Empirical calibration of the clinopyroxene–garnet magnesium isotope geothermometer and implications

The large equilibrium Mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high-precision geothermometer, but calibration of this thermometer by natural samples is still limited. Here, we report Mg isotopic compositions of eclogite whole rocks as well as Mg and O isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the Dabie orogen, China. The whole-rock δ 26 Mg values vary from −1.20 to +0.10 ‰. Among them, 11 samples display limited δ 26 Mg variations from −0.36 to −0.17 ‰, similar to those of their protoliths. The mineral separates exhibit very different δ 26 Mg values, from −0.39 to +0.39 ‰ for clinopyroxenes and from −1.94 to −0.81 ‰ for garnets. The clinopyroxene–garnet Mg isotope fractionation (Δ 26 Mg clinopyroxene–garnet = δ 26 Mg clinopyroxene –δ 26 Mg garnet ) varies from 1.05 to 2.15 ‰. The clinopyroxene–garnet O isotope fractionation (Δ 18 O clinopyroxene–garnet = δ 18 O clinopyroxene –δ 18 O garnet ) varies from −1.01 to +0.98 ‰. Equilibrium Mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the δ 26 Mg clinopyroxene versus δ 26 Mg garnet plot and the state of O isotope equilibrium between clinopyroxene and garnet. The equilibrium Δ 26 Mg clinopyroxene–garnet and corresponding temperature data obtained in this study, together with those available so far in literatures for natural eclogites, are used to calibrate the clinopyroxene–garnet Mg isotope thermometer. This yields a function of Δ 26 Mg clinopyroxene–garnet = (0.99 ± 0.06) × 10 6 / T 2 , where T is temperature in Kelvin. The refined function not only provides the best empirically calibrated clinopyroxene–garnet Mg isotope thermometer for precise constraints of temperatures of clinopyroxene- and garnet-bearing rocks, but also has potential applications in high-temperature Mg isotope geochemistry.