The Serpentinization of Peridotite from Cedar Valley, Jamaica

Abstract

Previously undescribed, microscopic textures in partially serpentinized peridotite shed light on the process of serpentinization. Fracturing and fragmentation of the peridotite protolith produced a modest increase in volume and opened channels to reactive aqueous fluids. Prior to, or in conjunction with, the first appearance of serpentine, micrometer-scale grains of magnetite crystallized on the surfaces of fractures. This decoration of fracture surfaces faithfully preserved the original, angular outlines of fragments of forsterite and pyroxene through all stages of serpentinization. Simple geometric considerations indicate that the volume of a given fragment did not change during replacement. Partially serpentinized forsterite shows a penetrative, micrometer-scale, sieve or sponge texture. Fifty percent or more of the volume of optically continuous forsterite can consist of micrometer-scale perforations, filled with serpentine. This sponge texture has not been described elsewhere. The delicate connections between different parts of the same forsterite grain further support strict constant-volume replacement of the fragments. Serpentinization stopped before completion (mid-reaction) when the otherwise open, magnetite-decorated channels themselves filled with serpentine, effectively sealing the rock from further access by aqueous fluids.