EXCESS SILICA IN OMPHACITE AND THE FORMATION OF FREE SILICA IN ECLOGITE

Silica lamellae in omphacite are widely interpreted as evidence of exsolution during decompression of eclogite from high or ultrahigh pressures. The interpretation of the first appearance of free silica in eclogite depends upon the reaction by which it was produced, yet mechanisms other than exsolution have not been considered. We find at least three broad classes of silica-producing reactions in eclogite: (1) vacancy consumption in non-stoichiometric pyroxene; (2) dissolution of Ti-phases in pyroxene or garnet; (3) reactions between an accessory phase and either pyroxene or garnet. The possible mechanisms depend in part on the nature of excess silica, which may occur in either stoichiometric or non-stoichiometric pyroxene. We use a three-component, condensed composition diagram to illustrate the relationship between excess silica in stoichiometric and non-stoichiometric pyroxenes and to demonstrate that all common definitions of excess silica in non-stoichiometric clinopyroxene are equivalent. We explored all possible net-transfer reactions in model eclogite using a Thompson reaction space. From this, we find that formation of free silica by consumption of vacancies can only occur if the omphacite is non-stoichiometric. Because vacancies can be consumed by silica-conserving reactions, the vacancy content of pyroxene cannot be used as an independent monitor of the potential for producing free silica. Dissolution of Ti-phases and reactions with accessory phases can operate with either stoichiometric or non-stoichiometric pyroxene. However, if pyroxene is stoichiometric, with or without excess silica, all mechanisms of producing free silica require the involvement of accessory phases. We suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility, but also titanium solubility, and the possible effects of accessory phases and inclusions on the balance of free silica.