Calcium isotopic composition of the lunar crust, mantle, and bulk silicate Moon: A preliminary study

Calcium isotopes have the potential to explore genetic links between the Moon and Earth. Here, we constrain calcium isotopic composition of the bulk silicate Moon (BSM), using petrological, geochemical and calcium isotopic data obtained from five lunar meteorites (two basalts and three feldspathic breccia rocks), and on four anorthite crystals from two feldspathic breccia meteorites. The delta Ca-44/(40) of lunar feldspathic breccias are 0.76 +/- 0.06 parts per thousand (2SD, n = 3), 0.78 +/- 0.10 parts per thousand (2SD, n = 6) and 0.82 +/- 0.02 parts per thousand (2SD, n = 3), respectively, consistent with previously determined calcium isotopic composition of feldspathic breccias. Four anorthite crystals yield a mean delta Ca-44/(40) value of 0.75 +/- 0.13 parts per thousand (2SD, n = 4), inferred to represent the Ca value of the lunar crust. The delta Ca-44/(40) values of the two lunar basalts are 0.90 +/- 0.07 parts per thousand (2SD, n = 3) and 0.96 +/- 0.11 parts per thousand (2SD, n = 4), respectively, slightly heavier than the feldspathic breccias and anorthites. The delta Ca-44/(40) of the studied lunar basalts and literature data show negative correlations with CaO, Al2O3, and anorthite mode, pointing to the effect of contamination by lunar crust rocks. Using the heaviest delta Ca-44/(40) found in the least contaminated lunar basalts and a 0.10-0.20 parts per thousand fractionation of calcium isotopes during partial melting, we estimate the delta Ca-44/(40) value of the lunar mantle to be 0.96-1.11 parts per thousand. The delta Ca-44/(40) values of the BSM is estimated to be 0.89-0.95 parts per thousand, using a two-end member mixing model. The Ca isotopic composition of the BSM is within error to that of the bulk silicate Earth (0.94 +/- 0.05 parts per thousand), providing insights into the information of the comparison of the Earth-Moon system and the planets of inner Solar System. (C) 2019 Elsevier Ltd. All rights reserved.