Oxygen isotope composition of a section of lower oceanic crust, ODP Hole 735B
نویسندگان
چکیده
[1] The oxygen and hydrogen isotope compositions of vein minerals and dO values of 64 whole rocks from the deeper kilometer of the 1.5 km lower crustal section of ODP Hole 735B in the SW Indian Ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust. Vein amphiboles have dD values of 35 to 65%, indicating formation from seawater fluids and hydrothermal fluids containing a component of low-dD magmatic fluid. Crystal-plastic shear zones reacted with O-enriched fluids at high temperatures and have dO values of 6.0–7.0%, indistinguishable from surrounding rocks. Reaction with seawater-dominated fluids over a range of temperatures ( 350–700 C) led to local O-depleted rocks, but much of the section exhibits O enrichment (dO up to 7.6%) as the result of static alteration of plagioclase over a range of temperatures (<250 to 600 C) and fluid compositions. Chloritic veins and associated alteration by O-enriched fluids occurred at temperatures of 150–250 C, and late carbonates and Fe-oxyhydroxides formed from cold ( 10 ) seawater associated with open fault-related fractures. Bulk rock dO values of the lower 1 km range from 3.7 to 10.2% but are mostly 5.6–7.6%, and the section has a mean bulk dO of 6.5 ± 1.0%. In contrast, rocks from the shallower 500 m exhibit net O depletion (5.1 ± 1.0%) reflecting high-temperature deformation and seawater hydrothermal alteration (Stakes, 1991). The weighted average dO for the entire gabbro section of Hole 735B is 6.0%, exhibiting little or no change compared to unaltered crust. Although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low-temperature alteration and O enrichment, there is no evidence for a consistent difference in the bulk dO of the plutonic sections of oceanic crust formed at fast versus slow spreading rates.
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