High‐temperature seawater circulation throughout crust of oceanic ridges: A model derived from the Oman ophiolites

Abstract

We describe here the high‐temperature (HT ∼ 700°C) and very high temperature (VHT ∼ 1000 °C) recharge and discharge systems which are responsible for HT‐VHT hydrothermal alterations at the oceanic ridge of origin in the crust of Oman ophiolites. The recharge system is a regular network of parallel microcracks, a fraction of a millimeter wide, which carries seawater to the walls of the magma chamber where hydrous melting is triggered. The discharge system is a network of clinopyroxene, pargasite gabbroic dikes issued from the hydrous melting; upsection, these HT‐VHT gabbroic dikes grade into the well‐documented low‐temperature (LT) (400–500°C) hydrothermal veins. Microcracks, gabbroic dikes, and veins are preferentially parallel to the diabase‐sheeted dikes and normal to magmatic lineations in enclosing gabbros. Thus the fractures of both the recharge and discharge systems are parallel to the paleoridge plane. In the brittle field, fracturing is induced by tensional stress related to seafloor spreading. In the ductile field and up to temperatures of ∼900°C, opening of microcracks is ascribed to the anisotropy of thermal contraction during gabbros cooling. Gabbro fabrics and anisotropy are such that the induced microcracks are also parallel to the ridge plane. Thermal contraction between 1200°C (temperature of magma chamber) and ∼700°C (brittle‐ductile limit) produces a differential strain of 0.1% comparable to that deduced from width and spacing of microcracks in the gabbros. Simple estimates on activity time, fluid velocity, and fluid flux in microcracks account for all available data and suggest that in microcracks, activity was short lived and episodic.