A precise olivine-augite Mg-Fe-exchange geothermometer

Olivine and augite that were experimentally equilibrated in the temperature interval 1175-1080°C at 1 bar in natural basaltic and andesitic bulk compositions are used to calibrate an Mg-Fe2+ cation-exchange geothermometer. Within its temperature interval of experimental calibration, and over a broad range in olivine Mg/Fe ratio, the geothermometer has a standard error of ±6°C. In compositionally simpler synthetic systems, the same calibration retrieves appropriate experimental temperatures up to at least 1250°C. In application to intermediate and felsic volcanic rocks erupted at ∼1080 -800°C (below the range of experimental calibration), calculated olivine-augite temperatures are in good agreement with Fe-Ti oxide thermometry in the same samples. These results encourage confidence in the olivine-augite geothermometer over at least the 800-1250°C interval at low pressures. Sparse experimental data up to 1250°C at higher pressures for olivine + augite in the assemblage olivine + plagioclase + augite ± pigeonite or orthopyroxene suggest that the low-pressure calibration recovers experimental temperatures without systematic bias to pressures of 10 kbar. Examples illustrate applications to determining igneous equilibration temperatures in holocrystalline extrusive and intrusive rocks, and to estimating intratelluric H2O content dissolved in magmas.