Calcium Isotopic Composition of the Lunar Crust, Mantle, and Bulk Silicate Moon:A Preliminary Study

Wei Wu, Yi-Gang Xu, Zhao-Feng Zhang, XinLi

Geochimica et Cosmochimica Acta


“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 δ44/40Ca of lunar feldspathic breccias are 0.76 ± 0.06‰ (2SD, n=3), 0.78 ± 0.10‰ (2SD, n=6) and 0.82 ± 0.02‰ (2SD, n=3), respectively, consistent with previously determined calcium isotopic composition of feldspathic breccias. Four anorthite crystals yield a mean δ44/40Ca value of 0.75 ± 0.13‰ (2SD, n=4), inferred to represent the δ44/40Ca value of the lunar crust. The δ44/40Ca values of the two lunar basalts are 0.90 ± 0.07‰ (2SD,n=3) and 0.96 ± 0.11‰ (2SD, n=4), respectively, slightly heavier than the feldspathic breccias and anorthites. The δ44/40Ca 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 δ44/40Ca found in the least contaminated lunar basalts and a 0.10–0.20‰ fractionation of calcium isotopes during partial melting, we estimate the δ44/40Ca value of the lunar mantle to be 0.96-1.11‰. The δ44/40Ca values of the BSM is estimated to be 0.89-0.95‰, 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‰), providing insights into the information of the comparison of the Earth-Moon system and the planets of inner Solar System.”