Sr isotope geochemistry and hydrothermal alteration of the Oman ophiolite

Abstract

The Sr isotopic behavior in hydrothermally altered rocks of a complete section through the Wadi Fizh oceanic crust in the Oman ophiolite was investigated in order to evaluate seawater-rock interaction through the oceanic crust. On the basis of rock-types, secondary mineral assemblage, alteration temperature, average 87Sr/86Sr of hydrothermal fluids, and degree of alteration, the section is divided into the following 5 sequences: (1) mainly basalts and clay minerals, low alteration temperature, average 87Sr/86Sr 0.70634, and 78% degree of alteration; (2) mainly basalts and chlorite, clay minerals and calcite, fairly low alteration temperature, average 87Sr/86Sr 0.70584, and 76% degree of alteration; (3) basalts and sheeted dike diabase and prehnite-actinolite facies, moderate alteration temperature, average 87Sr/86Sr 0.70519, and 85% degree of alteration; (4) dike diabase, plagiogranite, metagabbro and epidosite and prehnite-free greenschist facies, high alteration temperature, average 87Sr/86Sr 0.70514, and 65% degree of alteration; and (5) noncumulate and cumulate gabbro and amphibolite facies, very high alteration temperature, average 87Sr/86Sr 0.70413, and 85% degree of alteration. Alteration (metamorphic) grade deduced from secondary mineral assemblages generally increases versus stratigraphic depth. Only 15% of the rocks have escaped hydrothermal alteration. The estimated strontium isotopic compositions of hydrothermal fluids are much more water-dominated than those reported previously from modern and ophiolitic hydrothermal systems. On the basis of ideal model analysis we conclude that the whole sequence of oceanic crust in the Wadi Fizh section has experienced seawater recharge (downflow zone). The evidence suggests that the Wadi Fizh section was located close to a segment boundary of the oceanic crust along a spreading axis. Copyright 2001 by the American Geophysical Union.