Pyroxene Phase Equilibria for Thermobarometry of FeldspathicMafic and Ultramafic Rocks

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Experiments to determine the compositions of pyroxenes in the assemblage enstatite + diopside + forsterite + anorthite will be performed in the systems CaO-MgO-Al2O3-SiO at temperatures from 925oC to 1175oC, and pressures of 5000 and 7500 bars. Results from these experiments will be combined with results obtained earlier in this experimental program at atmospheric pressure, and at 2500 and 5000 bars. The experimental data will be used to construct and to constrain empirical and thermodynamic models of subsolidus pyroxene phase equilibria, which may then be applied to methods of estimating temperatures and pressures of equilibration of felspathic, mafic and ultramafic rocks. Pyroxene compositions will be reversed, both in terms of Ca/Mg ratio and Al contents by approaching the equilibrium compositions from starting compositions with both higher and lower values. Experiments will use vanadium pentoxide as a high-temperature solvent to promote grain growth and chemical equilibration; several lines of evidence developed earlier in this experimental program demonstrate that this technique produces negligible effects upon the equilibria under investigation. Analysis of compositions and phase assemblages will be performed petrographically, by electron microprobe, and powder X-ray diffraction, supplemented by scanning and transmission electron microscopy and single crystal X-ray diffraction as required.