Calcium isotopic composition of mantle xenoliths and minerals from Eastern China

This study presents calcium isotope data for co-existing clinopyroxenes (cpx), orthopyroxenes (opx), and olivine (ol) in mantle xenoliths to investigate Ca isotopic fractionation in the upper mantle. delta(44/40) Ca (delta(44/40) Ca (parts per thousand) = (Ca-44/Ca-40)(SAMPLE)/(Ca-44/Ca-40)(SRM915a) - 1) in opx varies from 0.95 +/- 0.05 parts per thousand to 1.82 +/- 0.01 parts per thousand and cpx from 0.71 +/- 0.06 parts per thousand to 1.03 +/- 0.12 parts per thousand (2se). delta Ca-44/40 in ol (P-15) is 1.16 +/- 0.08 parts per thousand, identical to delta Ca-44/40 of the co-existing opx (1.12 +/- 0.09 parts per thousand, 2se). The Delta Ca-44/40(opx-cpx) (Delta Ca-44/40(opx-cpx) = delta Ca-44/40(opx)-delta Ca-44/40(cpx)) shows a large variation ranging from -0.01 parts per thousand to 1.11 parts per thousand and it dramatically increases with decreasing of Ca/Mg (atomic ratio) in opx. These observations may reflect the effect of opx composition on the inter-mineral equilibrium fractionation of Ca isotopes, consistent with the theoretical prediction by firstprinciples theory calculations (Feng et al., 2014). Furthermore, Delta Ca-44/40(opx-cpx) decreases when temperature slightly increases from 1196 to 1267 K. However, the magnitude of such inter-mineral isotopic fractionation (1.12 parts per thousand) is not consistent with the value calculated by the well-known correlation between inter-mineral isotope fractionation factors and 1/T-2 (Urey, 1947). Instead, it may reflect the temperature control on crystal chemistry of opx (i.e., Ca content), which further affects Delta Ca-44/40(opx-cpx). The calculated delta Ca-44/40 of bulk peridotites and pyroxenites range from 0.76 +/- 0.06 parts per thousand to 1.04 +/- 0.12 parts per thousand (2se). Notably, delta Ca-44/40 of bulk peridotites are positively correlated with CaO and negatively with MgO content. Such correlations can be explained by mixing between a fertile mantle end-member and a depleted one with low delta Ca-44/40, indicating that Ca isotopes could be a useful tool in studying mantle evolution. (c) 2015 Elsevier Ltd. All rights reserved.