EMPIRICAL CALIBRATION OF THE OMPHACITE–GARNET MAGNESIUM ISOTOPE GEOTHERMOMETER BASED ON ECLOGITES FROM THE DABIE OROGEN, CHINA

The large equilibrium Mg isotope fractionation between omphacite and garnet in eclogites may serve as a potential geothermometer but comprehensive calibration of this thermometer by natural samples is still lacking. Here, we report Mg isotopic compositions of whole rocks, Mg and O isotopic compositions of omphacites and garnets for twenty eclogites that formed at variable temperatures in the Dabie orogen, China. Fifteen out of twenty samples have whole-rock δ²⁶Mg values from -0.36 to -0.18‰, which is similar to those of their protoliths and, thus, consistent with limited Mg isotopic variation of rocks during eclogite-facies metamorphism. By contrast, δ²⁶Mg values vary largely in omphacite from -0.37 to +0.54‰ and in garnet from -1.94 to -0.66‰, with inter-mineral fractionation (Δ²⁶Mg_{omphacite–garnet} = δ²⁶Mg_omphacite – δ²⁶Mg_garnet) ranging from 1.05 to 2.15‰. According to the “δ_omphacite vs. δ_garnet diagram” and Δ¹⁸O_{omphactie–garnet} (= δ¹⁸O_omphacite – δ¹⁸O_garnet) values, the large fractionation reflect both equilibrium and disequilibrium inter-mineral Mg isotope fractionation in the investigated samples. The equilibrium fractionation equation between omphacite and garnet is calibrated by using the equilibrium Δ²⁶Mg_{omphacite–garnet} values and corresponding temperatures calculated by the clinopyroxene–garnet Fe–Mg thermometer for natural eclogites, which yields the following function: Δ²⁶Mg_{omphacite–garnet} = (0.99 ± 0.09) × 10⁶/T², where T is temperature in Kelvin. The novel omphacite–garnet Mg isotope thermometer has a better precision of temperature estimation than the clinopyroxene–garnet Fe–Mg thermometer (< 60 ºC vs. >100 ºC). Hence, it provides a new thermometer for precise temperature constraints of eclogite-facies metamorphism.