Abstract
The three-pyroxene assemblage, augite+orthopyroxene+pigeonite, has been observed in two different Japanese lavas of known crystallization temperatures. Based on the compositions of pyroxenes in these lavas and experimental data on the iron-rich inverted pigeonite, temperature of the lower stability limit of pigeonite (or “pigeonite eutectoid reaction line”) near 1 atm has been estimated; i. e., 1125°C, 1090°C and 1000°C for Fe/(Mg+Fe) ratios 0.300, 0.370 and 0.560, respectively. These temperature-composition relations are approximated by a line T=1270−480XFe (T is temperature in °C and XFe is atomic ratio Fe/(Mg+Fe)) and can be used as a geothermometer.
