As in the cores recovered during Leg 118, the most common sulfides present in the cores from the lower two-thirds of Hole 735B are globular igneous sulfides, although every thin section examined contains secondary sulfides in subordinate amounts. In this sample suite, the abundance of sulfide does not correlate with depth or with any specific lithology; however, this may be an artifact of sampling, as only the most sulfide-rich samples were collected. Unlike the cores from Leg 118, no obvious intervals are devoid of or significantly poorer in sulfide mineralization, although this might be expected because >70% of the material recovered is relatively fresh olivine gabbro.
Sulfide mineralization was subdivided into groups based on the three most common assemblages:
The third group, although generally the least abundant, was present in every section examined and probably represents secondary mineralization. The sulfides armored in plagioclase or pyroxene are the next most common morphology in terms of the number of thin sections where they were recognized, but sulfides armored in olivine are exceedingly rare. More common in terms of sheer numbers of occurrence, however, are the sulfides that are present in association with translucent brown amphibole.
Pyrrhotite is far and away the most common igneous sulfide phase and is present in every sample examined that contained igneous sulfide. In order of greatest to least abundance, other sulfide phases identified are chalcopyrite, pentlandite, troilite, sphalerite, and galena. Pyrite is the most common secondary sulfide phase. The most common association, seen in cores from throughout Hole 735B, is pinkish tan (in reflected light), pyrrhotite with blades of brassy yellow chalcopyrite.
Monomineralic grains of pyrrhotite are common; single-phase chalcopyrite grains are less so. The association of pyrrhotite, chalcopyrite, pentlandite, and troilite was reported in Cores 118-735B-45R, 54R, 68R, and 81R (216, 258, 335, and 434 meters below seafloor [mbsf], respectively) (Alt and Anderson, 1991). Intergrowths of these four phases were common throughout the cores recovered during Leg 176 but were significantly more abundant in sections containing fresh olivine (Fig. F4). Review of thin sections from Leg 118 indicates that this four-phase intergrowth is also present in Cores 118-735B-21R, 27R, 31R, and 34R (89, 122, 143, and 158 mbsf, respectively). Again, all of these samples are fresh olivine gabbro. Where present, the largest part of these multiphase grains is (in reflected light) pinkish tan pyrrhotite with faint, zigzag-shaped lamellae of lighter-hued troilite (Fig. F5). Pentlandite is light cream yellow and usually is present in one or two discrete patches with sharp, irregular margins. Brassy yellow chalcopyrite is present in blades and blebs. The troilite lamellae end abruptly at the contact with either pentlandite or chalcopyrite and do not taper to a fine point as these lamellae do when hosted only in pyrrhotite. Troilite was not seen in samples that contained only pyrrhotite or only pyrrhotite and chalcopyrite. In rare samples, troilite is significantly more abundant than pyrrhotite (Fig. F4).
Table T2 contains average sulfide mineral compositions from samples throughout the recovered section. These averages represent three to seven analyses per grain, and five to 10 grains per sample. Pyrrhotite is present in every sample analyzed. From 500 to 1165 mbsf, all samples analyzed are stoichiometrically close to Fe7S8 and contain from 0.2 to 0.9 wt% Ni. Between 1165 and 1195 mbsf, there is a radical change in the composition of the sulfides from predominantly pyrrhotite (Fig. F6) to approaching stoichiometric troilite, with all samples containing less than 0.5 wt% Ni (Fig. F7). No unit boundary was identified at this location during Leg 176; however, at this depth there is a radical drop in magmatic foliation intensity, concomitant with a sharp decrease in crystal-plastic foliation and a major reduction in bulk magnetic susceptibility (Dick, Natland, Miller, et al., 1999). With an eye of faith, there appears to be a shift in the trend of Mg number at this depth (Fig. F3), suggesting with the data above that there may be a lithologic boundary at this depth.
Pentlandite (stoichiometrically [Fe,Ni]9S8) compositions are variable downsection, but discounting the shallowest sample analyzed, Ni composition is relatively constant (Fig. F8) at about 29 wt% (±2 wt%). Co in pentlandite ranges from 3 to ~14 wt% (Fig. F9), substituting for Fe. There appears to be a rapid increase in Co in pentlandite at the base of the lithologic unit boundary at ~960 mbsf (see Fig. F3) and a steady decrease in Co below the proposed lithologic boundary between 1165 and 1190 mbsf.
Chalcopyrite is stoichiometric in all samples but contains as much as 1.15% wt% Zn in solid solution.
The primary mineralogy of the samples described consists of plagioclase, clinopyroxene, and olivine, which are present in nearly all of the samples, with orthopyroxene and amphibole scattered irregularly throughout the section. Plagioclase mode percent ranges between 35% and ~80%, with most of the samples ranging between 40% and 60%. Clinopyroxene ranges between 1.6% and 57%, most commonly in the range of 20% to 40%. Olivine mode ranges from nil to >50%, but generally falls between 2% and 20%. Brown amphibole (possibly igneous) is present in 28 of the samples described in this study and varies between 0.1% and 4.9% of the mode. The average proportions of olivine, clinopyroxene, and plagioclase are consistent with the cotectic proportions of these phases in the 2-kbar crystallization experiments of Grove et al. (1992) on mid-ocean-ridge basalt.
Alteration is variable and ranges from <1% to >55%. Most of the samples we looked at, however, are <10% altered. Of the primary minerals in this suite, olivine consistently shows the highest degree of alteration and is altered to amphibole, talc, smectite, and magnetite in variable proportions. Clinopyroxene is altered to amphibole. Plagioclase, although apparently the least altered primary phase, is in places replaced by secondary feldspar, actinolite, and chlorite; the last two alteration phases are particularly common where plagioclase abuts either clinopyroxene or olivine.