Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite

The Sr isotopic behavior in hydrothermally altered rocks of a complete sect ion through the Wadi Fizh oceanic crust in the Oman ophiolite was investiga ted in order to evaluate seawater-rock interaction through the oceanic crus t. On the basis of rock-types, secondary mineral assemblage, alteration tem perature, average Sr-87/Sr-86 (of hydrothermal fluids, and degree of altera tion, the section is divided into the following 5 sequences: (1) mainly bas alts and clay minerals, low alteration temperature, average Sr-87/Sr-86 0.7 0634, and 78% degree of alteration; (2) mainly basalts and chlorite, day mi nerals and calcite, fairly low alteration temperature, average Sr-87/Sr-86 0.70584, and 76% degree of alteration; (3) basalts and sheeted dike diabase and prehnite-actinolite facies, moderate alteration temperature, average S r-87/Sr-86 0.70519, and 85% degree of alteration; (4) dike diabase, plagiog ranite, metagabbro and epidosite and prehnite-free greenschist facies, high alteration temperature, average Sr-87/(86)n 0.70514, and 65% degree of alt eration; mid (5) noncumulate and cumulate gabbro and amphibolite facies, ve ry high alteration temperature, average Sr-87/Sr-86 0.70413, and 85% degree of alteration. Alteration (metamorphic) grade deduced from secondary miner al assemblages generally increases versus stratigraphic depth. Only 15% of the rocks have escaped hydrothermal alteration. The estimated strontium iso topic compositions of hydrothermal fluids are much more water-dominated tha n those reported previously from modem and ophiolitic hydrothermal systems. On the basis of ideal model analysis we conclude that the whole sequence o f oceanic crust in the Wadi Fizh section has experienced seawater recharge (downflow zone). The evidence suggests that the Wadi Fizh section was locat ed close to a segment boundary of the oceanic crust along a spreading axis.