HIGH-TEMPERATURE ALTERATION OF OCEANIC GABBROS BY SEAWATER (HESS DEEP, OCEAN DRILLING PROGRAM LEG-147) - EVIDENCE FROM OXYGEN ISOTOPES AND ELEMENTAL FLUXES

Two major stages of high-temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the Hess Deep Rif t Valley at the East Pacific Rise (2 degrees N, 101 degrees W). The 15 4 m of plutonic rocks cored at site 894G show mineralogies and texture s that suggest that they represent the roof of an approximate 1 Myr ma gma chamber. A late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite - upper greenschist facies co nditions. The hydration of the gabbros occurred locally through the de velopment of well-crystallized green amphiboles. Significant chemical fluxes were associated with the fluid flow and precipitation-dissoluti on mechanisms. Oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the Hess Deep gabbros at temperatures abov e 500 degrees C. Hydration was limited with only minute amphibole lame llae in pyroxenes that trapped Cl, Na, and minor K from seawater. Ca-p lagioclases remained stable and preserved their magmatic cationic comp ositions and zoning patterns. We propose that this high-temperature ev ent lowered the delta(18)O of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion. Although it i s unclear whether the O-18 depletion of Hole 894G gabbros is represent ative of the entire lower oceanic crust, it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans. During this stage, water rock ratios calculated using Sr isotopes are underestima ted (0.1 - 0.5) because of insignificant diffusion of this cation in p lagioclases. Better estimates of water rock ratios are provided by oxy gen isotopes (0.2 - 1) which are important in unraveling the cooling h istory of the oceanic crust.