Alteration Processes and Related Minerals of the Oceanic Lower Crust and Upper Mantle

Abstract

Gabbroic rocks recovered from deep holes in the oceanic crust significantly vary in the abundance and assemblage of alteration minerals, showing a close association with the original lithol-ogy and distribution of dikes and veins. The mineralogical variation is considered to reflect the durability of primary minerals, accessibility and composition of alteration fluids, and alteration temperature. Textural relationships of alteration minerals suggest a common cooling history of oceanic gabbros from granulite or pyroxene hornfels facies to zeolite facies conditions. It is considered that regardless of spreading rate, the static formation of upper greenschist-to lower am-phibolite-facies minerals is the dominant alteration process at the lower crust near oceanic ridges , whereas subgreenschist-facies alteration represents the exhumation histories of gabbroic masses from depth. High-temperature plastic shear zones with almost anhydrous recrystalliza-tion of primary minerals develop locally at slow-spreading ridges, and possibly provide pathways for later hydrothermal fluids. In contrast to the gabbroic rocks, oceanic peridotites have a monotonous mineralogy formed during low-temperature serpentinization processes, making it difficult for us to depict their cooling histories or in-situ alteration processes at the upper mantle. The hypothesis that oceanic Moho represents a serpentinization front in peridotites is suitable for the uniformity of crustal thickness inferred from seismological observations, but lacks a rationale for supplying a constant amount of water to the upper mantle or for the cessation of serpentinization at a constant degree. Alternatively, preferential alteration of pyroxene at relatively high-temperature conditions might form the oceanic crust of uniform thickness.