RESULTS

Rocks

Gabbroic rocks from Hole 735B record high-temperature metamorphism that began at near solidus temperatures, as well as hydrothermal alteration that continued down to zeolite facies and lower-grade metamorphic conditions (Stakes et al., 1991; Vanko and Stakes, 1991; Shipboard Scientific Party, 1999). The uppermost 300 m of the Leg 118 section of Hole 735B is intensely altered at high temperatures, leading to its interpretation as the roots of an axial hydrothermal cell (Vanko and Stakes, 1991; Mével and Cannat, 1991; Dick et al., 1991; Alt, 1995). The veining and metamorphism observed deeper in the section reflect penetration of fluids into the lower crust in a waning axial cell and as the crust moved off-axis, as well as during later fracturing and uplift of the crustal block. The following petrographic summary is modified from the Shipboard Scientific Party (1999) based on our subsequent postcruise work.

The rocks from Hole 735B (Leg 176) are generally slightly altered (0%-20%), but are locally more intensely recrystallized as the result of high-temperature plastic deformation. Below 1000 meters below seafloor (mbsf) there are large intervals where the rocks are <10% recrystallized. In undeformed rocks, "static" alteration is related to fracturing at various scales and penetration of aqueous fluids into the rocks. Olivine alteration varies from cracks filled with talc and serpentine + magnetite in the least-altered grains to coronitic alteration and total replacement by talc + serpentine + colorless Mg amphibole + magnetite. Orthopyroxene is present in more evolved rocks and as local rims on olivine grains and is variably altered to colorless or green amphibole, minor smectite, chlorite/smectite, talc, and magnetite. Clinopyroxene is variably replaced by brown and green amphiboles and local trace chlorite and serpentine. Brown amphibole is present chiefly in sheared oxide gabbros, but also is present elsewhere, generally making up <1% percent of the rock, whereas green amphibole may locally comprise up to 50% of the rock. Plagioclase is the most stable phase, and over large intervals alteration is 1% or less to actinolite, chlorite/smectite, or smectite along cracks, although minor epidote, prehnite, and chlorite are also present associated with veins. The intensity of macroscopic alteration is related to visible veins (see below), but at a smaller scale, crystals are more altered along tiny veinlets (<200 µm) of amphibole, plagioclase, chlorite, and local epidote and prehnite.

Veins

Veins comprise nearly 1% of the Leg 176 core by volume, generally decreasing in abundance with depth. Higher-temperature veins include felsic veins, plagioclase veins, plagioclase + amphibole veins, clinopyroxene-bearing veins, and amphibole veins (Shipboard Scientific Party, 1999) (Fig. F1). Lower-grade veins are described below.

Carbonate veins in the Leg 176 section average 0.5 mm wide and are concentrated in the 500- to 600-mbsf interval (Fig. F1), where the rocks are altered to orange phyllosilicates, iron oxyhydroxide, and calcite. Fracturing related to a major cataclastic fault at 560 mbsf probably provided pathways for seawater solutions into this zone. Another cataclastic fault at 490 mbsf may also have contributed to fracturing. Although macroscopic carbonate veins are restricted to depths less <600 m, microscopic carbonate veins and elevated bulk rock CO₂ contents are present below this depth (Bach et al., 2001).

Smectite and chlorite/smectite veins were originally all described as smectite on board ship and are the most abundant veins by number. They are 0.1-7 mm wide and have generally moderate dips (mostly 20°-40°). These veins are concentrated in two zones: 575-835 and 1050-1500 mbsf (Fig. F1). Fracturing related to the fault at 560 mbsf and another major cataclastic fault at 690-700 mbsf may have provided pathways for seawater solutions to form smectite and chlorite/smectite in and along fractures. Phyllosilicate veins below 1050 mbsf are present in tensional fractures unrelated to faults. Throughout the core, olivine, plagioclase, and pyroxene that are in the host rocks for up to 1-2 cm along the smectite and chlorite/smectite veins are variably altered to smectite, chlorite/smectite, ± magnetite, pyrite, and pyrrhotite.

Zeolite and prehnite veins are present at 1300-1490 mbsf within the smectite-chlorite/smectite-rich zone near the base of the core (Fig. F1). These veins are mostly ~1 mm wide and contain natrolite, analcite, thompsonite, and prehnite. Also associated are smectite, chlorite/smectite, albite, and K-feldspar. All these minerals are present both in veins and in the adjacent altered host rock.

Preliminary shipboard identification of rare local chlorite veins (Fig. F1) are shown by our subsequent work to be in error; these vary from chlorite to chlorite/smectite. Alteration halos in the host rocks contain chlorite/smectite, albite, prehnite, natrolite, analcite, thompsonite, and K-feldspar. Other veins include six quartz veins, two of amorphous silica, and a single large (12 mm) epidote vein.

Phyllosilicate Mineralogy

In shipboard descriptions and the core log, phyllosilicates were logged as chlorite or smectite (Shipboard Scientific Party, 1999). In contrast, our results show that the mineralogy of phyllosilicates in Hole 735B is highly variable, commonly on the scale of a single thin section and even within individual veins. For example, Sample 176-735B-172R-7, 132 cm, contains a zoned vein that ranges continuously from chlorite at the margins to smectite-rich chlorite/smectite at the center (Fig. F2; Table T2). Other phyllosilicates present include various smectites, talc, serpentine, and phlogopite, as well as fine-scale intergrowths, mixtures, and partial replacement relations among these phases, all reflecting varying conditions of hydrothermal alteration.

Chlorite in the gabbroic rocks is relatively Mg rich, having Fe/Fe + Mg = 0.25-0.4 compared to values of 0.3-0.6 for chlorites in oceanic metabasalts and diabases (see review in Alt, 1999) (Fig. F2). Compared to chlorite, mixed-layer chlorite/smectite falls along a broad trend of increasing Si and interlayer cation contents and is locally Fe rich (Fig. F2).

Smectites in Hole 735B are predominantly Fe-bearing saponites, having compositions that are somewhat variable but generally similar to that of saponite in altered seafloor basalts (see review in Alt, 1999). Saponites have variable ratios of Fe/Fe + Mg and covarying contents of Si, Al, and interlayer cations and range from near talc through low-charge to higher-charge smectite (Fig. F2). Interlayer cations are dominated by Ca + Na (Fig. F3). Small amounts of chlorite interlayers are present in many samples, as indicated by high octahedral totals (Schiffman and Fridleifsson, 1991). Talc and serpentine may be present in some analyses as well, as suggested by locally very low interlayer cation and Fe contents (Fig. F2).

Talc is locally present, commonly with small amounts of chlorite and smectite layering, as shown by high Al, Fe, and interlayer cation contents (Table T2; Fig. F2). A serpentine phase is also present locally and has high Mg content (up to 36 wt% MgO) (Table T2). Elevated Al, Fe, and interlayer contents, however, suggest the presence of chlorite and smectite layers.

Nontronite was identified in one sample at 592.8 mbsf (Sample 176-735B-102R-3, 1 cm). Compared to saponite, it is Fe and K rich. The octahedral cation occupancy is high for nontronite (Fig. F4) and may be related to the presence of trioctahedral saponite layers. Relatively high interlayer cation contents (Fig. F4) suggest the presence of celadonite (mica) as well.

Mg montmorillonite was identified in a vein at 1241.7 mbsf (Sample 176-735B-181R-2, 7 cm) and is distinguished from saponite by high Al content (19 wt% Al₂O₃) and low octahedral site occupancy (Fig. F4). Some saponite analyses trend slightly to higher Al and lower octahedral occupancy, indicating the presence of a montmorillonite component.

Biotite/phlogopite is present in felsic veins and in reaction coronas around olivine. Many analyses of olivine and clinopyroxene alteration products fall along a trend from phlogopite to chlorite or chlorite/smectite (Fig. F2). This may reflect fine intergrowths of phlogopite and chlorite or loss of K and Na from interlayer positions during hydrothermal alteration of early-formed phlogopite.

There are some changes in phyllosilicate mineralogy below ~1300 mbsf (Fig. F5). Below this depth, smectite, talc, and serpentine have not been identified. The presence of chlorite and chlorite/smectite and the absence of smectite below 1300 mbsf suggest that alteration occurred at higher temperatures here than shallower in the core. Two samples from deep in the hole (>1475 mbsf) also exhibit relatively high abundances of greenschist-facies minerals (albite, chlorite, and amphibole), also consistent with more abundant higher-temperature (>250°) alteration (but still low grade) at depth.

Other Minerals

Other minerals include prehnite, albite, K-feldspar, analcite, natrolite, and thompsonite, which partly replace plagioclase and fill small veins associated with chlorite/smectites. Representative electron microprobe analyses of these phases are given in Table T3. Pyrite and titanite are also commonly associated in pseudomorphs after olivine and replacing titanomagnetite, respectively. Prehnite and zeolites are most abundant below ~1300 mbsf where smectite is absent (Figs. F1, F5).

Isotopic Data

Phyllosilicates separated from veins have ¹⁸O values of 7.1 to 19.9 (Table T1), which indicate formation from seawater at temperatures of ~60°-110°C (Savin and Lee, 1988). Application of temperature-dependent mineral-water isotopic fractionation factors in the remaining cases is problematic because of the mixtures of minerals that are present. Using the chlorite-water oxygen isotopic fractionation from Wenner and Taylor (1971) gives temperatures of <100°C for four chlorite-rich samples. More realistic temperatures of 150°-300°C for chlorite-rich mixtures implies formation from evolved, ¹⁸O-enriched fluids. The high Sr content (105 ppm) of one chlorite/smectite sample suggests the presence of small amounts of a contaminant (e.g., prehnite), which could in part explain the high ¹⁸O value of this sample (Table T1).

Strontium isotopic data for phyllosilicate veins indicate highly rock-dominated fluids, with six ⁸⁷Sr/⁸⁶Sr ratios of 0.70312-0.70338 and two higher values ranging up to 0.70492 (Table T1).

Carbonates have uniform ¹⁸O values of 31.2-32.5 (Table T1), which translate into formation temperatures of ~10°C if formed from normal seawater (O'Neil et al., 1969). ⁸⁷Sr/⁸⁶Sr ratios are mostly 0.70865-0.70891, similar to 0- to 10-Ma seawater (0.7091-0.7088) (Hodell et al., 1991), but two lower values suggest more rock-dominated fluids (Table T1). Sr contents of carbonates are also roughly consistent with formation from seawater (150-280 ppm for calcite and 3200 ppm for aragonite). However, one calcite having low ⁸⁷Sr/⁸⁶Sr also has a very low Sr content (23 ppm). Carbonate ¹³C values of ~0 to +3 are consistent with formation from seawater, with one lower value of -2.7 suggesting more evolved fluids (Table T1). The low ⁸⁷Sr/⁸⁶Sr values of carbonates correspond with low ¹³C and ¹⁸O, which is consistent with more evolved fluids.