Experimental study of subsolidus phase relations and mixing properties of pyroxene in the system CaO-Al 2 O 3 -SiO 2

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Subsolidus phase relations in the system CaO-Al 2 O 3 -SiO 2 (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-CaAl 2 SiO 6 ) and Ca-Eskola (CaEs-Ca 0.5 AlSi 2 O 6 ) endmembers. Reversals were obtained on the following reactions (bar, °C): 3 An = Gr + 2 Ky + Q ( P = 22 T - 700), 3 An + Cor = Gr + 3 Ky ( P = 21.8 T - 950), 3 CaTs = Gr + 2 Cor ( P = 55 T - 53900), and 6 CaTs (1 - x ) CaEs x = 2(1 - 2 x ) Gr + 4(1 - 2 x ) Cor + 9 xAn . 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 3 An = 2 CaTs + 2 CaEs ( J , bar , K ): 2 RT ln ( X CaTs · X CaEs ) + 60200 + 86.4 T - (5.06 + 13 × 10 -7 P ) P = 0, resulting in H 0 j = -39.8 kJ / mol and S 0 = 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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