Rapid formation of eclogite in a slightly wet mantle

Abstract

A model, in which dissolved ions migrate through water films surrounding mineral grains to sites of reaction, predicts the geologically rapid occurrence of the gabbro-eclogite phase change in the earth's mantle at temperatures less than 600–800°C. In a water-undersaturated mantle, interstices within the rock can contain water vapor in equilibrium with small amounts of hydrous phases such as chlorite, tremolite or talc and in the presence of other gases such as CO_2, at H_2O pressures less than the lithostatic pressure of the rock. The solubility of ions in this interstitial water vapor is strongly dependent on pressure and is the rate-limiting process in the model; reaction occurs rapidly if the water pressure is at least 0.5–1 kbar. The 5 km of oceanic gabbroic crust can transform to eclogite upon subduction into the mantle at depths of several tens of kilometers, depending on the rate of heating of the descending crustal material and the nature of the minor hydrous phases present. The downward body force on the descending slab due to the eclogitization of oceanic crust is comparable to the downward forces associated with thermal contraction of the slab and the elevation of the olivine-spinel phase boundary.

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