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

Abstract

Abstract 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 Ca O bond in garnet than clinopyroxene. Agrt–cpx ( A grt - c p x = Δ 44 / 42 Ca grt - c p x × T 2 / 10 6 , 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. Eight I-type non-adakitic granitoids with CaO contents ranging from 1.59 wt.% to 6.96 wt.% have homogeneous Ca isotopic compositions with an average δ44/42Ca of +0.37 ± 0.03 (2SD, N = 8), identical to the mean value of the local basement (+0.36 ± 0.06, 2SD), indicating insignificant Ca isotope fractionation during crustal melting and magma differentiation at low pressures (e.g., without a role of garnet). According to the negligible isotope fractionation between hornblende and plagioclase ( Δ 44 / 42 Ca hbl - p l g = ( + 0.10 ± 0.02 ) × 10 6 / T 2 ) at the liquidus temperatures, the Ca isotope fractionations between melt and hornblende, plagioclase are very small. Nine low-Mg adakitic granitoids have δ44/42Ca ranging from +0.24 ± 0.02 to +0.38 ± 0.02. Given their low chemical index of alteration (