HYDROTHERMAL ALTERATION, METAMORPHISM, AND METASOMATISM
Alteration mineral assemblages record cooling of the plutonic rocks, from magmatic conditions to zeolite facies, during unroofing and uplift of Atlantis Massif. The alteration intensity is moderate (Fig. F16) and tends to decrease downcore in general. Locally, there are exceptions to this, where alteration intensity correlates with the modal abundance of olivine in the intercalated dunitic troctolites, olivine gabbros, and gabbros between ~1090 and ~1230 mbsf and in the lowermost gabbros and olivine gabbros (Fig. F17). The coarser grained gabbro intervals generally appear to be more altered than the medium- to coarse-grained gabbros and olivine gabbros. In dunitic troctolite intervals, alteration is mostly restricted to heterogeneous serpentine networks, with strong alteration gradients from the contact with intensely veined intercalated gabbros to the fresher cores of the dunitic troctolite units. The latter locally contain intervals of very fresh (as low as 1% serpentinization) olivine-rich (as much as ~90%) rocks. Alteration is commonly more intense in the vicinity of metamorphic veins. The vein assemblages change with lithology (local control) and with depth, likely reflecting changes in late fluid chemistry.
The overall metamorphic history is divided into a high-temperature phase and, by far the most important, a low-temperature phase, each with multiple events that reflect alteration at temperatures <500°C. The several stages of metamorphism and alteration appear as follows.
The first high-temperature (granulite/upper amphibolite facies) event is commonly associated with the limited occurrences of high-temperature ductile deformation. It is characterized by recrystallization of plagioclase, clinopyroxene, and, more rarely, brown hornblende (which may or may not be igneous in origin).
Static replacement of pyroxene by green to brown hornblende occurs most commonly in oxide gabbros.
The main mode of alteration of the most olivine-rich rocks is serpentinization. The degree of serpentinization varies widely from >90% to <10% of the olivine. In the simplest case, serpentinization proceeds via the development of kernel texture (O'Hanley, 1996) (Fig. F18). In hand sample, areas of more intense serpentinization form a conspicuous, but generally irregular, foliation (Fig. F19). The veinlets that define the foliation appear black because of included opaque phases (Fig. F20), mostly magnetite and also some sulfides (pyrrhotite and, more rarely, pyrite, as identified by reflected light microscopy). Plagioclase in the serpentinized rocks ranges from virtually unaltered to partially altered (usually to prehnite) to complete replacement by hydrogarnet and/or prehnite. Hydrogarnet appears to be most common in highly serpentinized rocks. A commonly observed feature in serpentinized rocks is the development of microfracture sets that radiate or extend into plagioclase from neighboring serpentinized olivine grains (Fig. F21). These fractures are commonly filled with chlorite and/or amphibole.
A widespread greenschist facies event is characterized by (1) the development of corona texture in olivine-plagioclase rocks (Fig. F22), (2) bleaching and epidote growth that may or may not be related to late leucocratic intrusions, (3) widespread emplacement of amphibole-rich veins and accompanying halo alteration, (4) talc-carbonate metasomatic alteration of olivine-rich rocks (restricted to the upper part of the core; see Shipboard Scientific Party, 2005), and (5) relatively late emplacement of slip-fiber amphibole veins and associated metasomatism.
The most common corona texture consists of partial or complete replacement of olivine by talc/tremolite and replacement of the edges of neighboring plagioclase by chlorite. Tremolite may either entirely replace olivine or be restricted to its margins (Fig. F23). Talc is present, mostly in the cores of completely or nearly completely altered olivine, although, where relatively fresh olivine cores are present, an annulus of talc may separate it from the outer tremolite zone. Serpentine is generally present only as relics within fractures in olivine, as a late replacement of olivine cores, or, rarely, as a replacement of tremolite. In a few cases, carbonate is also found associated with serpentine, talc, and tremolite.
Dark green amphibole veins are the most common vein type in Hole U1309D (~48%); they are the major contributor to the overall observed alteration. Corona textures are commonly well developed near dark green amphibole veins. In hand sample, these veins are usually relatively straight, but also commonly braided, and typically associated with an alteration halo 5 mm to 2 cm wide (Fig. F24). In thin section, they are filled with green amphibole (probably actinolite)
Late light green amphibole veins, commonly in a slip-fiber configuration, cut the dark green veins. They contain mostly tremolite and/or actinolite and, in more olivine-rich rock types, talc and/or serpentine. These veins can contain a substantial amount of isotropic and/or very fine grained material that may include hydrogarnet. The veins are commonly associated with alteration halos as thick as a few centimeters, especially in olivine-rich rocks containing talc and/or tremolite, amphibole, chlorite, albite, prehnite, zeolite, or hydrogarnet. As with the more abundant dark green veins, the signature of this alteration seems to be the transport of silica into olivine-rich lithologies.
The latest stage of alteration involves emplacement of a variety of veins, postdating greenschist facies alteration and containing variable mineralogy that, at least in part, correlates with depth. The veins contain quartz, carbonate, zeolite, prehnite (Fig. F25), and, in one case, anhydrite. Overprinting all of these vein types is a late, low-temperature set of clay-rich (probably saponite) veins. The clay is present along with calcite and/or zeolites (especially thomsonite) in broad, uneven fractures in the rock. The clay veins appear most commonly associated with olivine-rich rocks and are notably abundant in gabbroic rocks intercalated with dunitic troctolite between ~1090 and ~1230 mbsf.
The mineralogy of common "white" veins varies significantly with depth. Above ~400 mbsf, sulfide-, quartz-, and, possibly, zeolite-bearing veins are abundant. Quartz is rare below ~400 mbsf, and little sulfide is found in veins below ~800 mbsf. Zeolite, absent from the vein assemblage below 300 mbsf, reappears at ~700 mbsf and increases in abundance downhole to the deepest penetration of Hole U1309D (Fig. F26). Carbonate occurs in veins associated with ultramafic rocks throughout the hole.
Fluid flow along fracture zones, including zones of brecciation or cataclasis, or associated with late magmatic intrusions commonly results in higher degrees of alteration in the surrounding gabbro. In cores recovered during Expedition 305, it was apparent that zones of plastic and, especially, brittle deformation provide fluid pathways and thus cause greater alteration of the adjacent rocks. However, not all plastic or brittle deformation zones are highly altered.
The overall trends in alteration and the changes in secondary mineralogy suggest that two separate secondary processes affected the footwall in the vicinity of Hole U1309D. In the upper ~840 m, seawater–rock interactions pervade the sequences. Below that depth, the nature of and the fluctuations in degree and style of metamorphism are related to fluids of a different composition percolating along fault zones and zones of deformation. Hence, the rocks record an extensive history of gabbroic rock/fluid interaction, possibly including magmatic fluids.