A total of 107 hard-rock samples from Ocean Drilling Program Leg 147 drill cores were analyzed for sulfur content and, where possible, for sulfur isotope composition and sulfide mineralogy. At Site 894, plutonic rocks (gabbros and gabbronorites) from the roof of an intrarift magma chamber were sampled, whereas mafic and ultramafic rocks (gabbroic rocks, troctolites, dunites, and harzburgites) belonging to a transition zone between the lower magmatic crust and upper residual mantle were sampled at Site 895.
The average (and range) of sulfur content in analyzed rocks are as follows: gabbros (14 samples): 1590 ppm (<100–10340 ppm); gabbronorites (39 samples): 760 ppm (<100–3100 ppm); troctolites (9 samples): 680 ppm (<100–1700 ppm); harzburgites (20 samples): 200 ppm (<100–700 ppm); and dunites (19 samples): 400 ppm (<100–1200 ppm).
About 65% of delta34S values from 45 isotope analyses are between –2‰ and +4‰, which is similar to reported values of about 0‰ ± 2‰ for magmatic rocks and upper mantle material. Extreme deviations (up to –29‰) from this range were found in three extremely serpentinized dunite samples.
In gabbroic rocks, sulfur-bearing phases were identified as pyrrhotite, pyrite (marcasite), and chalcopyrite by microscopy and electron microprobe. In troctolites, dunites dominate, and in harzburgites pentlandite dominates with some Ni-Fe alloys and native copper as alteration products.
Date of initial receipt: 5 August 1994
Date of acceptance: 12 January 1995
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