Effect of Al on enstatite solubility in CMAS clinopyroxenes: 1 - Experimental results in the clinopyroxene-orthopyroxene two phase field at P = 18 kbar
Tribaudino, Mario; Bruno, Emilianο
European Journal of Mineralogy Volume 5 Number 1 (1993), p. 123 - 132
published: Mar 4, 1993
manuscript accepted: Apr 17, 1992
manuscript received: Oct 31, 1991
ArtNo. ESP147050501010, Price: 29.00 €
Abstract Clinopyroxene coexisting with orthopyroxene in the univariant (at constant P and T) two pyroxene field in the CMAS system was reversed at P = 18 kbar and T = 1350 and 1430°C, in order to compare the En content in Al-rich clinopyroxenes of CMAS system at the same P and T and in corresponding Al-free clinopyroxenes of the CMS system. Reversal runs were performed with compositions from 5 to 20 mole % CaTs to determine the solubility variation of En vs CaTs in clinopyroxenes at constant P and T. The obtained results show that the En solubility in clinopyroxenes at T = 1430°C is progressively lower than that observed along the Al-free join in the CMS system at the same P-T conditions (10 mol % less in enstatite for a CaTs content close to 20 mol %). At T = 1350°C, however, the change in En solubility in clinopyroxenes with increasing CaTs content is not significant. Therefore, at higher T, mixing on M2 and M1 sites in clinopyroxenes is not independent. As suggested by previous authors the presence of Al affects the En solubility in clinopyroxenes. As a consequence, geothermometers based on clino-orthopyroxene equilibria in the CMS system - for- a P-T range close to that of spinel lherzolite stability field - can be used to a good approximation for Al-rich two pyroxene assemblages only at T < 1300°C. At higher T, the CaTs content strongly affects the En solubility and limits its thermometric application.