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Calculated phase equilibria in high pressure metamorphic rocks and the deep mantle

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It is now possible to calculate quite complex phase diagrams for natural rocks of basic, ultrabasic, pelitic and carbonate bulk compositions with the possibility of inferring the P −T − t history within orogeny. In other ways we still have far to go. The improvement in internally- consistent thermodynamic datasets for the end-members of minerals in real solid solutions has advanced our knowledge and ability to understand metamorphic rocks. Improvements in activity-composition relations have brought much needed realism to such calculations, but these models are still in their infancy. It is increasingly clear that mineral solid solutions need to be modelled in larger chemical systems to yield reliable information, and so the mixing properties will require significant work in future. In addition, the thermodynamic models are being extended to include a number of deep mantle phases and should be well-suited to predicting pseudosections for realistic bulk compositions at deep mantle conditions.

References Green, E., Powell, R. & Holland, T. 2007. An order-disorder model for omphacitic pyroxenes in the system jadeite–diopside-hedenbergite–acmite, with applications to eclogitic rocks. American Mineralogist, 92, 1181–1189.

Harte, B (2010) Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones. Mineralogical Magazine, 74, 189–215.

Holland, T.J.B. & Powell, R. 2010. An improved and extended internally-consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids. Journal of Metamorphic Geology, in press.

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