FOOTWALL SITE (continued)

Intrusive Sequence

Igneous rocks recovered from Holes U1309B and U1309D on the central dome of Atlantis Massif record a series of intrusions likely to have occurred at a variety of depths within the subaxial zone of the spreading center and, perhaps, into young lithosphere as it was incorporated in the western flank of the rift axis (Fig. F5). Mafic rocks recovered at Site U1309 fall into six major rock types: basalt and diabase, gabbroic rocks including oxide gabbro, gabbro, olivine and troctolitic gabbro, and troctolite. Ultramafic rocks recovered at Site U1309 are, in general terms, serpentinized peridotite, their composition ranging from residual mantle harzburgite and dunite to cumulate dunite, wehrlite, lherzolite, and olivine-rich troctolite. All ultramafic rocks have undergone hydrothermal alteration and are multiply intruded by later gabbro dikes and/or veins. In the upper 100 m interval cored in both Holes U1309B and U1309D, gabbroic rocks make up 50%–60% of the recovered section, basalt and diabase 40%–50%, and serpentinized peridotite ~0.5%–3% total (Fig. F6). Of the 401 m penetrated in Hole U1309D, 87% of the rock recovered is gabbroic, <10% is basalt and diabase, and 4% is serpentinized peridotite.

Basalt and diabase are restricted to the upper part of the footwall at Site U1309. No in situ diabase was recovered below ~127 mbsf during Expedition 304 (Fig. F5). Their intrusive contacts, taken with the relative intensity of alteration and vein development, suggest that the diabase bodies were emplaced late in the intrusive history of the footwall at Site U1309. Subhorizontal magmatic foliation, taken with paleomagnetic and logging data, suggest that diabase in Holes U1309D and U1309B forms groups of subhorizontal sheets or sills (Fig. F7). Unit boundaries are locally marked by chilled margins, and in some cases magnetic susceptibility varies systematically toward the top/base of a unit (Fig. F8). Together, these observations suggest individual sheet thicknesses on the order of 2–8 m. The basalt and diabase from Site U1309 are tholeiitic basalts to basaltic andesite, with compositions that overlap basaltic glasses from the entire MAR (Fig. F9). All samples analyzed are slightly CaO and Al₂O₃ poor, and Na₂O rich, compared to average MAR basaltic glass compositions. These differences may be related to the pervasive greenschist facies alteration.

Gabbroic rocks recovered at Site U1309 during Expedition 304, including troctolite, olivine gabbro, gabbro, and oxide gabbro, are grouped into 10 zones based on olivine content and the proportion of intercumulus phases present. Within each zone, one rock type is dominant, although others are commonly present as magmatic and/or intrusive layers ranging from a few centimeters to many tens of centimeters in thickness (Fig. F5). Grain sizes vary from fine to very coarse and locally pegmatitic. The thickness of each zone varies from ten to many tens of meters. This gabbroic section (>300 m thick) is cut by numerous thin late-magmatic leucocratic dikes, representing a late episode of intrusion of fractionated magma (Fig. F10).

The gabbroic rocks from Site U1309 have compositions that are among the most primitive sampled by drilling along the MAR (23°N and 15°20'N, Agar et al., 1997; Kelemen, Kikawa, Miller, et al., 2004) and on the Southwest Indian Ridge (ODP Hole 735B) (Dick, Natland, Miller, et al., 1999, Fig. F11). This is reflected in Mg numbers ranging from 74 to 90 and low TiO₂, Na₂O, and trace element contents. Most of the compositional variation observed in gabbroic rocks from Site U1309 is consistent with a simple mass balance involving increases in the proportion of clinopyroxene (or olivine in the troctolite) with decreases in the oxides that correspond to plagioclase (CaO, Al₂O3, Na₂O). Site U1309 gabbroic rocks are therefore interpreted as cumulates related to the basalt and diabase through crystal fractionation processes and a common parental magma.

Thermal demagnetization data show multiple components of remanence in several olivine gabbro and troctolite samples. The reversed magnetization component that characterizes most of the rocks from Site U1309 is overprinted by a normal polarity component with moderate inclination. The highest stability magnetization component in these samples is typically of reversed polarity and is isolated at temperatures above 520°–550°C. A lower stability normal polarity overprint is typically removed over the temperature range of ~350°–520°C. Such overprinting indicates reheating of the rocks, most likely associated with magmatic intrusion. Shipboard analyses show that this occurred in several intervals cored in Hole U1309D; more detailed onshore analyses may provide paleomagnetic limits for the timing of such events.

Ultramafic rocks recovered at Site U1309 are of two types: those that may have been formed by cumulate igneous processes and, thus, may be part of the gabbroic section discussed above (Fig. F12A) and those that lack cumulate textures (Fig. F12B). The latter have bulk rock characteristics that suggest a residual mantle origin (high Mg and Fe content and low SiO₂, Al, and Na content), but these sections are too serpentinized for this conclusion to be certain based on shipboard observations; onshore analysis can provide key data.

Two short intervals of ultramafic rock were recovered in the upper 100 m of both Holes U1309B and U1309D. Four intervals of serpentinized peridotite (three of which are clearly in place) at ~61, 132.5, 172–173, and 224 mbsf were recovered in Hole U1309D and include both harzburgite and dunite. Harzburgites from Hole U1309B have Mg# (100 x molar Mg/[Mg+Fe]) of 90 and 91. The low CaO and Al₂O₃ contents of these harzburgites suggest that, prior to alteration, these peridotites were more refractory than those collected during Leg 153 at 23°N on the MAR (Casey et al., 1997). Ultramafic rocks from Hole U1309D include wehrlite from ~61 mbsf and dunite and harzburgite recovered from ~171 mbsf. The wehrlite is characterized by Mg# of 83, low Ti concentrations (<1 wt%), and high Fe₂O₃ (15 wt%). In contrast, ultramafic samples from deeper in the hole (Sections 304-U1309D-31R-1 and 31R-2 at 171 mbsf) have high Ca concentrations (1.15 wt%), Fe₂O₃ contents (10.03–10.84 wt%) and low Mg# (88.9–89.6). Barring possible calcium carbonate alteration, the two samples from intermediate depths in Hole U1309D are geochemically comparable to peridotite in Hole U1309B. As with Hole U1309B harzburgites, they are enriched in Fe compared to a global compilation of both abyssal peridotites and those tectonically emplaced with the same MgO concentrations.

Alteration mineral assemblages in rocks from Site U1309 record cooling of mafic plutonic rocks from magmatic conditions (>1000°C) to the temperatures of zeolite facies (<200°C) during unroofing and denudation of Atlantis Massif. Many samples from the site, particularly those collected from below 350 mbsf in Hole U1309D, display little to no alteration (Fig. F13). Individual samples generally display a range of superposed metamorphic conditions, but no single sample records the entire cooling history of the site. The mineral assemblages encountered in any sample depend on at what point in its cooling history the rock underwent deformation and/or hydration.

The most extensive alteration event recorded at Site U1309 is hydration that took place at and below greenschist facies. Three distinct processes that occurred at temperatures below 500°C dominated this hydration. Static hydration at 400°–450°C formed characteristic tremolite-chlorite-talc corona textures (Fig. F14) along olivine/plagioclase interfaces and resulted in the formation of tremolite/actinolite from pyroxene in both gabbro and diabase. Plagioclase was stable except in the presence of olivine. The intensity of this hydration decreases with depth and is localized in and around small gabbroic dikes below ~350 mbsf. In another stage, localized fluid flow led to intense alteration associated with breccia zones above 60 mbsf and with late magmatic leucocratic dikes below ~160 mbsf. This alteration characteristically produced secondary plagioclase in addition to actinolite and, below 370 mbsf, epidote. This alteration episode occurred under conditions similar to the static alteration but generally appears to postdate it. In ultramafic rocks, this alteration produced talc-tremolite veins. Finally, in rocks where the coronitic reaction did not go to completion, fracture-controlled serpentinization of olivine is accompanied by replacement of plagioclase by prehnite and hydrogrossular at <350°C. This assemblage is rare above 300 mbsf, and apparently did not occur in rocks that are now above 130 mbsf, because either olivine or plagioclase was completely removed by the corona-forming reaction.

In addition to these dominant processes, pyroxene and plagioclase locally recrystallized in response to granulite-grade deformation. Hydration in the amphibolite facies is associated with ductile deformation in gabbro and in the margins of late magmatic leucocratic dikes. Hydration at clay grade (<100°C) appears along faults and fractures throughout the core but is not a pervasive phenomenon, and clays are only rarely seen in X-ray diffraction spectra of unfractured rock.

Talc-tremolite assemblages are common at contacts between ultramafic and mafic rocks (Fig. F15) and at dike contacts within the serpentinite. These contacts show greenschist facies metasomatism of peridotite by fluids rich in Si and Ca that have passed through and equilibrated with mafic rocks. The reciprocal metasomatic reaction in mafic rocks leads to complete chloritization of plagioclase and replacement of actinolite by tremolite. The same metasomatic reactions most likely produced the talc-tremolite-chlorite schists found in the uppermost cores of Holes U1309B, U1309D, and U1309F and by previous workers on the south side of Atlantis Massif (Schroeder and John, 2004). In addition, hydrothermal fluids may be preferentially channeled along gabbroic intrusions as previously observed on the Mid-Atlantic Ridge (Cannat et al., 1992). Carbonate veins and replacement zones also appear in serpentinite close to contacts with gabbroic rocks and may reflect flow of fluid involved in serpentinization. The focused, serpentinization-driven fluid venting at the Lost City hydrothermal field, 5 km to the south of Site U1309, clearly indicates that this process takes place at Atlantis Massif. Variations in the strength of such flow would be expected as this oceanic core complex evolved.