Tin isotopes indicative of liquid–vapour equilibration and separation in the Moon-forming disk

Abstract

The depletion in moderately volatile elements in the Moon relative to Earth and comparison of the isotope compositions of the Moon and Earth have placed important constraints on models of lunar formation. A liquid–vapour protolunar disk from a high-energy giant impact has been proposed to explain some of these constraints. Here we present high-precision tin isotope data for lunar rocks, measured by double-spike MC-ICP-MS (multicollector inductively coupled plasma mass spectrometry). The lunar rocks are enriched in light tin isotopes compared to the Earth (Δ124/116SnMoon–Earth = −0.48 ± 0.15‰). On the basis of our data and constraints on tin speciation, we show that this tin isotope fractionation is inconsistent with volatile loss from a lunar magma ocean. Instead, we propose a scenario with vigorous mixing between the protolunar disk and the Earth in high-energy conditions during the impact, followed by liquid–vapour equilibration and phase separation at around 2,500 K while the disk was cooling. This scenario is consistent with the depletion in moderately volatile elements and isotope composition of the Moon. Vigorous mixing between the protolunar disk and Earth followed by processes in the cooling disk may explain the enrichment in light isotopes of tin on the Moon relative to Earth, as found by analysis of lunar rocks and geochemical calculations.