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

Ian J. Parkinson,Mahesh Anand,Pierre Bonnand

doi:10.1016/j.gca.2015.11.041

Ian J. Parkinson, Mahesh Anand + Show 1 more

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

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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.

Highlights

IntroductionMars-sized impactor (e.g. Hartmann and Davis, 1975; Cameron and Ward, 1976) there is still debate over whether the Moon is formed predominantly from impactor material or from the Earth’s mantle (e.g. Gessmann and Rubie, 2000; Chabot and Agee, 2003; Pahlevan, 2014; Dauphas et al 2014)
It is widely accepted that the Moon formed after the collision between the proto-Earth and aMars-sized impactor (e.g. Hartmann and Davis, 1975; Cameron and Ward, 1976) there is still debate over whether the Moon is formed predominantly from impactor material or from the Earth’s mantle (e.g. Gessmann and Rubie, 2000; Chabot and Agee, 2003; Pahlevan, 2014; Dauphas et al 2014)
In this study we focus on the geochemistry of chromium (Cr) an element that has several geochemical properties that make it an ideal tool to investigate the formation of the Moon and its relationship to the Earth

Summary

Introduction

Mars-sized impactor (e.g. Hartmann and Davis, 1975; Cameron and Ward, 1976) there is still debate over whether the Moon is formed predominantly from impactor material or from the Earth’s mantle (e.g. Gessmann and Rubie, 2000; Chabot and Agee, 2003; Pahlevan, 2014; Dauphas et al 2014). Several models have been proposed to explain the isotopic similarities between the Earth and Moon: both bodies were formed at similar heliocentric distances (Wiechert et al 2001); the lunar mantle is made of Earth material (Zhang et al.2012); the proto lunar disc equilibrated with the Earth (Pahlevan and Stevenson 2007; Armytage et al 2012); the solar system is homogeneous inward of Earth’s orbit (Dauphas et al 2014; Mastrobuono-Battisti et al 2015) or the canonical simulations are incorrect (Cuk and Stewart, 2012; Canup, 2012; Reufer et al 2012).

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