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.

¹Y.Zhao,^1,2J.de Vries,^1,2A.P.van den Berg,³M.H.G.Jacobs,¹W.van Westrenen
Earth and Planetary Science 511, 1-11 Link to Article [https://doi.org/10.1016/j.epsl.2019.01.022]
¹Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
²Dept. Earth Sciences, Utrecht University, the Netherlands
³Institute of Metallurgy, Clausthal University of Technology, Germany
Copyright Elsevier
The ilmenite-bearing cumulates (IBC) formed from the solidification of the lunar magma ocean are thought to have significantly affected the long-term evolution of the lunar interior and surface. Their high density is considered to trigger Rayleigh–Taylor instabilities which allow them to sink into the solidified cumulates below and drive a large-scale overturn in the lunar mantle. Knowledge of how the IBC participate in the overturn is important for studying the early lunar dynamo, chemistry of surface volcanism, and the existence of present-day partial melt at the lunar core–mantle boundary. Despite early efforts to study this process as Rayleigh–Taylor instabilities, no dynamical models have quantified the degree of IBC sinking systematically. We have performed quantitative 2-D geodynamical simulations to measure the extent to which IBC participate in the overturn after their solidification, and tested the effect of a range of physical and chemical parameters. Our results show that IBC overturn most likely happened when the magma ocean had not yet fully solidified, with the residual melt decoupling the crust and IBC, resulting in 50–70% IBC sinking. Participation of the last dregs of remaining magma ocean melt is unlikely, leaving its high concentrations of radiogenic elements close to the surface. Our simulations further indicate that foundered IBC can stay relatively stable at the core–mantle boundary until the present day, at temperatures consistent with the presence of a partially molten zone in the deep mantle as inferred from geophysical data. 30–50% of the primary IBC remain at shallow depths throughout lunar history, enabling their assimilation by rising magma to form high-Ti basalts.