Iron isotopic composition of very low-titanium basalt deduced from the iron isotopic signature in Luna 16, 20, and 24 soils

Satoki Okabayashi, Tetsuya Yokoyama, Takafumi Hirata, Kazuo Terakado, Erik M.Galimov

Geochimica et Cosmochimica Acta, Available online 23 October 2019


“Isotopic analysis of Fe has proved to be a productive tool for the study of the formation and evolution of planetary bodies. The Fe isotopic studies on the Moon revealed that low-Ti and high-Ti basalts are positively correlated with TiO2 contents, indicating the occurrence of Fe isotopic fractionation during lunar magma ocean differentiation (Weyer et al., 2005, Liu et al., 2010, Wang et al., 2015, Sossi and Moynier, 2017). If true, very low-Ti basalts may be more representative of the lunar mantle than high-Ti and low-Ti basalts. Thus, an Fe isotopic study of very low-Ti basalts is required to investigate the Fe isotopic evolution scenario during lunar magma ocean differentiation.

In this study, we measured Fe isotopic ratios of soil samples from the Soviet Luna missions, Luna 16, 20, and 24, using MC-ICP-MS. Bulk soil samples without isolation of components and separated monomineralic grains (olivine, pyrixene, and agglutinate) were dedicated for the Fe isotopic study. Among our samples, Luna 24 soil is recognized as very low-Ti basalt. Evidence of non-mass-dependent Fe isotopic fractionation was not found in bulk soils and separated monomineralic olivine, pyroxene, and agglutinate grains. Lunar regolith samples are weathered by meteorite impacts on the Moon’s surface. A mature regolith sample, L1613, showed clearly distinguished Fe isotopic compositions between the fine-grained fraction (δ57Fe = 0.54 ± 0.06‰) and the coarse-grained fraction (δ57Fe = 0.20 ± 0.06‰). Such Fe isotopic differences among different size fractions were smaller in the submature sample (L2001) and the immature sample (L24130.3). The Fe isotopic ratios of separated grains show that agglutinates have higher δ57Fe values than bulk and monomineralic grains, indicating abundant nanophase Fe in agglutinates and partial evaporation of Fe during its melting.

The δ57Fe value for the coarsest-grained fraction of L24130.3 was found to be 0.08 ± 0.04‰, which is the first Fe isotopic composition reported for lunar samples with very low-Ti mare basalt composition. This value is clearly lower than previously reported δ57Fe values for high-Ti and low-Ti basalts and is nearly identical to those of Mg-suite rocks. This observation is consistent with the Fe isotopic evolution model during lunar magma ocean differentiation (Sossi and Moynier, 2017). Our results related to very low-Ti basalt showed that the bulk silicate Moon has lower Fe isotopic composition than the value estimated from averaged low-Ti or high-Ti basalts.

Given the data known to date, including our results, we conclude that it is most likely that Earth and the Moon have the same Fe isotopic composition. We believe that isotopically heavy Fe of silicate Earth is balanced by isotopically light Fe of Earth’s core, as suggested by Galimov’s (2011) model for the formation of the Earth-Moon system.”