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 Delta Ca-44/42(hbl-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 degrees C according to the plagioclase-hornblende geothermometry. Delta(44/42)Ca(grt-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. A(grt-cpx) (A(grt-cpx )= Delta(4)(4/42) Cagrt-cpx 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 delta Ca-44/42 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 (Delta Ca-44/42(hbl-plg) = (+0.10 +/- 0.02) x 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 delta Ca-44/42 ranging from +0.24 +/- 0.02 to +0.38 +/- 0.02. Given their low chemical index of alteration (<51), limited CaO variation (1.86-3.07 wt.%), and mineral assemblage similar to that of non-adakitic granitoids, delta Ca-44/42 variation in low-Mg adakitic samples cannot be explained by either weathering or fractional crystallization. delta Ca-44/42 is negatively correlated with (Dy/Yb)(N), which most likely reflects the role of residual jadeite-rich clinopyroxene and garnet. Model calculation suggests that the lowest delta Ca-44/42 adakitic sample requires 30% garnet and 70% clinopyroxene (with 28% jadeite) in its residue, consistent with a local crust thickness greater than 50 km. Our results indicate that Ca isotopes can be used as a potential tool to constrain the magma generation depth, because garnet and jadeite-rich clinopyroxene tend to be rich in heavier Ca isotopes. Combined with literature data, the upper continent crust delta Ca-44/42 is estimated to be +0.35 +/- 0.05 (2SE, 95% c.i.). (C) 2019 Elsevier Ltd. All rights reserved.