Large sulfur isotope fractionation in lunar volcanic glasses reveals the magmatic differentiation and degassing of the Moon

Sulfur isotope variations in mantle-derived lavas provide important constraints on the evolution of planetary bodies. Here, we report the first in situ measurements of sulfur isotope ratios dissolved in primitive volcanic glasses and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. The new data reveal large variations in 34S/32S ratios, which positively correlates with sulfur and titanium contents within and between the distinct compositional groups of volcanic glasses analyzed. Our results uncover several magmatic events that fractionated the primordial sulfur isotope composition of the Moon: the segregation of the lunar core and the crystallization of the lunar magma ocean, which led to the formation of the heterogeneous sources of the lunar magmatism, followed by magma degassing during generation, transport, and eruption of the lunar lavas. Whether the Earth’s and Moon’s interiors share a common 34S/32S ratio remains a matter of debate.

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

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.