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.
Original language | English |
---|---|
Pages (from-to) | 208-221 |
Number of pages | 14 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 175 |
Early online date | 12 Dec 2015 |
DOIs | |
Publication status | Published - 15 Feb 2016 |
Keywords
- chromium isotopes
- Moon
- mare basalts
- magmatic differentiation