The vanadium isotopic composition of lunar basaltsOPEN ACCESS
S.S. Hopkins, J. Prytulak, J. Barling, S.S. Russell, B.J. Coles, A.N. Halliday
Earth and Planetary Science Letters
Volume 511, 1 April 2019, Pages 12-24
“Highlights
• We present the first measurements of vanadium (V) isotopes ratios in lunar samples.
• Lunar basalts show an isotopic range larger than all terrestrial igneous rocks.
• Magmatic processes on the Moon do not control isotopic variability.
• Cosmogenic processes control the isotopic variability observed.
• Pre-exposure V isotope ratios are similar for BSE, Moon, chondrites, and Mars.”
“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 (
). 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 ⁎ [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.”
