Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and differentiation of the Moon

Bonnand, Pierre; Parkinson, Ian J. and Anand, Mahesh (2016). Mass dependent fractionation of stable chromium isotopes in mare basalts: implications for the formation and differentiation of the Moon. Geochimica et Cosmochimica Acta, 175 pp. 208–221.

DOI: https://doi.org/10.1016/j.gca.2015.11.041

Abstract

We present the first stable chromium isotopic data from mare basalts in order to investigate the similarity between the Moon and the Earth’s mantle. A double spike technique coupled with MC-ICP-MS measurements was used to analyse 19 mare basalts, comprising high-Ti, low-Ti and KREEP-rich varieties. Chromium isotope ratios (δ⁵³Cr) for mare basalts are positively correlated with indices of magmatic differentiation such as Mg# and Cr concentration which suggests that Cr isotopes were fractionated during magmatic differentiation. Modelling of the results provides evidence that spinel and pyroxene are the main phases controlling the Cr isotopic composition during fractional crystallisation. The most evolved samples have the lightest isotopic compositions, complemented by cumulates that are isotopically heavy. Two hypotheses are proposed to explain this fractionation: (i) equilibrium fractionation where heavy isotopes are preferentially incorporated into the spinel lattice and (ii) a difference in isotopic composition between Cr²⁺ and Cr³⁺ in the melt. However, both processes require magmatic temperatures below 1200 °C for appreciable Cr³⁺ to be present at the low oxygen fugacities found in the Moon (IW −1 to −2 log units). There is no isotopic difference between the most primitive high-Ti, low-Ti and KREEP basalts, which suggest that the sources of these basalts were homogeneous in terms of stable Cr isotopes. The least differentiated sample in our sample set is the low-Ti basalt 12016, characterised by a Cr isotopic composition of −0.222 ± 0.025‰, which is within error of the current BSE value (−0.124 ± 0.101‰). The similarity between the mantles of the Moon and Earth is consistent with a terrestrial origin for a major fraction of the lunar Cr. This similarity also suggests that Cr isotopes were not fractionated by core formation on the Moon.

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