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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.

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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.

Item Type: Journal Item
Copyright Holders: 2015 The Authors
ISSN: 0016-7037
Project Funding Details:
Funded Project NameProject IDFunding Body
Secular evolution of water in the lunar mantle (SE-10-037-MA)ST/I001298/1STFC (Science & Technology Facilities Council)
Astronomy and Planetary Sciences at the Open UniversityST/L000776/1STFC (Science & Technology Facilities Council)
Keywords: mare basalts; Moon; chromium isotopes
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Research Group: Space
Item ID: 58369
Depositing User: Mahesh Anand
Date Deposited: 19 Dec 2018 11:52
Last Modified: 02 May 2019 14:02
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