Chemical control of Al-solubility in orthopyroxene and its implications on pyroxene geothermometry

Abstract

Two suites of spinel peridotites of the Westeifel/West Germany were found to have equilibrated in a narrow temperature interval each. Temperatures calculated from the CaO-solubility in orthopyroxene using experimental data of Lindsley and Dixon (1976) are 945° C to 980° C for the amphibole bearing Ia-suite and 1,150° C to 1,165° C for the Ib-suite. From the study of phase equilibria in both series it appears that Al-solubility in orthopyroxene is controlled by the composition of the coexisting spinel. Temperatures calculated from Al-solubility in orthopyroxene using an equation of Fujii (1976) derived from the univariant reaction en+sp↔fo+Al-en in the MgO-Al₂O₃-SiO₂ system show a clear dependence on the FeCr₂O₄ component in the spinel phase, although ideal solution correction allowing for additional components was made. Temperatures obtained for the most chromite rich spinel — orthopyroxene pairs are 1,370° C and 1,240° C, respectively. These temperatures are by 220° C higher than those from Cr-poor pairs and those obtained from Ca solution in orthopyroxene.

For practical purposes of geothermometry in spinel peridotites, an empirical equation $T({}^oC) = \frac{{4.59 + ln K - 1.552Y_{C_r }^{sp} }}{{0.0025}}$ allowing for the mole fraction of FeCr₂O₄ in spinel has been derived by a linear least squares fit to the orthopyroxene — spinel compositional data and temperatures calculated from the CaO-solubility of orthopyroxene in the present paper.