The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15聽GPa with possible implications for the formation of oriented SiO₂-inclusions in omphacite

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• 作者：J眉rgen Konzett ; Daniel J. Frost ; Alexander Proyer and Peter Ulmer
• 关键词：Ca ; Eskola pyroxene ; High ; P ; T experiments ; Oriented quartz inclusions ; Eclogite
• 刊名：Contributions to Mineralogy and Petrology
• 出版时间：February, 2008
• 出版年：2008
• 期刊代码：29_0010-7999
• 类别：ch
• 卷：155
• 期：2
• 页码：215-228
• 数据来源：sp

摘要

Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500掳C using bulk compositions in the systems SiO₂–TiO₂–Al₂O₃–Fe₂O₃–FeO–MnO–MgO–CaO–Na₂O–K₂O–P₂O₅ and SiO₂–TiO₂–Al₂O₃–MgO–CaO–Na₂O to investigate the Ca-Eskola (CaEs Ca_0.5□_0.5AlSi₂O₆) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO₂ 卤 TiO₂ 卤 kyanite as a function of P, T, and bulk composition. The results show that CaEs_ss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative anorthite and low diopside contents. In this study, a maximum of 18 mol%CaEs_ss was found at 6 GPa and 1,350掳C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison, no significant increase in CaEs_ss with increasing P could be observed. If the formation of oriented SiO₂-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites cooled from temperatures ≥750掳C. The presence of clinopyroxene with approx. 4 mol%CaEs_ss in an experiment conducted at 2.5 GPa/850掳C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes are not necessarily restricted to ultrahigh pressure metamorphic conditions.