5% olivine, and 5% oxides. Minor apatite in some samples is usually present within large, unusually clear plagioclase grains that are presumed to have recrystallized to more sodic compositions. The majority of the plagioclase is strained but relatively fresh prismatic grains that display zoned and sweeping extinction and are radially oriented to form a classic subophitic to ophitic texture with anhedral clinopyroxene. Euhedral prismatic to equant plagioclase phenocrysts are rare but present throughout. Most display strong zonation, and some are embayed, indicating resorption. Clinopyroxene is 20%–100% altered to amphibole, including tremolite, actinolite, hornblende, and, possibly, chlorite. Olivine is never fresh and is replaced by chlorite, amphibole, and opaques.

Upper gabbro sequence

• Depth: ~38–58 mbsf
• Units: 38–57
• Interval: Sections 304-U1309B-7R-1, 0 cm, through 11R-1, 86 cm

This sequence consists of coarse-grained gabbro with an average grain size of 10–20 mm. No internal contacts or consistent lithologic variations that might indicate multiple intrusions were recovered, and we interpret this to be a single intrusive body with a maximum thickness of ~20 m (recovered thickness = ~7.5 m). The gabbro is extensively altered and fractured throughout, with pervasive crystal-plastic deformation fabric dipping ~50°. Small anorthosite intrusions partially invade the fabric of the gabbro in several places, with prominent occurrences as thick as ~5 cm (Fig. F39) (interval 304-U1309B-8R-1, 83–100 cm). Magmatic layering defined by variations in mode and grain size is apparent in both hand samples and thin section beginning in Unit 29 (Section 304-U1309B-10R-1, 102 cm) (Fig. F40) and continuing through Unit 29 (Section 304-U1309B-10R-1, 121 cm), where several centimeter-scale troctolitic layers are present.

The primary mineralogy of the coarse-grained gabbros is dominated by subequal but variable proportions of plagioclase and clinopyroxene, with small amounts of olivine and, in rare cases, orthopyroxene and opaques. Olivine gabbros are rare or absent, and minor troctolite or troctolitic layers were only recognized in Unit 29. Alteration is pervasive with virtually complete conversion of clinopyroxene to green-brown actinolite in rough optical continuity and of olivine to randomly oriented clusters of actinolitic amphibole. Plagioclase remains relatively fresh in appearance. In places, lower grade alteration minerals, including chlorite, clays, and, possibly, prehnite are present along or close to fractures. The petrography of the upper gabbros is similar to that of comparable lithologies in the lower gabbros, which is described in “Lower gabbros and troctolites,” below.

Serpentinized harzburgite

• Depth: ~58–60 mbsf
• Unit: 32
• Interval: Sections 304-U1309B-11R-1, 86 cm, through 11R-2, 94 cm

Between the upper and lower intrusive series, a thin interval of serpentinized peridotite with a maximum thickness of ~3.5 m (recovered thickness = 1.4 m) begins at 57.86 mbsf (interval 304-U1309B-11R-1, 86 cm, through 11R-2, 94 cm) (Fig. F41). The upper boundary is in direct (intrusive) contact with the coarse-grained gabbro from Unit 31. The lower boundary was not recovered.

The unit consists of protogranular harzburgite with overall alteration degree ranging from 60% to >99%. The harzburgite contains ~20% subrounded orthopyroxene, and relict clinopyroxene grains cannot be identified. Equant spinels in Sections 304-U1309B-11R-1, 100 cm, and 11R-2, 88 cm, have a measured Cr# (Cr × 100/[Cr + Al]) of 42, which correlates to a degree of melting of 15%–16% (Hellebrand et al., 2001).

Indicators of deformation in the harzburgite are rare and only weakly developed; lattice-preferred orientation (LPO) and subgrain formation in olivine both record mantle flow (see “Structural geology”). Domains with recrystallized olivine are present close to the gabbro/​peridotite boundary (Fig. F42B). The harzburgite is intruded by narrow gabbroic dikes and dikelets at 109 and 111 cm in Section 304-U1309B-11R-1, as well as at 7, 40, and 76 cm in Section 11R-2. At the upper gabbro/​harzburgite boundary, a 2 cm wide zone parallel to the contact differs in texture from the rest of the serpentinized harzburgite. In thin section, this zone has a lower degree of serpentinization, a higher degree of deformation, and a relatively high modal clinopyroxene content that decreases away from the contact. Similar low degrees of serpentinization are a common feature of gabbro/​peridotite contacts observed in other oceanic rock samples (Bideau et al., 1991; Cannat, Karson, Miller, et al., 1995).

Talc-tremolite bands of hydrothermal origin formed along the upper harzburgite/​gabbro contact (Fig. F42A) and at dike contacts within the serpentinite. This suggests that hydrothermal fluids are preferentially channeled along gabbroic intrusions, as previously observed on the MAR (Cannat et al., 1992). Extensive serpentinization of harzburgite postdates both the intrusion of the gabbroic dikelets and their hydrothermal alteration, as the talc-tremolite bands are cut by serpentine tension cracks filled by lizardite. This is in contrast with results from Leg 209, where pronounced alteration of pyroxene to talc near gabbro contacts was found to postdate serpentinization in Hole 1268A (see Kelemen, Kikawa, Miller, et al., 2004). A variety of characteristics of the serpentinized harzburgite and its contacts with gabbroic intrusions suggest high emplacement temperatures. Both the absence of a chilled margin in the coarse-grained gabbro at the peridotite contact and the medium grain sizes in the dikes suggest emplacement in host rock without a significant temperature contrast to intruding liquid. Crystal-plastic deformation in the gabbro, minor amounts of recrystallization of olivine in peridotite, and deformation (undulose extinction and bent lamellae) of magmatic clinopyroxenes next to the gabbro/​peridotite contact (Fig. F42B) must have occurred after crystallization of clinopyroxene and above the olivine brittle–ductile transition, ~700°–1000°C, depending on strain rate (Kirby, 1983). Ductile deformation of peridotite away from the contact is subdued, and protogranular textures are preserved. These considerations give a lower limit for the deformation temperature of at least 600°C.

Key indicators of magmatic infiltration into peridotite suggest even higher temperatures. The relatively abundant clinopyroxene (~5%) close to the gabbro/​peridotite contact have textures that indicate crystallization as a result of melt infiltration. These include clinopyroxenes crosscutting mantle olivines (Fig. F42C) and networks of altered interconnected pyroxenes (Fig. F42D). Dark brown interstitial spinel grains in the melt reaction zone (Sample 304-U1309B-11R-1, 87–91 cm) have a Cr# of ~55 and are significantly higher than those in the remainder of the unit. Similar elevated Cr#s are often found in spinels from melt-impregnated peridotites (Dick and Bullen, 1984; Cannat et al., 1997), although they can also form by secondary hydrothermal alteration (Kimball and Evans, 1988).

Apart from a few rare grains close to dike contacts (Fig. F42E), clinopyroxene is not present in most of the recovered peridotite. The farthest location of clinopyroxene from a gabbro contact is ~6 cm (Sample 304-U1309B-11R-1, 98–100 cm), where a single relict pyroxene grain is completely altered to bastite + tremolite. A second indication of the extent of melt impregnation is the relative enrichment in iron, sodium, and titanium and depletion in nickel of a sample from interval 304-U1309B-11R-1, 100–104 cm, relative to one from farther downcore (see “Geochemistry”). Similar chemical enrichments are reported from peridotites close to gabbroic dikelets recovered during Leg 153 (MARK; Cannat et al., 1997). The apparent restricted range in infiltration depth of the melt suggests temperatures close to the peridotite solidus and potentially provides an upper limit for the emplacement temperature (Cannat et al., 1997; Kelemen, Kikawa, Miller, et al., 2004). Taken together, these indicators suggest a mantle temperature between 1000° and 1100°C is likely for the emplacement of the gabbro unit.

Lower intrusive series

Middle diabase

• Depth: ~62–67 mbsf
• Units: 33–35
• Interval: Sections 304-U1309B-12R-1, 0 cm, through 13R-1, 147 cm

The middle diabase sequence consists of narrow upper and lower fine-grained intervals (~70 and ~30 cm thick, respectively) separated by a uniform medium-grained interval that is 4–5 m thick. The internal contacts were not recovered, and no transitions in grain size are apparent in the core. The upper and lower contacts were not recovered, and there is no information in the core as to their nature (see further discussion of the lower boundary in “Lower gabbros and troctolites,” below).

The medium-grained unit (Unit 34) is mineralogically and chemically (see “Geochemistry”) very similar to those of the upper and lower diabase sequences. It is characterized by a well-developed ophitic texture, with plagioclase and clinopyroxene as the dominant minerals and only minor olivine. An opaque phase varies in abundance up to 5%. Plagioclase phenocrysts are present very sparsely throughout the core, with rare examples up to 1 cm in length. Plagioclase is relatively fresh, clinopyroxene is pervasively altered to green-brown actinolite in rough optical continuity, and olivine is altered to randomly oriented clusters of actinolitic amphibole.

Lower gabbros and troctolites

• Depth: ~68–93 mbsf
• Units: 37–59
• Interval: Sections 304-U1309B-13R-2, 12 cm, through 18R-3, 87 cm

The lower gabbro sequence is dominated by medium-grained olivine gabbros grading to frequent thin troctolite intervals. Average grain size is ~5 mm, ranging from 2 to 10 mm. Occasional intervals of coarse-grained olivine gabbro range in average grain size up to 20 mm. No internal contacts or consistent lithologic variations that might indicate multiple intrusions were recovered. At its upper boundary, the gabbro is in contact with and intruded by a fine-grained basalt dike (Fig. F43) (interval 304-U1309B-13R-2 [Piece 1, 0–10 cm]). At its lower boundary, the gabbro adjoins a similar fine-grained basalt, but no contact was recovered. However, ~30 cm above the boundary (Fig. F44) (interval 304-U1309B-18R-3 [Piece 9, 48–53 cm]), a small (<2 cm thick) dikelet of very similar basalt intrudes coarse-grained gabbro. This intrusive relationship suggests the lower gabbro boundary is also marked by basalt intrusion. No contacts were recovered between the middle and lower diabase sequences and either the lower gabbro or the intrusive basalts that are present at its boundaries. We interpret these relationships to indicate that the lower gabbro is a single intrusive body with a maximum downhole thickness of ~24 m (recovered thickness = ~15.6 m). The natures of its boundaries are not known, but they appear to have been loci for later basalt dike emplacement.

The lower gabbros differ from those of the upper series in their finer grain size, the predominance and variety of their olivine-bearing assemblages, and their common magmatic layering. Magmatic layers are typically between five and a few tens of centimeters in thickness. They are defined primarily by variations in clinopyroxene abundance and/or grain size. In some intervals (e.g., interval 304-U1309B-17R-2, 0–120 cm), patchy olivine gabbro and troctolitic domains predominate (Fig. F45). The dominant modal mineralogy of Hole U1309B gabbroic rocks is summarized in Table T2. Minor phases include opaque minerals, brown amphibole, and apatite.

Textures of gabbros from both the upper and lower gabbro sequences are normally equigranular and rarely seriate under the microscope. Plagioclase is present as anhedral to subhedral crystals and is almost always fresh relative to clinopyroxene and olivine, particularly away from shear and/or hydrothermal zones. Overall, plagioclase is 0%–40% altered to chlorite and amphiboles. Small polygonal recrystallized neoblasts are present in shear zones. Olivine was originally present as anhedral crystals in troctolite and troctolitic gabbro and as anhedral to subhedral crystals in olivine gabbro and gabbro. Throughout the lower and upper intrusive series, olivine is usually completely altered to randomly oriented clusters of actinolitic amphibole, typically surrounded by halos of chlorite that appear black in hand specimen. In the troctolites, this style of alteration is particularly well developed; the chlorite halos connect along grain boundaries, forming a distinctive and diagnostic network texture (Fig. F46). The only recognized unaltered olivines are in olivine gabbro (interval 304-1309B-16R-2, 71–73 cm) and troctolite (interval 304-1309B-15R-1, 110–112 cm). Clinopyroxene is anhedral in troctolite and troctolitic gabbro and, most commonly, granular and subhedral to anhedral in olivine gabbro and gabbro. In the most altered and/or foliated samples, clinopyroxene is 60%–100% altered to aggregates of amphibole, including tremolite, actinolite, and hornblende, which tend to maintain crystallographic continuity with the original pyroxene. In some places, primary clinopyroxene is replaced by clear clinopyroxene with wider spaced cleavages, possibly as a result of late magmatic fluid migration (Pettigrew, Casey, Miller, et al., 1999; Maeda et al., 2002) (Fig. F47) (interval 304-1309B-16R-2 [Piece 5, 26–29 cm]). The troctolites and troctolitic gabbros have higher whole-rock Mg# than gabbros from Atlantis Bank (SWIR) and the MARK area (see “Geochemistry”).

A strongly deformed and brecciated sample of oxide gabbro from interval 304-U1309B-16R-1, 29–48 cm (Unit 48), is characterized by a medium-grained equigranular texture. Major constituent minerals are 48% plagioclase, 48% clinopyroxene, and 3% ilmenite. Clinopyroxene and plagioclase are both strongly deformed, and recrystallized plagioclase neoblasts are present throughout the thin section. Ilmenite is interstitial between the plagioclase neoblasts and also intimately associated with greenish brown amphibole. In some places, ilmenite has been cut by greenish brown amphibole veins. Small grains of rutile are associated with, or included within, ilmenite (see “Metamorphic petrology”), suggesting that ilmenite is formed from magnetite during a late-stage process.

Alteration is pervasive throughout the gabbros, although plagioclase remains relatively fresh in appearance. Most clinopyroxene has been altered to green-brown actinolite in rough optical continuity. Adjacent to the basalt dike at the upper boundary of the lower gabbro, larger grains (5–10 mm) of brown hornblende are present within clusters of fine actinolitic amphibole (interval 304-U1309B-13R-2 [Piece 1, 0–9 cm]). Throughout the lower intrusive series, olivine is commonly altered to randomly oriented clusters of actinolitic amphibole that are usually surrounded by halos of chlorite that appear black in hand specimen. In the troctolites, this style of alteration is particularly well developed; the chlorite halos connect along grain boundaries, forming a distinctive and diagnostic network texture (Fig. F45).

Lower diabase

• Depth: ~94–101 mbsf
• Unit: 62
• Interval: Sections 304-U1309B-19R-1, 13 cm, through 20R-2, 83 cm

This sequence consists of a single intrusive unit of medium-grained diabase, mineralogically and texturally indistinguishable from those of the middle and upper sequences. Its contact with the overlying gabbro was not recovered (see “Lower gabbros and troctolites”).

Crosscutting fine-grained basalt dikes

• Depths: various short intervals
• Units: 36, 40, 42, 54, 57, and 60

Within the lower intrusive series, a distinctive dark-colored sparsely plagioclase (± olivine) phyric basalt was recovered at seven distinct locations. Recovered intervals range from individual small pieces to 50 cm in interval 304-U1309B-18R-3 (Pieces 13–17, 87–138 cm). Two intrusive contacts with underlying gabbro were recovered in intervals 304-U1309B-13R-1 (Piece 1, 0–10 cm) and 18R-3 (Piece 9, 49–51 cm). Both dip at ~60° (Figs. F43, F44).

In thin section, fine randomly oriented plagioclase needles are seen to be set in an indeterminate, altered granular groundmass dominated by actinolitic amphibole. Patchy concentrations of rounded altered olivine are present in several locations and are most prominent in the lowermost unit (Unit 60; Section 304-U1309B-18R-3 [Pieces 16–17, 111–139 cm]).

Hole U1309D (Expedition 304)

In the upper 132 m of Hole U1309D, the range of primary igneous rock types is very similar to that of Hole U1309B (Fig. F35; Table T3). In broad outline, Hole U1309B and the upper part of Hole U1309D are dominated by two gabbroic intrusive series that are separated by a narrow interval of serpentinized peridotite. The gabbroic and ultramafic series are intruded, in turn, by a series of diabase sills and several narrow, steep, fine-grained basaltic dikes. Invasive and discrete felsic veining of magmatic and/or hydrothermal origin is common and accounts for a significant fraction of total volume in some parts of the core. In detail, the upper gabbroic series (gabbro Zone 1) in Hole U1309D is more extensively layered and more variable in composition than the one in Hole U1309B. As in the upper series in Hole U1309B, gabbro Zone 1 in Hole U1309D becomes increasingly more altered and fractured downhole. It is occasionally cut by small (10 cm) dikes of coarse-grained gabbro. Pervasive, irregular yellow-green actinolitic veins are abundant in Hole U1309D but rare or absent in Hole U1309B. The upper basalt sequence of Hole U1309B has no equivalent in Hole U1309D, perhaps because the uppermost 20 m of Hole U1309D was not cored. In both holes, the short serpentinized peridotite interval is intruded by the gabbros above; the lower serpentinite boundaries were not recovered. In Hole U1309D, both gabbro Zones 1 and 2 are complex, layered sequences that include troctolites, troctolitic and olivine gabbros, and gabbros. Both series can be divided into lithologic zones, each with a dominant lithology. Throughout Zone 1, cataclastic deformation and hydrothermal alteration effects are ubiquitous, whereas rocks of Zone 2 appear less altered and significantly less fractured. Gabbro Zone 1 in Hole U1309D, which comprises olivine-rich rock types with fine-scale (5–10 cm) layering, is distinctly different from the upper gabbro sequence of Hole U1309B, which consists mostly of fairly uniform gabbro, becoming layered only close to its lower boundary. Gabbro Zone 2 and the Hole U1309B lower series are, however, quite similar on a scale of one to a few meters, with each series divided into five correlated lithologic zones that are layered on the scale of ten to a few tens of centimeters and internally variable in mineralogy and texture (Fig. F35). Finer scale correlations between individual layers or distinctive packets of layers have not been recognized. Correlations between the six diabase intervals recovered in Hole U1309D and the three in Hole U1309B have proven elusive. MS data suggest that upper and lower diabases of Hole U1309B are equivalent to Diabases D-2 and D-5, respectively (see “Physical properties”), but petrographic and geochemical data neither support nor refute this hypothesis.

As drilling in Hole U1309D proceeded beyond the reentry depth of 132 m (see “Operations”), no further basaltic or diabase intervals were encountered during Expedition 304 and gabbroic rock types predominated, with only minor serpentinized peridotite. As in the shallower part of Hole U1309D, the gabbroic units can be divided into gabbro zones characterized by their dominant rock types (Figs. F35, F48). Overall, there are 10 such zones in the core recovered during Expedition 304.

Basalts

Rock types equivalent to the upper basaltic sequence of Hole U1309B were not recovered in Hole U1309D. However, the interval in which correlative lithologies would be expected was not cored above ~20 mbsf (see “Operations”).

Aphanitic basalt and breccia

• Depths: ~23 and ~32 mbsf
• Units: 2, 4, and 7
• Intervals: Sections 304-U1309D-1R-2, 2–119 cm, 1R-3, 3–18 cm, and 4R-1, 62–65 cm

In Hole U1309D, basalts and basalt breccias equivalent to the distinctive aphanitic to fine-grained basalt sequence of Hole U1309B (~21–29 mbsf) are present in three short intervals with a total recovered (and maximum) thickness of ~1 m, significantly less than in Hole U1309B. The basaltic intervals are interspersed between intervals of Diabases D-1 and D-2. Even though some of the basalt breccias contain small diabase clasts, these are finer grained than the adjacent diabase units, which appear to have intruded, and perhaps displaced, the basaltic sequence.

Diabase

There are six “diabase” intervals in Hole U1309D. The majority are medium grained with well-developed ophitic textures, closely resembling those of Hole U1309B, and we have not been able to identify distinctive petrographic characteristics that could establish correlations between the two holes.

Diabase D-1

• Depth: ~20–23 mbsf
• Unit: 1
• Interval: Sections 304-U1309D-1R-1, 0 cm, through 1R-2, 118 cm

This is a single intrusive unit with fine-grained margins grading to a medium-grained interior. Recovered thickness is ~1 m, but because coring began in this interval, the maximum thickness is not known but must be ≤23 m.

Diabase D-2

• Depth: ~24–32 mbsf
• Units: 5 and 6
• Interval: Sections 304-U1309D-1R-3, 55 cm, through 4R-1, 59 cm

Diabase D-2 includes two lithologic units. The diabase of Unit 5 is medium grained throughout, but it is more strongly (almost completely) altered approaching its top and bottom margins than at its center. The diabase of Unit 6 is finer grained, but because of the gap between Cores 304-U1309D-2R and 4R, it is unclear whether it represents a marginal phase of the same intrusion.

Diabase D-3

• Depth: ~43–48 mbsf
• Units: 12 and 14
• Interval: Sections 304-U1309D-6R-2, 50 cm, through 7R-3, 5 cm

Diabase D-3 is 5–6 m thick (5.2 m recovered). It has fine-grained margins at the top and bottom grading over ~0.5 m to a typical medium-grained diabase. Toward the center of the unit, grain sizes are as coarse as 3 mm. Between Units 12 and 14, a narrow interval of olivine gabbro rubble (Unit 13; Section 304-U1309D-7R-1, 6–48 cm) may represent a narrow screen between separate intrusions, but because of the fine-grained top and bottom margins in Units 12 and 14, respectively, it appears more likely that the gabbro pieces are out of place, derived from Unit 11. At the lower boundary of Diabase D-3, an intrusive contact with the underlying gabbro is preserved in interval 304-U1309D-7R-3 (Piece 3, 9–19 cm) (Fig. F49).

Diabase D-4

• Depth: ~55 mbsf
• Unit: 22
• Interval: Section 304-U1309D-9R-1, 9–44 cm

Diabase D-4 is a narrow interval (<50 cm recovered; <2 m maximum) within olivine gabbro. It is represented in the core by several small pieces, of which only the lowermost appears to have been cut by the drill bit. It is fine grained and very similar to Unit 6 (Diabase D-3). It is possible that this unit is entirely derived from Unit 6 and out of place.

Diabase D-5

• Depth: ~82–94 mbsf
• Units: 42, 44, and 46
• Interval: Sections 304-U1309D-14R-2, 112 cm, through 16R-4, 124 cm

Diabase D-5, with a recovered thickness of ~10 m (maximum = ~16 m), is the thickest of all Hole U1309D diabases and is comparable to the lower diabase of Hole U1309B. This similarity is consistent with the suggestion, based on MS profiles, that these units are correlated (see “Physical properties”). A fine-grained interval (~0.5 m) is present at each margin, whereas the interior has a uniform fine to medium grain size. The upper fine-grained margin, recorded as Unit 13 and described as basalt, was recovered at the base of interval 304-U1309D-14R-2, 112–149 cm, and is not contiguous with the remainder of the diabase. The lower margin is contiguous with the coarser interval above, whereas its lower boundary has been intruded by a fine-grained black basalt dike (see “Late basalt dikes”). The contact between this dike and the diabase that lies immediately above was not recovered, but at the lower margin of the dike, a short contact with a very small fragment of fine-grained basalt (very similar to the marginal phase of the basalt above) is preserved (Fig. F50) (interval 304-U1309D-16R-4 [Piece 13, 144–147 cm]). At the very top of the next section (Section 304-U1309D-16R-5 [Piece 1, 0–2 cm]; archive half only), what appears to be a very small fragment of the same fine-grained marginal diabase has intrusive contacts, both with an equally small fragment of the basaltic dike material and with the underlying short interval of cataclastically deformed, fine-grained basalt. If this interpretation is correct, this lowermost deformed diabase was emplaced and deformed before the emplacement of the main body of Diabase D-5 above.

Diabase D-6

• Depth: ~117–127 mbsf
• Units: 52 and 54
• Interval: Sections 304-U1309D-20R-1, 7 cm, through 22R-1, 112 cm

Diabase D-6 is finer grained overall than the other diabases. Sparse aphyric plagioclase phenocrysts are scattered throughout. Recovered thickness is ~4 m, but the maximum possible thickness could be as much as 10 m. At its lower margin, it grades from fine grained to microcrystalline over ~1 m. A finer grained upper margin was not recovered.

Gabbros

Gabbro Zone 1

Gabbro Zone 1 in Hole U1309D is a layered intrusive sequence that includes troctolite, troctolitic gabbro, olivine gabbro, and gabbro (Fig. F48). The layered sequence extends from ~32 to 61 mbsf (Fig. F35) and is intruded by Diabases D-3 and D-4. The overall recovered thickness (not including the diabase intervals) is ~12 m, with a possible maximum thickness of almost 25 m. Olivine-rich rock types are dominant. Estimated volume proportions, based on lithologic interval lengths, are 28% gabbro, 40% olivine gabbro, and 32% layered troctolite and troctolitic gabbro.

The ~25 m drilled thickness of this upper gabbroic sequence can be divided into five lithologic zones (G1-1 to G1-5 in Fig. F35), each between ~2 and 10 m in thickness. Within each zone, one lithology is dominant, although other lithologies are commonly present as gradational, magmatic, and/or intrusive layers ranging from a few centimeters to ten or more centimeters in thickness. In order downhole, the lithologic zones are as follows.

• Zone G1-1: gabbro
• Depth: ~32–34.5 mbsf
• Unit: 8
• Interval: Sections 304-U1309D-4R-1, 67 cm, through 4R-3, 8 cm

Zone G1-1 is dominated by coarse-grained, brecciated gabbro lithologically similar to but significantly more altered than the upper gabbro sequence of Hole U1309B. Cataclastic textures and networked yellow-green alteration veins are abundant. The contact between the brecciated gabbro and overlying diabase (D-2) was not recovered, but a single small piece of brecciated, fine-grained basalt (Unit 7) appears to record an intrusive contact of diabase into gabbro.

• Zone G1-2: troctolitic gabbro
• Depth: ~34.5–40.4 mbsf
• Unit: 9
• Interval: Sections 304-U1309D-4R-3, 8 cm, through 5R-4, 21 cm

Zone G1-2 is a ~6 m thick interval of medium-grained troctolitic gabbro, within which grain size generally increases downhole and modal compositions are slightly variable. Gabbroic domains (2–3 cm in size) with slightly larger grain size are present in Section 304-U1309D-5R-3 (Pieces 6 and 7, 43–66 cm).

• Zone G1-3: olivine gabbro
• Depth: ~46–51 mbsf
• Units: 11, 13, 16, and 17
• Interval: Sections 304-U1309D-7R-1, 6 cm, through 8R-1, 119 cm

Zone G1-3 is characterized by medium- to coarse-grained olivine gabbro (Units 11 and 13) with interlayered gabbro (Units 16 and 17). It is intruded by Diabase D-3 (Units 12 and 14). In several units, downhole coarsening from medium to coarse grain is observed, and there is an irregular, abrupt transition from medium- to coarse-grained gabbro (Units 16–17) in interval 304-U1309D-8R-1 (Piece 4, 19–23 cm). Weak magmatic foliation is overprinted by crystal-plastic deformation in Units 11 and 13. An intrusive contact of Diabase D-3 into gabbroic rocks was recovered in Section 304-U1309D-7R-3 (Piece 3, 11–20 cm) (Fig. F49).

• Zone G1-4: troctolite and troctolitic gabbro
• Depth: ~52–53 mbsf
• Units: 18 and 19
• Interval: Sections 304-U1309D-8R-1, 120 cm, through 8R-2, 133 cm

Zone G1-4 is characterized by interlayered troctolite and troctolitic gabbro with minor gabbro layers in Unit 18. Unit 18 has been deformed and has a well-developed mylonitic texture. Its contact with the overlying olivine gabbro was recovered in interval 304-U1309D-8R-1 (Piece 15, 126–130 cm). In intervals 304-U1309D-8R-2 (Pieces 1 and 2, 0–27 cm, and Piece 7, 66–79 cm), both magmatic layering and faulted contacts between troctolite and olivine gabbro were recovered (Fig. F51).

• Zone G1-5: olivine gabbro and gabbro
• Depth: ~56–61 mbsf
• Units: 23–27
• Interval: Sections 304-U1309D-9R-1, 45 cm, through 10R-1, 99 cm

Zone G1-5 is characterized by broader scale interlayering of medium-grained olivine gabbro (Units 23, 25, and 26) with coarse-grained gabbro (Units 24 and 27). Both lithologies are heterogeneous in modal composition and grain size.

Gabbro Zone 2

Gabbro Zone 2 extends from ~61 to 138 mbsf and comprises the same range of rock types as Zone 1, including troctolite, troctolitic gabbro, olivine gabbro, and gabbro (Fig. F48). Estimated volume proportions are 13% gabbro, 15% olivine gabbro, and 72% layered troctolite and troctolitic gabbro, suggesting that olivine-rich lithologies are significantly more common than in the first sequence. Diabase intrudes the lower gabbro series at ~82–94 mbsf (Diabase D-5) and ~117–127 mbsf (Diabase D-6).

Gabbro Zone 2 is subdivided into five lithologic zones (G2-1 to G2-5 in Fig. F35).

• Zone G2-1: layered troctolite (with olivine gabbro and gabbro)
• Depth: ~62–75 mbsf
• Units: 32–34
• Interval: Sections 304-U1309D-10R-2, 0 cm, through 13R-1, 10 cm

This thick (~13 m recovered; 15 m maximum) zone of troctolite and olivine gabbro is characterized by alternating 10–50 mm thick layers of medium-grained troctolite and olivine gabbro with occasional gabbroic bands (Fig. F52) (interval 304-U1309D-11R-2, 0–20 cm; see also intervals 10R-2, 55–57 cm, and 12R-1, 69–84 cm).

• Zone G2-2: gabbro
• Depth: ~75–77 mbsf
• Units: 35–37
• Interval: Sections 304-U1309D-13R-1, 11 cm, through 13R-2, 142 cm

Gabbros of Zone G2-2 are characterized by coarse grain size (e.g., interval 304-1309D-13R-2, 45–65 cm). Olivine is rare. Orthopyroxene is sporadically present as subhedral to anhedral grains as large as 6 mm in the lower part of Unit 35 (interval 304-U1309D-13R-1, 50–107 cm). In a few patches, modal orthopyroxene exceeds 5%, defining the host rock as gabbronorite (e.g., interval 304-U1309D-13R-1, 68–70 cm).

• Zone G2-3: layered troctolite (with olivine gabbro and gabbro)
• Depth: ~77–82 mbsf
• Units: 38–41
• Interval: Sections 304-U1309D-13R-3, 0 cm, through 14R-2, 110 cm

The principal rock types in Zone G2-3 are interlayered troctolites (~30 mm thick) and (olivine) gabbros (~10 mm thick) (e.g., interval 304-1309D-14R-1 [Piece 11, 61–73 cm]). Coarse-grained clinopyroxene as large as 10 mm is observed in interval 304-U1309D-14R-1 (Pieces 8 and 9, 46–56 cm).

• Zone G2-4: olivine gabbro
• Depth: ~99–103 mbsf
• Unit: 49
• Interval: Sections 304-U1309D-17R-1, 17 cm, through 13R-3, 131 cm

This 3.5 m thick zone is characterized by coarse-grained gabbro with very variable modal mineral proportions.

• Zone G2-5: layered troctolitic gabbro
• Depth: ~127–130 mbsf
• Unit: 55
• Interval: Sections 304-U1309D-22R-1, 112 cm, through 22R-3, 89 cm

Zone G2-5 is dominated by troctolitic gabbro. Its recovered thickness is ~2.5 m. Modal proportions of the dominant minerals are variable, and bands of anorthositic olivine gabbro (e.g., interval 304-U1309D-22R-2, 27–30 cm), anorthositic troctolite (e.g., interval 22R-2, 48–51 cm), and typical troctolitic gabbro (e.g., interval 304-U1309D-22R-3, 39–42 cm) are observed. Narrow, branching, clinopyroxene-rich gabbroic dikes cut the section at relatively high angles in interval 304-U1309D-22R-2, 72–84 cm. Two kinds of clinopyroxene can be distinguished in hand specimen: one is dark in color; the second has brown rims and an unusual pastel green core with apparent exsolution lamellae visible under the binocular microscope.

Gabbro Zone 3

Zone 3 extends between 138 and 180 mbsf (interval 304-U1309D-24R-1, 18 cm, through 33R-3, 108 cm) and is dominated by medium- and coarse-grained olivine and olivine-bearing gabbro that grades locally to troctolitic intervals and patches and, in places, to troctolite (Fig. F48). Neither zone boundary was recovered, and the exact relationships of Zone 3 gabbroic units to those of the adjacent zones are unknown. The uppermost unit is dunite that is almost completely serpentinized and grades downhole into serpentinized troctolite, and below this narrow (<2 m) upper subzone, there is an overall downhole increase in modal olivine (Fig. F53) within the interval 150–175 mbsf. In several intervals, orthopyroxene, which was only rarely recognized macroscopically elsewhere at Site U1309 during Expedition 304, is present. These include a gabbronorite at ~145.4 mbsf and two orthopyroxene-bearing intervals (145.4–148.3 and ~160–161 mbsf) in which relatively large (up to ~1 cm) orthopyroxene grains are sporadically present within otherwise olivine-bearing gabbro (Fig. F54). Beginning at ~148 mbsf, an unusual “salt-and-pepper” mottled matrix appears in the core as an interstitial phase between clinopyroxene and/or relict olivine grains (Fig. F55). This feature, which is present at least through gabbro Zone 7, was initially interpreted as invasive fine-grained dioritic material. In thin section, however, it is clear that most or all of it is plagioclase that has recrystallized into small, uniform, 0.1–0.2 mm neoblasts. Pervasive fine fractures in this material are filled with green actinolite. The principal visual effect of this texture appears to be one of contrast between larger transparent plagioclase grains that appear black in hand specimen and smaller aggregates of neoblasts that appear white.

The gabbroic sequence of Zone 3 is frequently interrupted by narrow, late-stage magmatic leucocratic dikes (see also “Metamorphic petrology”) that frequently invade and impregnate the surrounding gabbroic material, possibly contributing locally to the development of salt-and-pepper plagioclase alteration. Although dikes of this type are present throughout the gabbroic sections at Site U1309, they appear to be particularly common in Zone 3. Occasional narrow dikes of both coarse gabbro and microgabbro are also present throughout Zone 3. Also interrupting the olivine-bearing sequence are two serpentinite intervals. The first, at ~155 mbsf, is a ~20 cm interval of completely serpentinized harzburgite, for which the contacts with the surrounding gabbroic rocks were not recovered. The second, between 171.5 and 173.5 mbsf, is a complex interval of serpentinized dunite, harzburgite, and olivine-rich troctolite lithologies that appears to have been intruded by a coarse-grained gabbro dike (see “Peridotites”).

Gabbro Zone 4

Zone 4 extends from 186 to 208 mbsf (interval 304-U1309D-34R-1, 0 cm, through 34R-1, 58 cm) and is dominated by heterogeneous medium-grained gabbro grading to olivine gabbro and, rarely, to troctolitic gabbro (Fig. F48). Coarser grained patches are common, and, in places, a weak magmatic layering is defined by variations in mode and/or grain size. Coarser clinopyroxene is present, intermittently dispersed in a medium-grained matrix.

A 6 m thick coarse-grained oxide gabbro, the thickest encountered at Site U1309 during Expedition 304, is present in the middle of this zone (Fig. F56). The upper contact is not preserved, but the lower contact appears to be intrusive into the olivine gabbro with a narrow medium-grained chilled margin (Fig. F57). Alternating coarse-grained and pegmatitic intervals are a prominent characteristic of this unit. Oxide minerals throughout the interval are mainly disseminated. The top of Core 304-U1309D-36R has the most abundant oxide, with as much as 20% modal content. Emplacement of this and other oxide gabbros appears to be among the last igneous events recorded, at least in the deeper part of the section recovered during Expedition 304.

Gabbro Zone 5

Zone 5 extends from ~210 to 265 mbsf (interval 304-U1309D-39R-1, 58 cm, through 50R-3, 122 cm) and is dominated by medium- and coarse-grained olivine gabbro that commonly grades to olivine-bearing intervals, to troctolitic intervals and patches, and, occasionally, to troctolite (Fig. F48). From ~255 to 256 mbsf, the olivine-bearing sequence is interrupted by a 1.2 m thick oxide gabbro with 2%–5% oxides, and, from 238 to 244 mbsf, intermittent oxide-bearing patches are present within an otherwise olivine-bearing host gabbro. The gabbro sequence is also interrupted at 224 mbsf by a single piece of serpentinized harzburgite. No contacts are preserved, and, because this irregular piece was recovered at the top of a core (Section 304-U1309D-42R-1), it is most likely not in place. It was noted that a section of alternating olivine gabbro and troctolitic gabbro in Cores 304-U1309D-39R and 40R preceded the occurrence of the harzburgite in Core 41R. Sporadic narrow dikes of both coarse gabbro and microgabbro are present throughout.

At the upper zone boundary, medium-grained olivine gabbro is in sharp intrusive contact with the coarse-grained gabbro of Zone 4. A few centimeters below the contact, a narrow, irregular gabbro dikelet only 1–2 cm thick intrudes the olivine gabbro (Section 304-U1309D-39R-1, 50–52 cm) (Fig. F58). Near the bottom of Zone 5, in Core 304-U1309D-49R, a section of highly deformed olivine gabbro with a minimum thickness of 1.3 m occurs. This deformed section is located between undeformed rocks above and below. The lower boundary of Zone 5 was not recovered.

Gabbro Zone 6

Zone 6 extends from ~268 to 282 mbsf (interval 304-U1309D-51R-1, 17 cm, through 54R-2, 62 cm) and is dominated by coarse-grained gabbros (Fig. F48), characterized by the presence of large (~1 cm) clinopyroxene grains. In some places, the crystal distribution becomes patchy; in others, coarse-grained gabbro grades into a medium-grained matrix with sparsely and randomly dispersed grains (Fig. F59); in a few places, sharp contacts define short, apparently intrusive intervals (Figs. F60, F61). Similar intrusive gabbros, ranging from the width of a single crystal to 10 cm or more, are present throughout the section below ~180 mbsf and are common in gabbro Zones 8 and 10.

At the base of Zone 6 is a ~1 m thick coarse-grained gabbro. The upper boundary of this unit was not recovered, but a lower intrusive contact with troctolite of gabbro Zone 7 is preserved in interval 304-U1309D-54R-2, 53–62 cm (Fig. F62).

Gabbro Zone 7

Zone 7 extends from ~281 to 310 mbsf (interval 304-U1309D-54R-2, 63 cm, through 59R-4, 16 cm) and is dominated by coarse- and medium-grained olivine gabbro that grades intermittently into both troctolitic and olivine-bearing gabbro (Fig. F48). At the upper zone boundary, troctolite of Zone 7 is in intrusive contact with the apparently younger gabbro of Zone 6. At the lower zone boundary, olivine-bearing gabbro with a narrow, finer grained margin intrudes serpentinized dunite of Zone 8. (Fig. F63).

Modal olivine decreases overall downhole in Zone 7 (Fig. F53); troctolitic variants are more common near the top of the zone. The gabbro sequence is interrupted at ~291 and ~301 mbsf by intervals of serpentinized dunite that are 0.25 and 0.7 m thick, respectively. Beneath the upper dunite, a 3 m interval of troctolitic gabbro that closely matches similar intervals in the underlying zone (Zone 8) represents the shallowest limit of pervasive serpentinization of gabbroic rocks in Hole U1309D. The olivine-bearing gabbro sequence is also interrupted by a number of gabbro dikes ranging in thickness from a few centimeters to ~1 m (Fig. F64).

Gabbro Zone 8

Zone 8 extends from ~312 to 344 mbsf (interval 304-U1309D-60R-1, 0 cm, through 67R-1, 60 cm) and is dominated by troctolitic gabbros (Fig. F48) that are distinctly more olivine rich than those of the other zones (Figs. F53, F65). The upper zone boundary is an interfingering, intrusive contact with olivine-bearing gabbro of Zone 7 (see “Gabbro Zone 7,” above). The lower boundary was not recovered. Throughout the zone, where olivine is dominant it has been replaced by serpentine. The troctolitic gabbros are characterized by modal olivine >50% with common interstitial plagioclase (Fig. F66). They belong to the olivine-rich troctolite group (although they were called dunitic troctolites in the Expedition 305 Preliminary Report) as defined below in “Hole U1309D (Expedition 305).”

Clinopyroxene, frequently occurring as large oikocrysts, is also a common interstitial phase. In several places, troctolitic gabbro grades over a few centimeters to dunite. A 3 m thick oxide gabbro interval (321–324 mbsf; interval 304-U1309D-62R-1, 132 cm, to 63R-1, 10 cm) interrupts, and most likely intrudes, the troctolitic gabbro sequence. There are two significant coarse-grained gabbro intervals, each ~2 m thick, at ~317 and ~336 mbsf. Several narrow coarse-grained dikes and a single microgabbro also intrude the troctolitic gabbro sequence (Fig. F67).

Gabbro Zone 9

Zone 9 extends from ~344 to 356 mbsf (interval 304-U1309D-67R-1, 61 cm, through 69R-2, 124 cm) and consists of a coarse-grained gabbro that is intermittently olivine bearing and an underlying oxide gabbro (Fig. F48). The gabbro, which is 8–9 m thick, is coarser grained (~3 cm) than that of gabbro Zone 6 but otherwise very similar. The upper boundary of Zone 9 was not recovered, but intrusive contact with the underlying troctolitic gabbro of Zone 10 is present in Section 304-U1309D-69R-2, 118–133 cm (Fig. F68). As with other gabbro zones (Zones 4 and 6), an oxide gabbro interval is present at the base of Zone 9. The intrusive relationships of this oxide gabbro are complex; it is separated into two units by intervening short intervals of gabbro (upper) and troctolite (lower), between which an intrusive contact is preserved (Fig. F69). The contact between this troctolite and underlying oxide gabbro is obscured by subsequent alteration, and the contact between this gabbro and the upper part of the oxide gabbro was not recovered.

A tentative interpretation is that coarse-grained gabbro (Zone 9) originally intruded troctolitic gabbro from Zone 10 and a later oxide gabbro intrusion bifurcated at the location of Hole U1309D, isolating the original gabbro/troctolite contact as a narrow screen within the oxide gabbro.

Gabbro Zone 10

Zone 10 extends from ~358 to 402 mbsf (interval 304-U1309D-69R-2, 124 cm, through 78R-4, 100 cm), and the upper portion is dominated by relatively uniform plagioclase-rich (leucocratic) troctolitic gabbros. Beginning in Section 71R-3, olivine gabbro becomes the dominant rock type, with frequent transitions to layers or irregular domains of troctolitic gabbro (Fig. F48). Modal plagioclase exceeds 70% in the leucocratic subzone, but below ~375 mbsf there is a steady downhole decrease in modal plagioclase with this zone. Olivine abundances increase downhole throughout the zone (Fig. F53). In the uppermost troctolitic gabbros, the serpentinization of residual olivine that characterizes gabbro Zone 8 (overprinting the typical high-temperature alteration of olivine to actinolitic amphibole and chlorite) continues through Section 304-U1309D-71R-2, 75 cm; below this level, the intensity of serpentinization appears to be significantly diminished.

The olivine gabbros are medium to coarse grained with large (2–3 cm) prismatic clinopyroxene crystals having a patchy distribution though much of the zone (Fig. F70); presences range from small individual grains and clumps to small dikes with essentially the width of a single crystal to discrete, apparently intrusive small dikes that are typical of those that are present through much of Hole U1309D (Fig. F71).

Late basalt dikes

• Depths: ~48, 94 mbsf
• Units: 15 and 45
• Intervals: Sections 304-U1309D-7R-3, 8–12 cm, and 16R-4, 124–146 cm

Black fine-grained basalt dikes with a distinctive irregular fracture pattern on the outer core surface intrude gabbros from the upper part of Hole U1309D in two places. Lithologically, these dikes appear identical to the more numerous similar intrusions in Hole U1309B. The upper Hole U1309D dike is the only place where this lithology is present in the upper series of either hole. This dike is narrow (~1 cm thick) and branching, with an irregular contact geometry that is best observed in the working half of the core (Fig. F49) (interval 304-U1309D-7R-3, 11–21 cm). It cuts across the boundary between Diabase D-3 and the underlying gabbro of Zone G1-3. The lower dike is actually a composite, with at least one internal intrusive contact displaying both a chilled margin and local brecciation in thin section (e.g., interval 304-U1309D-16R-4, 143–146 cm). In hand specimen, small remnants of basalt at the lowermost end of Section 16R-4 and of both aphanitic basalt and black diabase at the very top of Section 304-U1309D-16R-5 indicate that this basalt dike intruded an earlier intrusive contact between the main body of Diabase D-5 and an earlier, now deformed, finer-grained basalt (see “Diabase D-5”).

Peridotites

Ultramafic rocks of probable mantle origin

Ultramafic rocks recovered at Site U1309 are of two types: those that have clearly cumulate textures and those that lack cumulate characteristics. Many of the latter commonly have characteristics that suggest that they originated in the mantle, but some are too completely serpentinized for their origin to be reliably inferred.

Ultramafic rocks that are interpreted to be of mantle origin were recovered at ~60 mbsf in Hole U1309B and at four different depths in Hole U1309D. Total recovered thickness in Hole U1309D is 140 cm. Compositions of these rocks range from dunite to lherzolite, and all show evidences of intensive melt impregnation. The degree of alteration varies from 10% to 100%, but most are extensively altered; dunites are the most severely altered.

• At 61.1 mbsf, close to the depth of the harzburgite recovered in Hole U1309B, ~20 cm of plagioclase-lherzolite (Fig. F72A) was drilled. It has an intrusive contact with the overlying troctolite. The underlying unit is a coarse-grained gabbro dike, but the boundary between these units was not recovered. The sample is cut by several dikes and veins.
• At 155.08 mbsf, plagioclase harzburgite (Fig. F72B) is present at the bottom of Core 304-U1309D-27R. The presence of fault rocks recovered directly above suggests a tectonic emplacement.
  
• At 171.61 mbsf, serpentinized harzburgite (Fig. F72C) and dunite (Fig. F72D) were recovered in Core 304-U1309D-31R. These ultramafic rocks are intruded by a coarse-grained gabbro dike that shows evidence of brittle deformation. The lower contact to either gabbro or ultramafic rocks is gradational, and cataclastic textures are observed again in the gabbro.
• At 224.3 mbsf, a single piece of serpentinized harzburgite (Fig. F72E) was recovered at the top of Core 304-U1309D-42R. Because of its small size, this sample is probably not in place and may have fallen downhole. However, it does not closely match any of the earlier recovered material and is described individually.

Within all of these samples, distinct modal variations provide evidence of intensive melt impregnation. These include interstitial plagioclase, scattered melt-derived clinopyroxenes, and olivine chadacrysts in larger clinopyroxene oikocrysts. Similar textural features in samples from the MAR (DSDP Leg 37; ODP Legs 153 and 209), Hess Deep, and the Oman ophiolite have been interpreted as originating from impregnation with a plagioclase-wehrlite melt.

• Impregnated lherzolite
• Gabbro Zone 1
• Depth: 61.1–61.31 mbsf
• Interval: Section 304-U1309D-10R-1, 90–111 cm

The uppermost mantle peridotite in Hole U1309D resembles the harzburgite from Hole U1309B in hand specimen. It contains abundant (35%), rounded pyroxenes that form clusters in the (former) olivine matrix. The degree of serpentinization varies from <10% to as much as 100%. Modal amount of spinel is low (<1%).

The lherzolite is in direct contact with the overlying medium-grained troctolite. Several crosscutting dikes and veins intrude the peridotite. The earliest is of gabbroic composition, followed by magmatic hornblende. Both are cut by a steeply dipping talc-tremolite vein and by randomly oriented carbonate veins. In thin section, the contact between peridotite and the adjoining troctolite interfingers with distinct alteration styles in each lithology. The troctolite appears to be intrusive into peridotite. Olivine in the peridotite is serpentinized (Fig. F73A), whereas olivine in the troctolite is completely replaced by talc (Fig. F73B). Interstitial plagioclase in both areas is almost completely altered to chlorite that appears dark in hand specimen. Residual olivine often has undulose extinction, and subgrain boundary formation is common. These characteristics are typical of mantle peridotites that have undergone mantle flow.

Several textural characteristics are typically indicative of melt impregnation in mantle peridotites. Unstrained olivine grains, and sometimes equant chromite, are often enclosed by poikilitic clinopyroxene that is, in turn, surrounded by interstitial plagioclase (Fig. F73C). Other pyroxene and plagioclase grains are heterogeneously distributed but increase in abundance toward the contacts with both the troctolite country rock and the gabbro dike (Fig. F73D). This is consistent with impregnation by intruding magmatic fluid(s). Finally, in one thin section (Sample 304-U1309D-10R-1, 99–102 cm), clinopyroxene forms a vermicular intergrowth with plagioclase (Fig. F73E) and sometimes replaces earlier clinopyroxene (Fig. F73F).

• Plagioclase-harzburgite
• Gabbro Zone 3
• Depth: 155.08–155.26 mbsf
• Interval: Section 304-U1309D-27R-3, 0–18 cm

A single piece and small pebbles of plagioclase-bearing peridotite were recovered at the bottom of Core 304-U1309D-27R (interval 27R-3, 0–18 cm). Plagioclase and olivine are completely altered, but pyroxenes are commonly fresher. No evidence for crystal-plastic deformation was observed. Coarse orthopyroxene often contains clinopyroxene exsolution lamellae. Fresh clinopyroxene is present as single grains as well as blebs and exsolution lamellae in orthopyroxene (Fig. F74A). One clinopyroxene contains inclusions that may have been olivine prior to alteration (Fig. F74B). Those are completely altered and often contain euhedral oxides. Spinel is equant and almost opaque. Plagioclase is present in small amounts, mostly along a discontinuous train in the matrix.

• Dunite and plagioclase harzburgite
• Gabbro Zone 3
• Depth: 171.61–173.63 mbsf
• Intervals: Sections 304-U1309D-31R-1, 11 cm, and 31R-2, 67 cm

The longest recovered section of peridotite in Hole U1309D (overall = 88 cm) was drilled at 171.61 mbsf (interval 304-U1309D-31R-1, 11 cm, to 31R-2, 67 cm). It consists of two intervals separated by a 0.7 m interval of deformed coarse-grained gabbro. The uppermost peridotite interval consists of dunite that is almost completely serpentinized, with almost no relict features. However, some coarse spinel grains remain, and these often contain olivine inclusions that have been partly protected from alteration.

The lower interval of harzburgite is in direct contact with gabbro, and talc-tremolite veins are present along the contact and within the harzburgite. Two thin section billets were cut from the harzburgite interval, one next to the contact to gabbro in Section 304-U1309D-31R-1 (and within the talc-tremolite zone) and the other in Section 31R-2; the latter was chosen in an attempt to avoid most of the alteration veins. The thin section from the contact area reveals strong talc alteration of all silicate phases. Carbonate is abundant and present in the serpentine mesh. Pyroxene grains are heavily altered to talc except for rare clinopyroxene exsolutions and blebs in altered orthopyroxene. Spinels are equant to interstitial in shape and often have olivine inclusions. Plagioclase is widespread in thin section but is invariably altered to chlorite and seems to be randomly distributed. Pyroxenes and olivine appear to be undeformed, but static replacement during alteration may have obliterated deformation textures. Some less altered pyroxenes show no preferred orientation and straight extinction, and spinel is undeformed with serrated grain boundaries.

Thin section Sample 304-U1309D-31R-2, 46–48 cm, from the bottom part of the interval, contains fresh cores of orthopyroxene and olivine that are mostly cut by serpentine veins but are not completely altered. Olivine exhibits subgrain boundaries and a weak LPO indicative of mantle flow. Two grains of clinopyroxene are fresh and contain olivine inclusions. They show corrosive boundaries with each other (Fig. F75A). Coarse spinels have mostly equant shapes and dark red colors. Plagioclase is confined to a narrow zone along the long axis of the thin section, where it is present in the matrix (Fig. F75B). Within this zone, spinels are often rimmed by plagioclase, whereas farther away, no plagioclase coronas could be observed. Pyroxenes have very elongated grains with length:width ratios >1:3.

• Impregnated harzburgite
• Gabbro Zone 5
• Depth: 224.3–224.39 mbsf
• Interval: Section 304-U1309D-42R-1, 0–9 cm

This sample is serpentinized (~70%) harzburgite with a talc-tremolite vein cutting along one edge. Olivine grains are commonly cut by serpentine veins, but grain boundaries, including some subgrain boundaries, are still recognizable. Pyroxene is present as oikocrysts enclosing olivine (Fig. F76A) and often forms heterogeneously distributed clusters (Fig. F76B) or trails. It has been mostly replaced by alteration phases. No plagioclase or plagioclase relics could be identified. The significant number of clinopyroxenes enclosing olivine and the absence of plagioclase are notable features in this sample, which differs from the peridotites previously described.

Hole U1309D (Expedition 305)

The gabbroic rocks recovered during Expedition 305 seamlessly form the continuation of the lithological sequence at the bottom (401.3 mbsf) of Hole U1309D from the end of Expedition 304. The downhole igneous stratigraphy for the entire hole is shown in Figure F77. The rock types and their proportions for the entire hole (and separated by expedition) are shown in Figures F77 and F78. Between 401.3 and 1415.5 mbsf, the most common lithology is medium- to coarse-grained (rarely pegmatitic) gabbro, which forms 60.8% of the rocks recovered during Expedition 305. The gabbroic rocks have a broad range in modal composition, commonly including minor quantities (rarely exceeding 10%) of olivine, Fe-Ti oxides, and/or orthopyroxene. Intercalated with these gabbros are medium-grained olivine gabbros, which have modal olivine that nearly always exceeds 20%. The modal composition of olivine-bearing rocks is highly variable on a submeter scale as well and, in places, frequently grades into rocks of troctolitic gabbro composition. Together, olivine and troctolitic gabbro form 23.1% of the section, the second most abundant lithology (Figs. F78, F79A). Spatially associated with the olivine- and troctolitic gabbros are troctolites with a dominantly seriate texture. The troctolites are less common and only contribute 2.2% of the lithologies recovered during Expedition 305.

A rarely recovered lithology at mid-ocean ridges is olivine-rich troctolite that contains >70% olivine and, in contrast to troctolite, has a cumulate-like texture with subhedral to rounded olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions. In places, these rare primitive rocks are very fresh. The downhole abundance of the olivine-rich troctolites is 5.4%, and they are restricted to very confined intervals.

Oxide gabbro (7.7% of the rocks recovered) is defined by the occurrence of >2% modal Fe-Ti oxide minerals (Fig. F79B). The oxide-bearing gabbros are most common within the gabbro sequence, and the Fe-Ti oxides are present as heterogeneously distributed patches or seams. The oxide gabbros can be discrete dikelets crosscutting other rock types or can display strong enrichment in ductile deformation zones. In general, oxide gabbros appear to be a common lithology along slow-spreading mid-ocean ridges, and in addition to Hole U1309D, they have been reported from other drilling locations along the MAR (Legs 153 and 209) and the SWIR (Hole 735B).

Finally, the gabbroic rocks have been crosscut by a number of diabase intrusions.

Olivine-rich troctolites

The term “olivine-rich troctolite” is assigned to olivine-rich rocks with relatively low modal abundance of plagioclase and clinopyroxene and a classic cumulate texture. They form a volumetrically minor component (5.4%) of the section of Hole U1309D recovered during Expedition 305 (Fig. F78). Their modal composition is 70%–90% olivine, 5%–20% plagioclase, 0%–15% clinopyroxene (Fig. F80), and, in places, up to 2% chromian spinel. Their modes commonly change on a decimeter scale, mainly as a result of variable plagioclase and clinopyroxene proportions. Typical features of each olivine-rich troctolite interval are shown in Figures F81, F82, F83, F84, F85, F86, F87, and F88. Representative photomicrographs are shown in Figures F89 and F90. Applying the International Union of Geological Sciences (IUGS) terminology, they can be classified as olivine gabbro, troctolite, wehrlite, or, locally, even dunite. Texturally, they consist of a medium-grained subhedral to rounded olivine matrix and interstitial to poikilitic plagioclase and clinopyroxene (e.g., Figs. F89, F90).

We have adopted the informal descriptive term “olivine-rich troctolite” as a practical tool to facilitate core description and subsequent statistical analysis. This approach is needed for four reasons:

1. To avoid recombining four different rock names that would inevitably result from an IUGS-conforming classification;
2. To clearly distinguish olivine-rich troctolites from true dunites from the olivine gabbros and associated “modifier-free” troctolites. The latter have far less modal olivine and clearly distinct equigranular to seriate texture;
3. To avoid the process-related term “cumulate,” which imposes a subjective and premature interpretation; and
   
4. To avoid the much-debated term “plagioclase-wehrlite,” as seen in many deep crustal sections of ophiolites, which is commonly inferred to have process-related implications.

Olivine-rich troctolites are present in eight discrete intervals recovered during Expedition 305 (Fig. F77):

1. Interval 305-U1309D-100R-1, 0–128 cm (495.30–496.58 mbsf),
2. Sections 305-U1309D-136R-1 to 136R-2, 58 cm (669.44–671.35 mbsf),
3. A complex interval intruded by a highly altered network of oxide gabbro and diabase dikes in Sections 305-U1309D-140R-2, 56 cm, to 140R-3, 84 cm (689.51–690.98 mbsf),
4. Sections 305-U1309D-227R-2, 32 cm, to 228R-4, 58 cm (1093.75–1101.28 mbsf),
5. Sections 305-U1309D-233R-1, 51 cm, to 238R-1, 123 cm (1093.75–1101.28 mbsf),
6. Sections 305-U1309D-240R-1, 115 cm, to 243R-1, 39 cm (1154.65–1168.29 mbsf),
7. Sections 305-U1309D-247R-1, 2 cm, to 248R-4, 58 cm (1187.12–1196.50 mbsf), and
8. Sections 305-U1309D-256R-1, 4 cm, to 257R-1, 74 cm (1230.34–1235.84 mbsf).

Olivine-rich troctolites form the dominant lithology between 1092 and 1236 mbsf. In the remainder of the hole drilled during Expedition 305, they represent only a minor component (Figs. F77, F78). In the sequence dominated by olivine-rich troctolite, the rocks are mainly intercalated with olivine and troctolitic gabbro and, in places, crosscut by sharply bounded medium- to coarse-grained olivine gabbro dikes. The olivine gabbro consists of subequal proportions of clinopyroxenes and plagioclase with variable low olivine and/or orthopyroxene (e.g., Fig. F85A). A second set of crosscutting gabbro dikes have similar modal contents, but their contacts with the surrounding olivine-rich troctolite are diffuse and difficult to discern.

In hand specimen, most olivine-rich troctolites are easily identified by the presence of fine black serpentine veins or a fine serpentine network, due to their (originally) high modal olivine (e.g., Fig. F88A). Most troctolites contain abundant, irregularly distributed, large (up to 30 mm), undeformed clinopyroxene oikocrysts that enclose olivine and, to a lesser extent, plagioclase (Figs. F81, F90A). Plagioclase commonly fills the interstitial spaces between olivine and pyroxene. In places, the interstitial plagioclase displays a preferred dominantly subhorizontal orientation.

Thin section observations generally confirm the coarse grain size and optical continuity of the clinopyroxene oikocrysts. Within the oikocrysts, plagioclase abundance is generally very low. Occasionally, plagioclase also forms oikocrysts enclosing olivine, but its predominant mode of occurrence is a continuous network of moderately annealed individual grains. Olivine is generally only partially surrounded by plagioclase; 10%–40% of olivine grain boundaries are olivine/olivine contacts. Olivine connectivity is significantly reduced among clinopyroxene-hosted olivines, where olivine/olivine contacts are scarce. Overall, the olivine grain size does not change significantly between clinopyroxene- and plagioclase-hosted texture.

Chromian spinel is a common accessory phase in olivine-rich troctolites. It is present as fine-grained (50–100 µm) subhedral to euhedral, rarely holly leaf shaped crystals, either enclosed within olivine grains or within the plagioclase matrix. In plagioclase-rich, clinopyroxene-free zones, the modal content of chromian spinel is as high as 2%, whereas in clinopyroxene-rich zones, spinel as olivine-hosted inclusions is rare. Roughly one out of twenty chromian spinels is orbicular, containing olivine, plagioclase, or, rarely, brown amphibole inclusions, around which it presumably nucleated.

Each olivine-rich troctolite has different contact relationships with the surrounding lithologic units. Given their potentially crucial importance for the interpretation of the igneous evolution of Atlantis Massif, each contact is described below in stratigraphic order.

Contact 1

The first occurrence of olivine-rich troctolite during Expedition 305 is in Section 305-U1309D-100R-1. Its contact with the lowermost coarse-grained gabbro of Section 305-U1309D-99R-3 at 494.92 mbsf (Fig. F81) was not recovered. The exact length of this interval is not accurately constrained, as a result of an overrecovery of Core 305-U1309D-101R that exceeded the cored interval. In order to avoid two cored intervals with the same depth, the overlying Core 305-U1309D-100R was shifted upward accordingly. The effect of this shift is that the nonrecovered interval is reduced from 168 to 38 cm. Neither this lowermost gabbro nor the uppermost olivine-rich troctolite are significantly deformed. Nevertheless, there is no evidence to either support or discount stratigraphic continuity.

Contact 2

A medium-grained olivine-rich troctolite in Section 305-U1309D-100R-1 grades through troctolitic gabbro to coarse-grained olivine gabbro (Fig. F81B), and details of the transition are obscured by a pervasive metamorphic overprint. The distribution of corona textures suggests that modal olivine decreases gradually as modal plagioclase increases.

Contact 3

The upper contact of the olivine-rich troctolite at 669 mbsf in Section 305-U1309D-136R-2 is missing, but the lower one is well preserved (Fig. F82). Between the lowermost undeformed coarse-grained gabbro of Section 305-U1309D-135R-2 at 667.05 mbsf and the underlying, equally undeformed olivine-rich troctolite of Section 136R-1, there is a no-recovery zone of 230 cm. The lower contact between the olivine-rich troctolite and an almost anorthositic clinopyroxene-poor gabbro is sharp. Included in this underlying plagioclase-rich gabbro are irregular 1–5 cm sized troctolitic patches (Fig. F82B). Along the contact between the anorthositic interval and the olivine-rich troctolite, a trail of well-preserved chromian spinel can be seen in thin section (Fig. F91). The primary igneous plagioclase microstructure in the anorthositic interval is obliterated by plastic deformation. In the olivine-rich troctolite, plagioclase is poikilitic and only weakly strained. In contrast, clinopyroxene in both intervals is largely undeformed.

Contact 4

The upper contact between the olivine-rich troctolite of Section 305-U1309D-140R-2 (Fig. F83) and the oxide-bearing gabbro of the section above is not well preserved; it is marked by deformed and highly altered serpentine-rich rubble. The recovery of Core 305-U1309D-140R is >100%, suggesting that the contact between two lithologies does not represent a significant displacement zone. This conclusion is supported by the presence of a network of crosscutting, small oxide-bearing gabbro and diabase dikes postdating the strong metamorphic overprint within the olivine-rich troctolite. In other words, the olivine-rich troctolites at 670 and 690 mbsf may represent a continuous lithologic “package.”

Contact 5

At 1127 mbsf, a well-preserved contact between coarse-grained olivine gabbro and fine- to medium-grained clinopyroxene-free, olivine-rich troctolite (85% modal olivine) is sharp, wavy, and undeformed (interval 305-U1309D-227R-2, 34–36 cm) (Fig. F84A). There is no gradual transition toward the contact within the olivine-rich troctolite, but within the overlying olivine gabbro modal olivine gradually increases from <5% at the top of the section to 30% near the contact without any significant change in grain size. Thirteen 0.5–20 cm wide gabbroic dikes crosscut this nearly 6 m long section of olivine-rich troctolite (Fig. F84B). Within the fine-grained olivine-rich troctolite, the plagioclase content increases strongly toward the contact. The lower contact to a medium- to coarse-grained troctolitic gabbro (20%–30% olivine; 15% clinopyroxene) is similarly well preserved, subvertical, and relatively sharp (interval 305-U1309D-228R-3, 64–76 cm) (Fig. F84C).

Contact 6

The next olivine-rich troctolite interval >1 m thick starts at Section 305-U1309D-233R-2, 51 cm, and ends at Section 305-U1309D-238R-1, 123 cm (Fig. F85). A diffuse upper contact with coarse-grained troctolite is well preserved; the lower contact is missing. It is likely, however, that the upper contact is with a crosscutting dike that has sharp contacts similar to the other crosscutting gabbroic dikes within this and other olivine-rich troctolite intervals (Fig. F85A, F85C).

Contact 7

At Section 305-U1309D-240R-1, 115 cm, the upper contact is missing, but an olivine-rich troctolite has generally sharp boundaries with coarse-grained gabbro dikes (Fig. F86A, F86B). Olivine-rich troctolite fragments are seen within the gabbro dike (Fig. F86C). The lower contact with olivine gabbro is similarly well preserved (Section 305-U1309D-243R-1, 34 cm) and relatively sharp (Fig. F86D). There is a gradual decrease of modal olivine approaching the contact within the olivine gabbro. This contact is distinct from those of the crosscutting gabbro dikes described above; it more closely resembles the gradual upper contact of Section 305-U1309D-227R-2.

Contact 8

The upper contact of an olivine-rich troctolite interval between the top of Section 305-U1309D-247R-1 and Section 248R-4, 59 cm (Fig. F87) was not recovered, and the lower contact with olivine-bearing gabbro is poorly preserved and strongly altered but appears to be sharp. Two coarse-grained gabbro dikes with sharp contacts crosscut the olivine-rich troctolites in Section 305-U1309D-247R-1 (Fig. F87A, F87B).

Contact 9

The lowermost olivine-rich troctolite interval recovered during Expedition 305 starts at the top of Section 305-U1309D-256R-1, below diabase rubble, and ends at Section 257R-1, 74 cm (Fig. F88). Neither the upper nor the lower contact was recovered, but the contacts with an olivine gabbro dike are relatively sharp (Fig. F88B, F88C). Thin gabbro dikes (<1 cm thick) intrude subvertically into the olivine-rich troctolite through interval 305-U1309D-256R-3, 64 cm, to 257R-4, 45 cm (Figs. F88C, F90B).

The overall olivine-rich troctolite-dominated interval from Section 305-U1309D-227R-2 through 257R-1 (1093.75–1235.84 mbsf) forms an integral lithologic package. The olivine-rich troctolites and minor associated lithologic units that comprise this package have subsequently been intruded, at temperatures below the olivine-rich troctolite solidus, by numerous crosscutting gabbroic dikes of variable thickness. The larger scale contacts with the surrounding olivine gabbro units seem, however, to be dominantly intrusive and to have formed under hypersolidus conditions. There is currently no evidence concerning possible genetic relationships between the olivine-rich troctolites and the surrounding gabbroic rocks.

Troctolite

Troctolite is relatively abundant in the interval 400–600 mbsf but occurs only sporadically through the remainder of the core (Fig. F79). Overall, troctolite constitutes only 2.7% of the rocks recovered from Hole U1309D. The deepest recovered troctolite is in interval 305-U1309D-268R-1, 78–93 cm (1290 mbsf). Primary mineral modes in troctolite are 30%–70% olivine, 30%–50% plagioclase, and up to 5% clinopyroxene. Plagioclase/​olivine ratios vary on a decimeter scale, and there are frequent irregular gradations into troctolitic gabbro and even olivine gabbro. Texturally, olivine-rich troctolites are characterized by cumulate textures defined by discrete rounded olivine grains with interstitial plagioclase and/or clinopyroxene. In contrast, troctolite textures are more irregularly seriate with local poikilitic clinopyroxene occurrences.

Olivine crystals vary in size. They are mostly present as continuous networks of millimeter-sized grains, but they are also present as subrounded centimeter-sized grains. Olivine chadacrysts, sometimes with aligned olivine kink bands, also occur in plagioclase and clinopyroxene.

Within the troctolite, clinopyroxene shapes range from interstitial to pegmatitic oikocrysts 2–50 mm in size. Their mode is highly variable, and troctolitic gabbro patches are developed locally. Interstitial clinopyroxene seems more abundant close to gabbroic dikes, suggesting that primary oikocrystic clinopyroxene is recrystallized during later intrusion of the crosscutting dikes. In many cases, interstitial orthopyroxene and clinopyroxene are present as fine rims around olivine or as thin films at olivine/​plagioclase contacts. Clinopyroxene oikocrysts enclose olivine and plagioclase chadacrysts. Less frequently, clinopyroxene is present as chadacrysts in plagioclase oikocrysts.

Plagioclase is present as subhedral to anhedral crystals and is fine to coarse grained. Plagioclase is present as interstitial crystals and oikocrysts enclosing clinopyroxene and olivine chadacrysts. In some cases, early plagioclase is possibly replaced by olivine. Both plagioclase-rich veins/​zones and olivine-rich patches are observed. Spinel is seen as euhedral to anhedral crystals enclosed in both plagioclase and olivine.

The troctolite units are commonly crosscut by late-stage dikes of both coarse-grained gabbro and microgabbro (Fig. F92A, F92B). Dike orientations are highly variable from horizontal to vertical, and contacts range from diffuse to sharp. Along the diffuse contacts, locally abundant sulfides and patches of clinopyroxene suggest infiltration into the host.

One particular example showing the textural contrast to the olivine-rich troctolites occurs in interval 305-U1309D-111R-3 through 112R-1, at 545–557 mbsf. Both the upper and lower contacts for the ultramafic interval around 650 mbsf are gradual and not clearly defined at all. The upper contact is probably characterized by the first presence of a pegmatitic troctolite patch enclosed in coarse-grained gabbro (interval 305-U1309D-110R-1, 113–128 cm). After an interval of interfingering coarse-grained gabbros and minor troctolitic patches in Sections 305-U1309D-110R-2 and 110R-3, olivine-rich troctolite with variable grain size becomes dominant. A zone of bimodally sized olivine in Sections 305-U1309D-112R-1 and 112R-2 accompanies the gradual change to more gabbro dominated zones. Occasionally, subrounded centimeter-sized olivines appear in addition to the millimeter-sized continuous olivine network that is typical of these troctolites. At the bottom of Section 305-U1309D-112R-3, the large olivines become dominant prior to a transition to coarse equigranular olivine gabbro.

Troctolitic gabbro and olivine gabbro

Olivine gabbro is abundant in Hole U1309D. Within this lithology, decimeter- to meter-scale variations in clinopyroxene content lead to local gradations to troctolitic gabbro. Together, olivine gabbro and troctolitic gabbro constitute 23.1% of the material recovered during Expedition 305 (Figs. F77, F78).

Olivine gabbro is the dominant lithology in the intervals 400–600, 1000–1100, 1200–1300, and 1380–1415 mbsf. The intervals are characterized by fine- to coarse-grained olivine and olivine-bearing gabbros that locally grade to troctolitic intervals and patches and, in places, to troctolite (Fig. F7). The modal composition varies widely from 5%–50% olivine, 25%–80% plagioclase, and 10%–70% clinopyroxene. Grain size, texture, and modal composition are heterogeneous and vary highly throughout this sequence.

Like the troctolites, the olivine gabbros are frequently interrupted by late-stage magmatic leucocratic dikes and by dikes of both coarse gabbro and microgabbro. Patches and zones of coarser grained clinopyroxene in a medium-grained matrix are also common. In the upper part of the interval cored during Expedition 305, intrusive contacts are sharp and commonly wavy, accompanied by tiny interstitial clinopyroxene grains that suggest infiltration into the host. Downhole, both obvious igneous contacts and more diffuse boundaries are seen. Owing to centimeter-scale variations in mineral proportions, lithologies determined from thin sections sometimes differ from those determined from hand samples. The olivine gabbros show strong modal variability. Plagioclase-, pyroxene-, and olivine-rich intervals and patches are irregularly distributed, leading to local variations in lithology ranging from troctolite to olivine-bearing gabbro. Occasionally, olivine is concentrated in variably oriented bands (vertical, oblique, and horizontal) separating medium- and fine-grained zones in the center of the core.

There is no systematic downhole variation in primary modes or textures of olivine gabbros. Olivine is present in the upper portion of the hole as subhedral to interstitial crystals. Down to ~1000 mbsf, olivine is usually pervasively altered by a hydration reaction with plagioclase to mesh-textured serpentine and/or fine-grained talc-tremolite intergrowths. In pervasively altered samples, the resulting coronitic texture delineates the original olivine grain boundaries, allowing a first-order approximation of primary olivine abundance (see also “Metamorphic petrology”) (Fig. F93A). In places, however, a similar texture surrounding altered clinopyroxene can be seen in thin section. In some places, this effect may lead to a small overestimation of olivine mode and a complementary underestimation of clinopyroxene in hand sample.

Olivine-rich zones also appear to correlate generally with MS highs (as measured with the whole-core multisensor track [MST]; see “Physical properties”). This correlation is presumed to reflect the formation of dispersed iron oxides during serpentinization of olivine. Olivines are seen as chadacrysts in plagioclase and clinopyroxene (Fig. F93B) but are also present as oikocrysts enclosing plagioclase. Below 1000 mbsf, olivine encloses spinel and is less altered, with abundant trails of (possibly) secondary fluid inclusions. Kink bands, which occur frequently in the upper part of the hole, are absent below 1000 mbsf.

Clinopyroxene most commonly occurs as anhedral primary grains ranging from 1 to 10 mm with an average grain size of 5 mm. In many samples, it is interstitial with interlocked contacts between plagioclases (Fig. F93C, F93D), as thin films between olivine and plagioclase, or as thin (50 µm) films rimming olivine. In places, large oikocrysts poikilitically enclose subhedral to rounded olivine (Fig. F93B), plagioclase, and spinel chadacrysts. Clinopyroxene is usually ~20% altered, with tremolite/actinolite forming rims around primary clinopyroxene (see “Metamorphic petrology”). In some cases, clinopyroxene is partially or completely replaced by acicular colorless amphibole, particularly along exsolution lamellae. Exsolved interlocking clinopyroxene/​orthopyroxene contacts are also abundant.

Plagioclase is present as euhedral to anhedral crystals and appears relatively fresh compared to clinopyroxene and olivine. It is rarely present as oikocrysts enclosing rounded clinopyroxene and olivine chadacrysts. The most altered plagioclases are ~20% altered to tremolite and chlorite rims. The plagioclase crystals commonly show recrystallized textures and abundant trails of fluid inclusions.

Orthopyroxene is sporadically present as interstitial films between olivine and plagioclase and rarely as clinopyroxene/​plagioclase contacts. The films are ~20–50 µm thick and up to 200 µm in length. They seem most abundant in the upper part of the hole.

Spinel appears as euhedral to anhedral small (~1 mm) crystals enclosed in olivine and plagioclase. In some cases, the spinels are oxidized, although some relics exist.

Trace amounts of sulfide and oxide are locally enriched, usually in the vicinity of nonmagmatic veins.

In the troctolitic gabbros, the olivines are, in some cases, widely spaced and show few weak kink bands compared to troctolite and olivine gabbro. In some cases, plagioclase is present as chadacrysts in olivine. Large olivines and clinopyroxene- and plagioclase-rich concentrations, together with grain size variation, lead to modal variations in the units.

Gabbro group (gabbro, olivine-bearing gabbro, microgabbro, and disseminated oxide gabbro)

In this section, we describe several rock types together. Gabbros are dominated by plagioclase and clinopyroxene with <5% olivine or orthopyroxene. Gabbronorites contain >5% modal orthopyroxene. Consistent with Expedition 304, we use the term “olivine-bearing gabbro” for modal olivine between 1% and 5%. These rock types are discussed together because modal olivine in the core fluctuates over short length scales. Modal variations from 0% to 5% are common in gabbros and from >5% to <5% olivine within olivine gabbro units. Thus, the term “olivine-bearing gabbros” is a descriptive term, not a genetic one. Microgabbros generally have subequal modal abundances plagioclase and clinopyroxene. It should be noted that many of the rocks identified as gabbros in the visual core descriptions actually contain significant modal orthopyroxene that could only be identified in thin section. Because the generally grayish orthopyroxene is very difficult to distinguish from clinopyroxene and groundmass plagioclase in hand sample, sections of core described as gabbro are in some places gabbronorites or olivine gabbronorites (see the more detailed discussion in “Gabbronorites and orthopyroxene-bearing gabbros”).

In terms of cumulative length, the gabbro group (gabbro, olivine-bearing gabbro, microgabbro, disseminated oxide gabbro, and gabbronorite) is the dominant lithologic group, encompassing 60.75% of material recovered during Expedition 305 and 55.7% over the entire depth of Hole U1309D (Figs. F67, F77, F78).

Gabbros vary considerably in grain size from microgabbro through seriate medium grained to pegmatitic, in places within a single core section. Gabbros commonly contain pegmatitic or coarse-grained clinopyroxene over tens of centimeters long (Fig. F94). Pegmatitic clinopyroxenes are commonly undeformed, between 50 and 150 mm in size, and can be oikocrystic. From 650 mbsf to the bottom of the hole, pegmatitic intervals are shorter; the grain size in gabbro is typically medium to coarse. Plagioclase crystals are generally anhedral and form interlocking textures with pyroxene and other plagioclase grains. Olivines are commonly interstitial in olivine-bearing gabbros and rimmed by alteration coronas (Fig. F95). Olivine-bearing gabbros show variable grain sizes and downhole distribution.

Microgabbro (including microgabbronorite) grades in grain size from fine grained to cryptocrystalline. It exhibits high-temperature equigranular crystallization textures (Fig. F96B), and along some contacts plagioclase has recrystallized at high temperature (Fig. F96C). In many cases, microgabbro and diabase are difficult to distinguish in hand sample because of similar grain size. In thin section, microgabbro is distinguished by its equigranular textures from diabase (Fig. F96), which has glassy, intersertal, or cryptocrystalline textures with acicular plagioclase. In some intervals, diabase intrusions have fine-grained chilled margins, whereas microgabbros do not exhibit chilled margins.

Microgabbros are also distinguishable from diabase on the basis of their chemical compositions. Plots of Na₂O against TiO₂ provide a useful discriminant; microgabbros plot with other cumulate rocks at lower TiO₂ for a given Na₂O content, whereas diabases follow a distinct liquid line of descent that is also followed by the basalts (Fig. F97).

Microgabbro is present intermittently as dikes throughout the hole but increases in abundance in association with a prominent ductile shear zone at ~575–675 mbsf. It occurs in three modes: in intrusive contact with surrounding rocks (Fig. F96A, F96B), in plastically deformed zones (Fig. F96D), and in zones that grade in grain size and apparent magmatic continuity to medium- and coarse-grained gabbro.

Where visible, the contact relationships of gabbro with other lithologies suggest that gabbro is generally intrusive into olivine-rich lithologies (olivine gabbros and troctolites) and is intruded by felsic (leucocratic) dikes and oxide gabbros. These relationships are more common between 400 and 650 mbsf than in the lower part of the hole, where gabbro contacts are diffuse. Contacts between troctolite and gabbro range from sharp, where thin intervals of gabbro crosscut the serpentine foliation in the troctolite, to gradational, where thicker intervals of gabbro are present. Contacts between oxide-bearing lithologies and gabbro range from sharp to gradational as well. True felsic dikes in gabbro generally show sharp contacts, but varying degrees of high-temperature reaction between dikes and wallrocks are also observed. These reaction zones commonly have oriented minerals such as oxides, pyroxenes, or plagioclase growing along and across them, indicating postemplacement reaction.

Gabbros are present in varying proportions throughout the hole, but there are long (tens of meters) intervals in which gabbro is the overwhelmingly dominant lithology. One of these gabbro zones is bounded above by oxide gabbro at ~600–650 mbsf and below by olivine gabbro and troctolite at ~1030 mbsf (Fig. F77). The gabbro sequence is punctuated only by a short interval in which oxide gabbros and more olivine rich lithologies appear at 870–900 mbsf. Gabbronorite abundance also peaks in the interval 760–1050 mbsf, tailing off where the abundance of olivine-rich lithologies begins to increase at ~1000 mbsf. A second, shorter gabbro-dominated interval from 1300 to 1360 mbsf is bounded above by a ~10 m interval of oxide gabbro and below by troctolite and olivine gabbro. It is punctuated at ~1340 mbsf by narrow intervals of oxide gabbro and olivine gabbro (Figs. F77, F79).

Downhole variations in lithology and modal composition define two similar intrusive packages (at ~600–1240 mbsf and ~1240–1415 mbsf) within the 600–1415 mbsf portion of Hole U1309D. In both packages, an interval of olivine-rich lithologies (troctolite and olivine-rich troctolite) occurs below a relatively thick interval of gabbro and gabbronorite with oxide gabbro and olivine-bearing gabbro units. These are overlain, in turn, by ~10–12 m upper intervals of oxide gabbro. Microgabbros are present as dikes in gabbro or in deformation zones. From 0 to 400 mbsf, olivine-rich lithologies are more abundant than below 400 mbsf.

Gabbronorites and orthopyroxene-bearing gabbros

Gabbronorites are defined as gabbroic rocks in which the total amount of orthopyroxene ± inverted pigeonite is >5%, but in practice, they are difficult to identify in the core because of significant local modal fluctuation and because orthopyroxene is difficult to consistently identify in hand specimen. For these reasons, reported gabbronorite abundances are minimum estimates and contact relationships are unconstrained. In the lower part of the hole (from Core 305-U1309D-243R through Core 272R), low-Ca pyroxenes include both orthopyroxene and inverted pigeonite (see below). Compositionally, gabbronorites partially overlap the gabbro field, with slightly higher average Na₂O (Fig. F97). This observation suggests that the parent liquids of the orthopyroxene-bearing gabbros are slightly more evolved than those of the orthopyroxene-free gabbros. Gabbronorites occur in contact with all other lithologies, but their contact relationships are not readily observable in the core.

Gabbronorites have similar textural relationships to those of the gabbros and olivine gabbros. Primary magmatic fabrics are usually absent but sometimes weakly developed. Weak high-temperature deformation is irregularly present as kink bands in olivine and rarely in orthopyroxene. Plagioclase usually shows wavy extinction and tapered twins. In strongly sheared to mylonitic gabbronorites, orthopyroxene is characteristically replaced by aggregates of tabular neoblasts (see “Structural geology”).

Plagioclase is the dominant phase; it is usually anhedral in the granular groundmass and is also present as rounded chadacrysts enclosed either in clinopyroxene and orthopyroxene oikocrysts.

Clinopyroxene is anhedral and rarely subhedral; it is frequently present as fine interstitial aggregates associated or directly connected with large oikocrystic grains.

Olivine is always anhedral, in places enclosed as rounded chadacrysts in large clinopyroxene and orthopyroxene oikocrysts.

Orthopyroxene occurs as large subhedral grains in the coarser gabbro and gabbronorite but is more commonly anhedral, particularly in the fine-grained microgabbro or microgabbronorite. Orthopyroxene oikocrysts enclose plagioclase with less clinopyroxene and olivine (Fig. F98). These chadacrysts are generally rounded and resorbed, suggesting that oikocrystic orthopyroxene is the last phase to crystallize. In the more olivine poor assemblages (<20% olivine), olivine and orthopyroxene are commonly present together with primary nonreactive contacts (Fig. F99A). A 100–400 µm thick orthopyroxene film that mantles olivine in almost all thin sections appears most commonly at olivine/​plagioclase contacts (Fig. F99B). In places, both orthopyroxene and clinopyroxene films occur. Rarely, orthopyroxene films extend from a large anhedral grain into clinopyroxene-olivine and plagioclase-clinopyroxene interstices (Fig. F99B).

Inverted pigeonite is present between Cores 305-U1309D-243R and 272R, dividing the lower gabbronorite into two zones (see below). It is identified on the basis of its characteristic exsolution pattern, following (001) and (100) (Fig. F99C). In places, inverted pigeonite is seen in thin section to be associated with late-stage magmatic impregnations (oxide-rich gabbroic veins). This association may suggest that pigeonite inversion occurs only under restricted thermal conditions. Because inversion simplifies pigeonite detection, this situation may lead to underestimation of pigeonite content.

Gabbronorite abundance increases downhole below ~400 mbsf (Fig. F100). Between Cores 305-U1309D-80R and 156R, gabbronorites occur only as rare fine- to medium-grained (micro)gabbronorite and orthopyroxene-bearing gabbro. The uppermost appearance of a gabbronorite is a narrow (8–10 mm) fine-grained dike that intrudes olivine-bearing gabbro (Unit 213, ~404 mbsf) (Sample 305-U1309D-80R-2, 123–143 cm). A narrower (up to 0.4 mm) amphibole-bearing gabbronorite dike cuts olivine gabbro at Unit 314 (582 mbsf).

Based on the continuity of gabbronorite recovery and modal mineralogy, six gabbronorite zones can be defined in Hole U1309D (Fig. F100).

Gabbronorite Zone 1

• Depth: ~400–625 mbsf
• Interval: Cores 304-U1309D-80R to 126R
• Lithology: occasional fine- to medium-grained (micro)gabbronorite and orthopyroxene-bearing gabbro.

Gabbronorite Zone 2

• Depth: ~650–682 mbsf
• Interval: Cores 305-U1309D-132R to 138R
• Lithology: fine- to coarse-grained olivine-bearing to olivine gabbro and gabbronorite in the upper part and fine- to medium-grained orthopyroxene-bearing microgabbro and gabbronorite in the lower part; Unit 379 shows mylonitic deformation and oxide impregnation during the late stage of deformation.

Gabbronorite Zone 3

• Depth: ~704–729 mbsf
• Interval: Cores 305-U1309D-143R to 149R
• Lithology: orthopyroxene-bearing gabbro and gabbronorite alternating with olivine gabbro in the upper part and with medium- to coarse-grained gabbro in the lower part.

Gabbronorite Zone 4: main gabbronoritic zone

• Depth: ~760–1100 mbsf
• Interval: Cores 305-U1309D-156R to 230R
• Lithology: plagioclase- and clinopyroxene-bearing gabbro. This zone constitutes the main body of the gabbro sequence. Plagioclase and clinopyroxene average 62% and 30%, respectively. Olivine and orthopyroxene average 7% and 5%, respectively. There is no correlation between olivine and orthopyroxene content. Their modes fluctuate downhole in an apparently random way. Relative enrichments in the two phases are commonly observed (Fig. F100). The maximum orthopyroxene content, indicated by the modal spikes in Figure F100, decreases downhole to the base of the main gabbronoritic zone, thus showing an inverse correlation with the increasing trend of the olivine-rich gabbros (Fig. F79).

Gabbronorite Zone 5: pigeonitic gabbronorite zone

• Depth: ~1168–1310 mbsf
• Interval: Cores 305-U1309D-243R to 272R
• Lithology: gabbronorite and olivine gabbronorite (olivine as high as 7%) are intercalated with troctolite and troctolitic gabbro in the upper part of this zone, with oxide gabbro in the central part and olivine-rich gabbro in the lower part. Low-Ca pyroxene is commonly represented by inverted pigeonite. In places, orthopyroxene, clinopyroxene, and inverted pigeonite coexist (Core 305-U1309D-264R).

Gabbronorite Zone 6: lower gabbronoritic zone

• Depth: ~1317–1415 mbsf
• Interval: Cores 305-U1309D-274R to 295R
• Lithology: in the lower part of the hole, gabbronorites are continuous; Orthopyroxene varies from a few percent to 45% in Core 305-U1309D-293R (norite).

Oxide gabbro

Oxide gabbros consist mainly of plagioclase and clinopyroxene but are defined by >2% modal Fe-Ti oxide; disseminated oxide gabbros contain between 1% and 2% modal oxides. Oxide gabbro forms 7.7% of material recovered from Hole U1309D during Expedition 305. Apart from their oxide contents, the mineralogical characteristics of the oxide gabbros are similar to those of the coarse-grained gabbros (Fig. F101A).

Oxide gabbros can be distinguished from other gabbros by their high bulk MS, and this property can be used to help constrain their contacts with surrounding oxide-free rock types. In some cases, ilmenite may be present in oxide-bearing zones of low MS.

In many oxide-rich gabbros, accessory apatite as large as 6 mm can be identified in thin section (Fig. F102). Bulk rock analyses indicate that the apatite content can locally exceed 7 wt%. Euhedral elongate prismatic zircons, up to 3 mm, also occur in oxide gabbros, but they are much less common than apatite (Fig. F102). Titanite is also associated with oxide-bearing rock types, particularly when the oxide gabbro is strongly altered. It is not clear whether titanite crystallized from a melt or is the product of a metamorphic reaction.

Oxide gabbros, and especially oxide-bearing dikelets, display a broad range of compositions and complex mineral assemblages and disequilibrium textures. They can be divided into five end-member magmatic types.

Type 1: normal oxide gabbros

Randomly dispersed oxide-bearing patches in undeformed, generally coarse-grained gabbro represent ~80% of all oxide gabbros. Their modal compositions are variable, with ~25%–75% clinopyroxene and complementary amounts of plagioclase, 0%–2% olivine, and, commonly, several percent orthopyroxene. Subhedral to anhedral plagioclase and clinopyroxene range in grain size from coarse to pegmatitic, as large as 50 mm. The oxide minerals are very irregularly distributed, commonly in interstitial patches between plagioclase and clinopyroxene. An example of an oxide-rich gabbro dikelet is shown in Figure F103. This sharply bound vertical dikelet crosscuts a gabbro that has undergone weak high-temperature recrystallization. The host gabbro contains coarse heterogeneously distributed oxide patches but is otherwise devoid of Ti-bearing oxides.

Type 2: amphibole-bearing gabbros

Amphibole-rich gabbros occur as intercalations within which are present amphibole-free oxide gabbros and orthopyroxene-bearing gabbros and gabbronorites. This second type of oxide gabbro is coarse grained and amphibole rich, with elongate, bladed green amphiboles and dominantly tabular plagioclase (Fig. F104). Amphibole is commonly saussuritized, giving it a leucocratic appearance in places. Sharp, abundant, arrow-shaped, low-angle contacts between two bladed amphiboles and plagioclase suggest an igneous origin. In thin section, secondary green to brown amphibole is commonly seen to have replaced original amphibole. A significant fraction of the secondary green amphiboles may, however, have replaced clinopyroxene, and it is unclear how much, if any, of the amphibole in most samples is of primary igneous origin. The total abundance of trace element–rich accessory minerals, including apatite, titanite, and zircon, can exceed 1%. The modal composition of coarse-grained amphibole-rich oxide gabbros includes 40%–70% plagioclase and as much as 50% possibly magmatic amphibole. Fe-Ti oxides are always present, from a few percent to >10%.

The presence of primary amphibole within this rock type was a matter of debate within the igneous and metamorphic petrology teams: resolution of this issue will require shore-based studies. In this report, the amphibole gabbros are not treated as a separate lithology but are grouped with the oxide gabbros. Between Cores 305-U1309D-128R and 140R, the amphibole-rich oxide gabbros form a significant portion (between 16% and 100% per core) of the core, but they contribute <4% of total core recovered during Expedition 305.

Type 3: oxide bands

Discontinuous oxide bands may be a special subset of the Type 1 normal oxide gabbros. They have been mainly observed in thin section. Figure F105B shows a whole thin section scan of a subvertical 3–8 mm wide oxide-rich band. The oxide minerals, along with minor sulfides, apatite, and secondary green amphibole occur along grain boundaries. It seems that silicates were not crystallized along with these oxides because there is no distinct crosscutting oxide-bearing lithology; the modal proportions and grain size of the primary silicate minerals appear to be entirely unaffected by the presence of the oxides. Zircon and titanite do not accompany oxides of this type. Type 3 oxide bands are very rare and represent far less than 1% of the oxide gabbros. Nevertheless, they may yield additional information about the formation of oxide gabbros in general and, in particular, why their occurrence is commonly not associated with variations in the modal composition of their host.

Type 4: oxide-enriched gabbros near undeformed contacts

Fe-Ti oxide minerals are strongly enriched at and near several undeformed contacts of Type 1 oxide gabbros with any of several other lithologies. This type includes the highest local oxide abundances and is associated with the highest local peaks in MS. For example, Figure F101C shows an undeformed contact between coarse-grained oxide-free gabbro and a Type 2 amphibole-bearing oxide gabbro. Within the Type 2 amphibole-bearing oxide gabbro, a 2–5 cm band is extremely oxide rich and there is no indication of significant deformation or displacement along the contact. A second example, in which sulfides accompany an extreme enrichment in Fe-Ti oxides, is shown in Figure F101B. This particular subtype of oxide enrichment is not very common.

Type 5: oxide gabbro shear zones

This type of oxide gabbro is characterized by oxide enrichment within narrow zones of intense ductile deformation. A distinctive characteristic of this type is the apparent replacement of clinopyroxene by Fe-Ti oxides (Fig. F106A, F106E). Figure F106A shows a whole thin section scan of a ductile shear zone containing 10% oxides and minor apatite that crosscuts a largely undeformed coarse-grained gabbro. Figure F106A and F106B shows a remarkably narrow oxide-bearing mylonite zone within an unstrained gabbro host. The mylonitic zone is lithologically distinct from its oxide-free host. Enclosed within the oxide gabbro matrix are neoblasts or porphyroclasts (20–50 µm) of clinopyroxene and plagioclase. A close-up of an apatite-clinopyroxene-plagioclase shear zone aggregate is shown in Figure F106D.

Type 6: Fe-Ti-bearing oxides

Relatively abundant oxides also occur in zones of focused metamorphic overprint, where the presence of albitized plagioclase and epidote give the rock a leucocratic appearance (see “Metamorphic petrology”). In this type, the oxides are commonly intergrown with stubby, irregularly shaped, strongly pleochroic titanite.

Diabase

Diabase and basalt intervals of variable thickness cut gabbroic rocks in several places throughout Hole U1309D and form 0.93% of the lithologies recovered during Expedition 305. Basalts occur as thin dikelets (<1 cm) or thin, discontinuous, irregularly shaped intrusive bodies with chilled margins to wallrock gabbro (Fig. F107). They are characterized by a fine-grained sparsely phyric to intersertal texture, few phenocrysts, and a glassy matrix (Fig. F108) with minor clinopyroxene. The modal phenocryst abundance never exceeds 1%–2%. Diabase intrusions range up to at least 3.5 m in thickness (Sections 305-U1309D-154R-1, 75 cm, through 155R-2, 124 cm) and are characterized by fine- to medium-grained ophitic to subophitic texture (Fig. F108) (Units 242 and 412). The dominant minerals are clinopyroxene and plagioclase with minor olivine. Modal compositions are variable (Unit 242 is 40% plagioclase, 10% olivine, 1% oxide, and 30% secondary amphibole, but Unit 412 is 40% plagioclase, 40% clinopyroxene, and 20% other, mainly altered, material). The degree of alteration is variable but usually high, making the estimation of primary modal composition difficult.

The following diabase intervals were cored during Expedition 305; in many cases, igneous contacts were not recovered and exact thicknesses are not known:

• 471–472 mbsf (Sections 305-U1309D-94R-3, 12 cm, through 95R-1, 7 cm),
• 626–628 mbsf (intervals 125R-2, 106–128 cm, and 127R-1, 44–73 and 108–150 cm),
• 751–761 mbsf (intervals U1309D-153R-1, 58–70 cm, and 154R-1, 75 cm, through 155R-2, 124 cm),
• 843 mbsf in a narrow (10 cm) interval (Section 304-U1309D-173R-1),
• 875 mbsf (Sections 179R-4 through 180R-1), and
• 990 mbsf (Sections 205R-4 through 206R-1).

The deepest diabase was recovered at 1377.6 mbsf (interval 305-U1309D-287R-1, 0–48 cm) without igneous contacts and a recovered thickness of only 50 cm. In this interval, the diabase includes wallrock xenoliths—aggregates of plagioclase, olivine, and pyroxene (Fig. F109).

Dikelets and veins of possible magmatic origin

Almost all gabbroic rock types are cut by dikelets or veins of variable thickness and composition. The origins of these features are complex, as they combine magmatic, metamorphic, and deformation-induced attributes. Both Expedition 304 and 305 igneous teams described many of these dikelets and veins using the term “leucocratic,” which encompasses a wide variety of light-colored veins and alteration fronts. Thus, the term “leucocratic” has no single meaning other than referring to color and general mode of occurrence. The centers of leucocratic veins commonly contain abundant subhedral to euhedral accessory minerals, including apatite, titanite, and zircon. However, these minerals are absent from the leucocratic halo of altered, commonly fractured, and albitized (saussuritized) plagioclase that is commonly present around veins or along some intrusive contacts. These accessory minerals are interpreted to have been derived from evolved melts that crosscut the gabbros, with the melt pathways subsequently acting as focusing zones for nonmagmatic fluids. However, less abundant, millimeter-sized crosscutting fine-grained amphibole-bearing dikelets also contain zircon and/or apatite, but they lack saussurite halos and were interpreted initially as nonmagmatic. A further complicating factor that inhibits a clear distinction of magmatic from metamorphic characteristics, especially in the smaller dikelets, is the commonly associated cataclastic deformation.

Finally, some “normal” vein-free gabbros, selected for inductively coupled plasma–atomic emission spectroscopy (ICP-AES) whole-rock analysis, contained extremely high phosphorus concentrations, possibly caused by localized concentrations of apatite. The following samples collected from Hole U1309D during Expedition 305 contain >2 wt% calculated apatite:

• 93R-1, 11–16 cm
• 116R-3, 67–77 cm
• 127R-2, 81–93 cm
• 128R-3, 38–48 cm
• 130R-1, 32–42 cm
• 137R-2, 132–136 cm
• 140R-3, 93–103 cm

These observations gradually became evident after at least a week of core description, although modal apatite was not visually identified in all samples that had high phosphorus. Unambiguously identifying all magmatic dikelets of this scale is not possible based on visual core description. The reported examples can only be interpreted as a minimum estimate.

Thin section descriptions of end-member vein or dike lithologies are summarized below.

Gabbro dikes

Crosscutting dikes of fine- to coarse-grained equigranular gabbro usually have sharp contacts with their hosts and are undeformed. They consist of subequal proportions of plagioclase and clinopyroxene, rarely with minor olivine or orthopyroxene, closely resembling the coarse-grained gabbros that predominate in Hole U1309D. Apatite and zircon are not present, suggesting that they crystallized from relatively unevolved melts. Dike thickness ranges from millimeter size to as much as several meters.

Gabbro dikelets intrude fine- to medium-grained olivine-rich troctolite, ranging in thickness from <5 mm to >1 m. Contacts range from diffuse, irregular, and discontinuous to sharp, straight, and continuous (Fig. F88C). Based on their textures, the diffuse dikelets can be interpreted as either local zones of melt segregation from a compacting olivine-supported mush or as focused infiltration zones that spread diffusely into the host from intrusive dikes or dikelets. Whether these features formed by infiltration or extraction, the temperature of the surrounding troctolite must have been at or near its solidus to allow for such pervasive grain boundary melt migration. The more sharply bound dikes may, in principle, have formed by the same process and subsequently traveled over a limited distance within the olivine-rich troctolite body as it cooled.

Medium-grained orthopyroxene (10% modal) occurs in the center of a diffuse olivine-free gabbro dikelet at 629 mbsf. In this and some other dikelets, clinopyroxene oikocrysts cross the boundary between the dikelet and the host olivine-rich troctolite. The apparent immediate crystallization of orthopyroxene from a liquid derived from a primitive troctolite can only have occurred at high pressure. Crystallization of the troctolite along an olivine-plagioclase cotectic requires relatively low pressures, whereas crystallization of magnesian orthopyroxene requires pressures exceeding 4 kbar (Ghiorso and Sack, 1995). If, on the other hand, the melt was significantly evolved prior to infiltration, orthopyroxene stability would be enhanced at crustal pressures. Resolution of this issue requires more detailed shore-based studies.

Oxide-bearing dikelets

The association of Fe-Ti oxides and their detection on the basis of MS peaks is described in the “Oxide gabbro” section. An example of a vertical oxide-rich dikelet crosscutting a gabbro with minor contents of heterogeneously distributed oxide patches is shown in Figure F103, where a vertical 2–3 cm wide oxide-rich dikelet is clearly visible. However, many oxide-bearing dikelets are too thin to be recognized in hand specimen.

Trondhjemites and late-magmatic leucocratic dikelets

Extremely evolved quartz-bearing granitoid rocks occur in Hole U1309D only in rare, narrow trondhjemite dikes. Six such dikes have been identified in thin section, three in the upper part of the hole (Samples 305-U1309D-89R-2, 130–133 cm, 89R-3, 42–44 cm, and 93R-1, 16–18 cm) and three in the lower part (Samples 212R-2, 50–53 cm, 216R-1, 73–76 cm, and 262R-4, 84–87 cm) (Fig. F110). The three trondhjemites in the upper part of the hole are affected by cataclastic deformation and a strong metamorphic overprint. The three lower ones are undeformed and their primary mineralogy is better preserved. These dikes consist of medium-grained albitic plagioclase, (magmatic?) amphibole, minor quartz, and accessory minerals. All samples contain apatite in various sizes and five contain zircon and allanite (identified optically and by energy dispersive X-ray analyzer in Sample 305-U1309D-212R-2, 50–53 cm).

Two possible trondhjemites were analyzed for whole-rock major and trace element compositions (Samples 305-U1309D-93R-3, 11–16 cm, and 158R-1, 11–17 cm, at 463 and 770 mbsf, respectively). They have the lowest MgO contents of the entire sample set and very low iron content, resulting in a misleadingly high Mg#. At the same time, their incompatible lithophile trace element concentrations (P, Zr, and Y) are exceedingly high, indicating significant apatite and zircon contents.

Downhole distribution of accessory zircon and apatite

Zircon and apatite are not restricted to quartz-bearing rocks. Apatite coexists with Fe-Ti oxides in oxide gabbros (see “Oxide gabbro”). Zircon is less common. Figure F111 shows the downhole occurrences of zircon and apatite, as observed in thin section. The presence of significant apatite (six samples exceed 2% modal) and zircon was also inferred from high P and Zr in whole-rock ICP-AES analysis. Apatite and zircon appear to be concentrated in four zones, based on thin section observation:

1. Sporadically above 200 mbsf;
2. 440–470 mbsf, where they occur in evolved dikelets that crosscut intercalated olivine gabbro and olivine-bearing gabbro;
3. 620–690 mbsf, where they occur in evolved Type 2 oxide gabbros, in association with possible primary magmatic amphibole (see “Oxide gabbro”); and
4. 840–910 mbsf, where they are found in oxide gabbro, which occurs both as massive bodies and as dikes crosscutting orthopyroxene-bearing gabbro.

Between 1020 and 1182 mbsf, neither zircon nor apatite has been observed, and below 1182 mbsf, apatite and zircon occur only sporadically within small, Type 5 oxide gabbro shear zones (see “Oxide gabbro”).

Discussion

The intrusive emplacement history of Hole U1309D is complex and is reflected in the relationships between the various rock types. Where visible, the contact relationships of gabbros with other rock types suggest that gabbro is generally intrusive into olivine-rich rock types (olivine gabbros and troctolites) and are themselves intruded by felsic (leucocratic) dikes and oxide gabbros. These relationships are more common between 400 and 650 mbsf than in the lower part of the hole, where gabbro contacts are diffuse. Contacts between troctolite and gabbro range from sharp, where thin intervals of gabbro cut the serpentine foliation in the troctolite, to gradational, where thicker intervals of gabbro are present. Contacts between oxide-bearing rock types and gabbro range from sharp to gradational as well. The sharpness of contacts probably reflects temperature contrast between the country rock and the intrusive unit. True felsic dikes in gabbro generally show sharp contacts, but varying degrees of high-temperature reaction between dikes and wallrocks are also observed. These reaction zones commonly have oriented minerals such as oxides, pyroxenes, or plagioclase growing along and across them, suggesting postemplacement reaction.

The olivine-rich troctolite interval from 1093.75 to 1235.84 mbsf forms an integral lithologic package. The olivine-rich troctolites and minor associated lithologic units that comprise this package have subsequently been intruded, at temperatures below the olivine-rich troctolite solidus, by numerous crosscutting gabbroic dikes of variable thickness. The larger scale contacts with the surrounding olivine gabbro units seem, however, to be dominantly intrusive and to have formed under hypersolidus conditions. Interstitial clinopyroxene seems more abundant close to gabbroic dikes, suggesting that primary oikocrystic clinopyroxene is recrystallized during later intrusion of the crosscutting dikes. There is currently no evidence concerning possible genetic relationships between the olivine-rich troctolites and the surrounding gabbroic rocks, but this will be a focus of postcruise research.

The presence of inverted pigeonite in the lower parts of the hole may have thermal implications for the intrusion. Inverted pigeonite is identified only in thin section and is associated with late-stage magmatic impregnations (oxide-rich gabbroic veins). This association may suggest that pigeonite inversion occurs only under restricted (lower temperature) thermal conditions of this lower intrusive sequence.

Ultramafic rocks from the upper portion of Hole U1309D appear to be true mantle peridotites that have been intensively impregnated by melts of variable composition. In the Hole U1309D samples, pyroxene oikocrysts enclosing olivine differ from textures commonly seen in impregnated peridotites. Similar textures have been previously described along the MAR (DSDP Leg 37, Girardeau and Mercier, 1992; Leg 153, Cannat, Karson, Miller, et al., 1995) and at Hess Deep (Francheteau et al., 1990), as well as ophiolites like Oman (Hopson et al., 1981; Boudier and Nicolas, 1995), where they are related to intrusions of or impregnation by wehrlitic melts. Similar textures were also reported from Site 1275 during Leg 209, where they were explained by addition of interstitial basaltic liquid to a peridotite (Kelemen, Kikawa, and Miller, 2004). A high melt/​rock ratio resulted in separation and isolation of most olivine grains that were surrounded by a gabbroic matrix and mimic a wehrlitic texture.

Several locations of wehrlites have been reported from the MAR, but the origin of wehrlitic intrusions into crustal gabbros is still a matter of debate. If their presence is more common than previously thought, we assume it has important implications for mid-ocean-ridge processes, as proposed, for example, in the Oman ophiolite (e.g., Jousselin and Nicolas, 2000). Clinopyroxenes from wehrlites are in equilibrium with mid-ocean-ridge basalt liquids (Koga et al., 2001), but it requires additional water in the melt to bring clinopyroxene before plagioclase on the solidus at a temperature of <1150°C. As normal tholeiites contain only ~1000 ppm water, an additional source is needed. Evidence for bursts of seawater into magma chambers has been proposed recently from Oman (Bosch et al., 2004), and this may provide the crucial water to form the wehrlitic melt.

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