The Ca isotope composition of mare basalts as a probe into the heterogeneous lunar mantle

The Ca isotope compositions of mare basalts offer a novel insight into the heterogeneous nature of the lunar mantle. We present new high-precision Ca isotope data for a suite of low-Ti and high-Ti mare basalts obtained using our collision cell MC-ICP-MS/MS instrument, Proteus. Mare basalts were found to have a Ca isotope composition resembling terrestrial basalts (CaSRM 915a =0.78–0.89‰) even though they are derived from a differentiated, refractory cumulate mantle source. Modelling of Ca isotope fractionation during crystallisation of a lunar magma ocean (LMO) indicates that the dominantly harzburgitic cumulates of the lunar interior should be isotopically heavier than Earth's mantle (CaSRM 915a =1.1–1.2‰ versus 0.93‰, respectively). These are balanced by an isotopically light lunar anorthosite crust, consistent with data for lunar anorthosite and feldspathic breccia meteorites.

We investigate the major element and Ca isotope composition of partial melts of various cumulate reservoirs by combining pMELTS models with equilibrium isotope fractionation mass balance calculations. The principal finding is that harzburgite cumulates alone are too refractory a source to produce low-Ti magmas. Partial melts of harzburgite cumulates have too low CaO contents, too high Al2O3/CaO and too high CaSRM 915a to resemble low-Ti magmas. From Ca isotope constraints, we find that the addition of 10–15% of late-stage, clinopyroxenite cumulates crystallising at 95% LMO solidification is required to produce a suitable source that can generate low-Ti basalt compositions. Despite the addition of such late-stage cumulates pMELTS finds that these hybrid sources are undersaturated in clinopyroxene and are thus consistent with experimental constraints that the mantle sources of the lunar magmas are clinopyroxene-free. High-Ti basalts have slightly lower CaSRM 915a (0.80–0.86‰) than low-Ti magmas (0.85–0.89‰) and clearly elevated TiO2/CaO. No suitable hybrid source involving ilmenite-bearing cumulates (IBC) was found that could reproduce melts with appropriate Ca and major element systematics. Instead, we suggest that metasomatism of low-Ti mantle sources by IBC melts is the most plausible way to generate high-Ti magma sources and the rich diversity in TiO2 contents of lunar basalts and pyroclastic glasses.