Experimental study of subsolidus phase relations and mixing properties of pyroxene in the system CaO-Al2O3-SiO2 |
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| Authors: | Tibor Gasparik |
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| Institution: | Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA |
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| Abstract: | Subsolidus phase relations in the system CaO-Al2O3-SiO2 (CAS) were experimentally determined with tight reversals of several univariant curves and with 14 equilibration experiments containing the assemblage pyroxene + anorthite, where pyroxene is a binary solid solution of Ca-Tschermak (CaTs-CaAl2SiO6) and Ca-Eskola (CaEs-Ca0.5AlSi2O6) endmembers.Reversals were obtained on the following reactions (bar, °C): 3An = Gr + 2Ky + Q (P = 22T ? 700), 3An + Cor = Gr + 3Ky (P = 21.8T ? 950), 3CaTs= Gr + 2Cor(P = 55T ? 53900), and 6CaTs(1 ? x)CaEsx = 2(1 ? 2x)Gr + 4(1 ? 2x)Cor + 9xAn. Observed slopes indicate 9.8 J/mol · K of Al-Si disorder in Ca-Tschermak pyroxene and 5.3 J/mol·K of Al-Si disorder in anorthite, at 1300°C. It is suggested that Al-Si disorder in anorthite increases by 1.9 J/mol · K from 700°C to 1300°C.Compositions of CaTs-CaEs pyroxene in equilibrium with anorthite and PbO-rich liquid were experimentally determined at 1400–1430°C and 22.7–30.8 kbar. Microprobe measurements gave compositions which are consistent with an ideal pyroxene solution and the following parameters for the reaction 3An = 2CaTs + 2CaEs (J, bar, K): 2RTln(XCaTs · XCaEs) + 60200 + 86.4T ? (5.06 + 13 × 10?7P)P = 0, resulting in ΔH0j = ?39.8 kJ/mol and S0 = 461.8 J/mol · K for the Ca-Eskola endmember at 1300°C. The obtained properties of the Ca-Eskola component are necessary for thermobarometry based on pyroxene bearing assemblages containing plagioclase, quartz, or kyanite. |
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