The vanadium isotopic composition of lunar basalts

1S.S.Hopkins,2,3J.Prytulak,1J.Barling,4S.S.Russell,2B.J.Coles,5A.N.Hallidaya
Earth and Planetary Science Letters 511, 12-24 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.008]
1Department of Earth Sciences, University of Oxford, OX1 3AN, United Kingdom
2Department of Earth Sciences and Engineering, Imperial College, London, SW7 2AZ, United Kingdom
3Department of Earth Sciences, Durham University, DH1 3LE, United Kingdom
4Natural History Museum, London, SW7 5BD, United Kingdom
5The Earth Institute, Columbia University, Hogan Hall, 2910 Broadway, New York, NY 10025, USA
Copyright Elsevier

We present the first high-precision vanadium (V) isotope data for lunar basalts. Terrestrial magmatic rock measurements can display significant V isotopic fractionation (particularly during (Fe, Ti)oxide crystallisation), but the Earth displays heavy V (i.e. higher 51V/50V) isotopic compositions compared to meteorites. This has been attributed to early irradiation of meteorite components or nucleosynthetic heterogeneity. The Moon is isotopically-indistinguishable from the silicate Earth for many refractory elements and is expected to be similar in its V isotopic composition.

Vanadium isotope ratios and trace element concentrations were measured for 19 lunar basalt samples. Isotopic compositions are more variable (∼2.5‰) than has been found thus far for terrestrial igneous rocks and extend to lighter values. Magmatic processes do not appear to control the V isotopic composition, despite the large range in oxide proportions in the suite. Instead, the V isotopic compositions of the lunar samples are lighter with increasing exposure age (te). Modelling nuclear cross-sections for V production and burnout demonstrates that cosmogenic production may affect V isotope ratios via a number of channels but strong correlations between V isotope ratios and te [Fe]/[V] implicate Fe as the primary target element of importance. Similar correlations are found in the latest data for chondrites, providing evidence that most V isotope variation in chondrites is due to recent cosmogenic production via Fe spallation. Contrary to previous suggestions, there is no evidence for resolvable differences between the primary V isotopic compositions of the Earth, Moon, chondrites and Mars.

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