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

• New, high-precision Ca isotope date obtained by collision-cell MC-ICP-MS. Lunar mare basalts have similar δ 44 / 40 to terrestrial basalts. Late-stage clinopyroxenite cumulates play a crucial role in magmatism. Notable mixing of the cumulate mantle is required form these hybrid sources. High-Ti require metasomatism melts ilmenite-bearing cumulates. The compositions offer novel insight into heterogeneous nature lunar mantle. We present new data for suite low-Ti and high-Ti using our collision cell MC-ICP-MS/MS instrument, Proteus. Mare were found composition resembling ( SRM 915a =0.78–0.89‰) even though they are derived from differentiated, refractory source. Modelling fractionation during crystallisation magma ocean (LMO) indicates that dominantly harzburgitic interior should be isotopically heavier than Earth's =1.1–1.2‰ versus 0.93‰, respectively). These balanced an light anorthosite crust, consistent with feldspathic breccia meteorites. investigate major element partial various reservoirs combining pMELTS models equilibrium mass balance calculations. principal finding harzburgite alone too source produce magmas. Partial low CaO contents, high Al 2 O 3 /CaO resemble From constraints, we find addition 10–15% late-stage, crystallising at 95% LMO solidification suitable can generate basalt compositions. Despite such late-stage finds sources undersaturated clinopyroxene thus experimental constraints magmas clinopyroxene-free. slightly lower (0.80–0.86‰) (0.85–0.89‰) clearly elevated TiO /CaO. No involving (IBC) was could reproduce appropriate systematics. Instead, suggest IBC most plausible way rich diversity contents pyroclastic glasses.