Calcium isotope fractionation during crustal melting and magma differentiation: Granitoid and mineral-pair perspectives

In order to investigate Ca isotope fractionation during crustal melting and magma differentiation, we report high-precision Ca isotopic data of 17 well-characterized granitoids from the Dabie orogen, China, and 22 mineral separates from two representative granitoids and nine local eclogites and garnet peridotites. Hornblende and plagioclase pairs have nearly constant Δ44/42Cahbl–plg, ranging from +0.09 ± 0.04 (in 2SE if not specified) to +0.12 ± 0.03, which likely reflects equilibrium inter-mineral isotope fractionation at 660–760 °C according to the plagioclase–hornblende geothermometry. Δ44/42Cagrt–cpx of eclogites and garnet peridotites range from +0.06 ± 0.03 to +0.38 ± 0.04, consistent with a stronger CaO bond in garnet than clinopyroxene. Agrt–cpx (Agrt-cpx=Δ44/42Cagrt-cpx×T2/106, T in Kelvin), representative of inter-mineral isotope fractionation corrected for temperature effect, decreases with increasing jadeite proportion in clinopyroxene, indicating a compositional control on equilibrium inter-mineral isotope fractionation.