Distribution of Fe2+ and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer

Multiple regression analysis of a compilation of the Fe2+–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln KD(Fe2+/Mg)Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln KD(Fe2+/Mg)Grt–Hbl–P–T compositional relationship (r2=0.93):T (°C)=1504+1784(XCaGrt+XMnGrt)ln KD(Fe2+/Mg)Grt–Hbl+0.720−273whereKD(Fe2+/Mg)Grt–Hbl=(Fe2+/Mg)Grt(Fe2+/Mg)M1–M3HblXCaGrt=CaCa+Mn+Fe2++Mg in garnetXMnGrt=MnCa+Mn+Fe2++Mg in garnet Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.