Experimental calibration of the roles of temperature and composition in the Ca-in-olivine geothermometer at 0.1 MPa

Abstract

The calcium content of olivine in equilibrium with a low-Ca pyroxene and clinopyroxene has previously been calibrated in several studies at elevated pressure as a geobarometer. However, the calcium content of olivine in such systems is also dependent on temperature and the Mg# (100*Mg/Mg+Fe) of the system which is represented here by the olivine forsterite (Fo) content. To isolate these variables a series of experiments has been performed at 0.1MPa between 1170°C and 1322°C in both, the simple CaO–MgO–Al2O3–SiO2 (CMAS) system and in Fe–Na–Ti–K bearing compositions. In the CMAS system the Ca content of pure forsterite coexisting with low-Ca pyroxene and clinopyroxene is strongly temperature dependent, decreasing by >0.1wt.% CaO between 1322°C and 1254°C. Addition of Fe to the system leads to a substantial increase in the Ca content of olivine coexisting with a low-Ca pyroxene and clinopyroxene at a given temperature. Empirical fitting of these data gives a geothermometer applicable at low pressure and in the compositional range Fo70–Fo100:T°C=−123682032lnXMo−6.3951.79+3.2350.51Fo−273where the values in parentheses are the standard error on the parameter. This reproduces all experimental data within 20°C.