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

Abstract

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.

The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon

“…Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses (Saal & Hauri 2021) which vary widely in S isotope compositions from -14.0 to 1.3‰ explained by extensive degassing of picritic magmas under high P/PSat values (> 0.9) during pyroclastic eruptions. The difference in the isotope compositions of picritic glass beads and mare basalts may result from differences in effusive (mare) and explosive (picritic) eruption styles wherein the high gas contents necessary for magma fragmentation would result in large effective isotopic fractionation factors during degassing of picritic magmas.…”

“…In total, these data provide further evidence for the idea that lunar volatile loss and volatile-element stable isotope fractionation largely occurred during lunar formation and that exceptionally high or low isotopic compositions likely resulted from localized phenomena influenced by reservoir effects. Faircloth et al, 2020;Gao and Thiemens, 1991;Gao and Thiemens, 1993a;Gao and Thiemens, 1993b;Labidi et al, 2013;Rees and Thode, 1974;Saal and Hauri, 2021;Wing and Farquhar, 2015;Wu et al, 2018). Green bar represents the estimated  34 S value of the silicate Earth (Labidi et al, 2013).…”

“…In contrast to the mare basalts, some lunar lithologies have been shown to have extreme variations in the Zn, S and Cl isotope compositions (15‰ in  66 Zn, 60‰ in  34 S, and 30‰ in values seen in 66095 (Rusty Rock) and some FAN samples (Gargano et al, 2020;Kato et al, 2015). Interestingly, while bulk-rock mare basalts do not retain such anomalous Zn, S, or Cl isotope compositions, the  34 S values of picritic glass beads (PGBs) range from -14.0 to 1.3‰ (Saal and Hauri, 2021) despite the fact that they are generally petrogenetically related to the more evolved mare basalts (Hauri et al, 2015). Picritic magmas are expected to have experienced limited differentiation, whereas mare basalts were produced following 20-30% fractional crystallization (Shearer and Papike, 1993).…”

“…We suggest these two styles or phases of volcanism represent starkly different degassing regimes. The former, sampled in PGBs is extensively degassed -as evidenced by marked differences in volatile contents when compared to melt inclusions (Chen et al, 2015;Hauri et al, 2011;Ni et al, 2019) and exceptionally low  34 S values (Saal and Hauri, 2021). The subsequently-emplaced mare ) + 1, where P/PSat is equal to the effective vapor pressure of a given element relative to the saturation vapor pressure (Saal and Hauri, 2021).…”

“…The former, sampled in PGBs is extensively degassed -as evidenced by marked differences in volatile contents when compared to melt inclusions (Chen et al, 2015;Hauri et al, 2011;Ni et al, 2019) and exceptionally low  34 S values (Saal and Hauri, 2021). The subsequently-emplaced mare ) + 1, where P/PSat is equal to the effective vapor pressure of a given element relative to the saturation vapor pressure (Saal and Hauri, 2021). In this scenario, a crossover at P/PSat ~ 0.86 results in the ′ Kin value deviating from <1 to >1, resulting in heavyisotope loss when degassing occurs in a near-saturated medium with P/PSat > 0.86 and a lightisotope loss when degassing occurs under low vapor pressure conditions.…”