Abstract

Hole 735B (Ocean Drilling Program leg 118) was drilled through 500 m of oceanic layer 3 gabbroic rocks. These plutonic rocks range in composition from ilmenite-rich gabbronorite to troctolite. The gabbroic rocks are relatively fresh and undeformed in lower units of the hole and are highly deformed and metamorphosed in upper units. Both primary and metamorphic facies occur interlayered. Igneous laminations are structurally transposed into a secondary foliation. Sulfide redistribution is accompanied by several stages of deformation and hydrothermal circulation, varying from high-temperature (800 degrees C) plastic deformation to low-temperature brittle deformation (300 degrees C). Sulfides are progressively liberated from host clinopyroxene and oxides, are disseminated in amphibolitized areas or injected into shear planes or later extensional fractures along with medium-grade amphiboles, and are reequilibrated to medium- and low-temperature assemblages. Sulfide redistribution involves hydrothermal fluids reacting with plutonic rocks, initially at low fluid/rock ratios, and later at higher fluid/rock ratios as the fracture system develops at continuously decreasing temperature. Compositions of hydrothermal fluids vary from Si poor and Na, Al, Ti rich to Si, Al, K, Na rich. Fe/Mg ratios are controlled by the bulk-rock compositions. Actual sulfide parageneses are the products of progressive reequilibration through hydrothermal circulation, starting from high temperature down to very low-grade metamorphism. The transfer of sulfides in the hydrothermal system is controlled by deformation which is the major factor that allows seawater penetration into layer 3.

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