SULFUR ISOTOPIC PROFILE THROUGH THE OCEANIC-CRUST - SULFUR MOBILITY AND SEAWATER-CRUSTAL SULFUR EXCHANGE DURING HYDROTHERMAL ALTERATION

Measurements of the abundance of anhydrite and pyrite veins and the di stribution of alteration types in oceanic basement enable estimates of global mass and isotopic fluxes of sulfur during hydrothermal alterat ion of oceanic crust. Fluxes of 2.5 x 10(12) g/yr S (delta(34)S = 2.7 parts per thousand) from the volcanic section to seawater and 2.1 x 10 (12) g/yr of seawater S (21 parts per thousand) into the crust result in an insignificant change in the S content of altered ocean crust. Th ese are clearly second-order fluxes compared to the riverine input of S to the oceans and the output of sedimentary pyrite. Most of the anhy drite that forms in the high-temperature axial convection cell must la ter be dissolved at lower temperatures during off-axis circulation. Th e mean delta(34)S of altered oceanic basement is 0.9 parts per thousan d, only slightly changed from the primary value of 0.1 parts per thous and. Unless S is preferentially mobilized from S-34-rich sulfide depos its or mineralized zones, subduction of altered basaltic crust is unli kely to be the source of high delta(34)S values of are volcanic rocks (similar to 4 parts per thousand), but could contribute to isotopic he terogeneities of S in the mantle.