Multiple sulfur isotopic reservoirs in the Moon and implications for the evolution of planetary interiors

Abstract Very few in situ lunar sulfur studies exist, with the major focus being on bulk-rocks in which a relatively restricted sulfur isotope fractionation is observed, leading to suggestions that the source of sulfur in the lunar interior is homogeneous. Using a novel approach, we present for the first time two complementary datasets combining in situ secondary ion mass spectrometry and X-ray absorption near-edge structure spectroscopy of lunar apatite, to investigate the late-stage behaviour of sulfur in lunar basaltic melts. Our measurements reveal varied sulfur contents of ~20–2,800 ppm and δ34S values of -33.3 ± 3.8‰ to +36.4 ± 3.2‰ (2σ). The apatites have S6+/ΣStot ratios of >0, with average values as high as 0.55, providing evidence for the existence of relatively oxidized late-stage silicate melts on the Moon. We propose the existence of multiple, previously unrecognised, distinct sulfur isotopic reservoirs in the lunar interior and atypical oxidizing conditions in late-stage silicate melts. These findings are important for our understanding of lunar formation processes and the evolution of redox conditions during the formation of terrestrial bodies.