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

1Martijn Klaver,1,2Tu-Han Luu,1Jamie Lewis,3Maximiliaan N.Jansen,4Mahesh Anand,5Johannes Schwieters,1Tim Elliott
Earth and Planetary Science Letters 570, 117079 Link to Article [https://doi.org/10.1016/j.epsl.2021.117079]
1Bristol Isotope Group, School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom
2Université de Paris, Institut de Physique du Globe de Paris. CNRS, F-75005 Paris, France
3School of Earth and Environmental Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom
4School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
5Thermo-Fisher Scientific, Hannah-Kunath Strasse, Bremen, Germany
Copyright Elsevier

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 (δ44/40CaSRM 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 (δ44/40CaSRM 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 δ44/40CaSRM 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 δ44/40CaSRM 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 δ44/40Ca 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.

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