Document Type

Article

Publication Date

1-2007

Keywords

Eclogite, Excess silica, Omphacite, Pyroxene, Ultrahigh-pressure

Abstract

Silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite. However, mechanisms other than exsolution might produce free silica, and the possible mechanisms depend in part on the nature and definition of excess silica. ‘Excess’ silica may occur in both stoichiometric and non-stoichiometric pyroxene. Although the issue has been debated, we show that all common definitions of excess silica in non-stoichiometric clinopyroxene are internally consistent, interchangeable, and therefore equivalent. The excess silica content of pyroxene is easily illustrated in a three-component, condensed composition space and may be plotted directly from a structural formula unit or recalculated end-members. In order to evaluate possible mechanisms for the formation of free silica in eclogite, we examined the net-transfer reactions in model eclogites using a Thompson reaction space. We show that there are at least three broad classes of reactions that release free silica in eclogite: (i) vacancy consumption in non-stoichiometric pyroxene; (ii) dissolution of Ti-phases in pyroxene or garnet; (iii) reactions between accessory phases and either pyroxene or garnet. 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 of titanium solubility, and the possible roles of accessory phases and inclusions on the balance of free silica.

Publication Title

Journal of Metamorphic Geology

Volume

25

Issue

1

First Page

37

Last Page

50

Required Publisher's Statement

Copyright 2007 Blackwell Publishing Ltd.

This article was published paid open access.

doi:10.1111/j.1525-1314.2006.00677.x

Subjects - Topical (LCSH)

Stoichiometry; Silicate minerals; Eclogite

Genre/Form

articles

Type

Text

Language

English

Format

application/pdf

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