Problem of Iron Isotope Composition of the Earth and Moon. Data on δ57Fe of Luna 16, Luna 20, and Luna 24 Lunar Soil Samples (Report on the XXII Symposium on the Geochemistry of Stable Isotopes, Moscow, October 29–31, 2019)

Abstract

The model of the formation of the Earth–Moon system during the compression and fragmentation of a gas–dust cloud (Galimov, 2011; Galimov and Krivtsov, 2012) suggests an unusual mechanism for the formation of the Earth’s core. In particular, the model predicts that iron in the Earth’s core should be enriched in the light isotope, and the Earth’s mantle should be, conversely, enriched in the heavy isotope compared to the primary iron (of chondrites). The alternative (megaimpact) model does not allow for this phenomenon. This problem could be solved by comparing the iron isotope composition of the Earth’s and Moon’s mantles. The best representation of the iron isotope composition of the lunar mantle (δ57Fe) is given by the Fe isotope composition of lunar basalts with very low titanium content (VLT), just as the best representation of the isotope composition of iron in the Earth’s mantle is given by the δ57Fe of the Earth’s mid-oceanic ridge basalts (MORB). VLT basalts are relatively scarce on the lunar surface, unlike high-titanium basalts, which fill lunar lowlands. Their iron isotope composition has not yet been measured. We were the first to analyze the iron isotope composition of very low-titanium lunar basalts (VLT) in material delivered by the Luna 24 space mission. Our data indicate that the δ57Fe of Earth’s mantle is higher than the δ57Fe of the bulk silicate Moon, which means (with a high degree of probability) that the Earth’s core is enriched in the light iron isotope. This, in turn, is in good agreement with the model of formation of the Earth–Moon system by the compression and fragmentation of the gas–dust cloud.