A reassessment of the iron isotope composition of the Moon and its implications for the accretion and differentiation of terrestrial planets

Abstract

The Fe isotope composition of planetary bodies may provide constraints on their accretion modes and/or differentiation processes, but to do so, the Fe isotope systematics of key planetary reservoirs needs to be determined. To investigate this for the Moon, we measured the Fe isotope compositions for a suite of 33 bulk lunar mare basalts and highland rocks. Combined with published data, a compendium of 73 different lunar bulk rocks reveals a statistically significant Fe isotope difference between low-Ti and high-Ti mare basalts, yielding average δ57Fe = 0.127 ± 0.012‰ (2SE; n = 27) and δ57Fe = 0.274 ± 0.020‰ (2 SE; n = 25), respectively, relative to the IRMM-14 isotopic reference material. As lunar basalts are thought to reflect the Fe isotope composition of their respective mantle sources, the estimated relative proportion of the low-Ti and high-Ti source mantle suggests that the lunar upper mantle δ57Fe value should be close to 0.142 ± 0.026‰. Whilst the composition of highland rocks (ferroan anorthosites and Mg-suite rocks) should provide a more global view of the Moon, the calculation of the mean δ57Fe value of 15 available highland rock analyses yields δ57Fe = 0.078 ± 0.124‰. Such a value is not defined precisely enough to be of critical use for comparative planetology. Ferroan anorthosites and Mg-suite rocks also give unresolvable means. It appears that Fe isotope heterogeneity among the lunar highland rocks is caused by non-representatively too small sample aliquots of coarse-grained rocks. It can also be the result of mixed lithologies for some. When the (kinetic) effect of olivine tending towards low δ57Fe and feldspar with predominantly high δ57Fe is cancelled, a more precise δ57Fe value of 0.094 ± 0.035‰ is calculated. It is indistinguishable from the mean δ57Fe of impact melts and is also similar to the upper lunar mantle estimate obtained from mare basalts. Collectively, this newly determined Fe isotope composition of the bulk Moon is indistinguishable from that of the Earth, and heavier than those reported for other planetary bodies. This planetary isotope relationship is only observed for silicon given the currently available mass-dependent stable isotope database. Because both iron and silicon reside in the Earth’s metallic core in significant quantities, this may point to the involvement of metallic cores of the Earth and Moon in the interplanetary Fe and Si isotope fractionation. Rather than via high-pressure metal–silicate fractionation at the core–mantle boundary, this would more likely be achieved by partial vaporization of the liquid outer metallic core in the aftermath of a Moon-forming giant impact.