Chemical and isotopic kinship of iron in the Earth and Moon deduced from the lunar Mg-Suite

Abstract

The Moon and the Earth's mantle share many chemical and isotopic traits, leading to the prevailing theory that they were formed from similar material. Iron is one element that shows apparent differences between the two bodies, with models for the composition of the Moon having ≈1.5 times more FeO (12–14 wt.%), relative to the Earth's mantle (8 wt.%). This difference is mirrored in their isotope compositions, where lunar mare basalts have δ57Fe (per mille deviation of the 57Fe/54Fe ratio from the IRMM-014 standard) 0.1–0.2‰ higher than peridotitic rocks representative of Earth's mantle, a feature initially attributed to loss of isotopically light Fe following a giant impact. However, whether basaltic rocks are suitable analogues for the Moon's composition is debatable in the light of their distinct source regions that reflect the extensive lithological stratification of the lunar mantle. Here, we evaluate the iron isotope composition of the bulk Moon through the study of igneous cumulate rocks of the lunar highlands Magnesium Suite (Mg Suite). The δ57Fe of Mg Suite rocks spans a limited range, from 0.05‰ to 0.10‰, with an average (+0.07±0.02‰) that overlaps with Earth's mantle (+0.05±0.01‰), similarities that extend to their Mg#s, where both reach 0.9. Numerical modelling of iron isotope fractionation during lunar magma ocean crystallisation shows that the Mg Suite should accurately reflect the composition of the bulk Moon, which is therefore +0.07±0.02‰, indistinguishable from Earth's mantle but heavier than chondrites (−0.01±0.01‰). Iron thus behaves coherently with other elements that condense at temperatures higher than Li in showing no isotopic difference between the Earth and Moon, suggesting element depletion on the Moon affected only the more volatile elements. Therefore, there is no cosmochemical basis for iron enrichment or depletion in the bulk Moon relative to the Earth's mantle, whose composition is an analogue for that of the Moon.