Abstract
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.
Highlights
The Earth and the Moon are remarkably similar in mass-independent isotope variations such as 54Cr (Lugmair and Shukolyukov, 1998; Mougel et al, 2018), 50Ti (Zhang et al, 2012), 17O (Young et al, 2016) and 96Zr (Akram and Schonbachler, 2016), mass-dependent isotope variations of moderately refractory element such as Si (Armytage et al, 2012), Cr (Bonnand et al, 2016), Ti (Millet et al, 2016), Fe (Sossi and Moynier, 2017; Liu et al, 2010; Wiesli et al, 2003), and in radiogenic 182W (Kruijer et al, 2015)
Comparison of ICPMS elemental concentrations to previous analyses of lunar basalts shows that samples lie within expected ranges for their mare basalt subtype (Table 1, Figs. 3c, S2, S3)
Chondrite-normalised REE patterns of the lunar samples match those of previous analyses and show the characteristic patterns expected for their sub-groups (Fig. S2)
Summary
The most widely-accepted theory of lunar origin is the socalled “giant impact” model whereby a smaller planetary object struck the proto-Earth in its final stages of accretion leading to an impact-generated disk from which the Moon formed (Cameron and Ward, 1976; Hartmann and Davis, 1975). The model satisfies constraints on the geodynamical properties, planetary densities, and broad chemical features of the Earth-Moon system. The Earth and the Moon are remarkably similar in mass-independent isotope variations such as 54Cr (Lugmair and Shukolyukov, 1998; Mougel et al, 2018), 50Ti (Zhang et al, 2012), 17O (Young et al, 2016) and 96Zr (Akram and Schonbachler, 2016), mass-dependent isotope variations of moderately refractory element such as Si (Armytage et al, 2012), Cr (Bonnand et al, 2016), Ti (Millet et al, 2016), Fe (Sossi and Moynier, 2017; Liu et al, 2010; Wiesli et al, 2003), and in radiogenic 182W (Kruijer et al, 2015)
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