^1,2Anthony Gargano,³James Dottin,⁴Sean S. Hopkins,^1,2Zachary Sharp,⁵Charles Shearer,^4,6lex N. Halliday,⁴Fiona Larner,^3,7James Farquar,⁸Justin I. Simon
American Mineralogist 107, 1985-1994 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1985.pdf]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-0001, U.S.A.
²Center for Stable Isotopes, University of New Mexico, Albuquerque, New Mexico 87131-0001, U.S.A.
³Department of Geology, University of Maryland, College Park, Maryland, 20742, U.S.A.
⁴Department of Earth Sciences, University of Oxford, OX1 3AN, U.K.
⁵Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131-0001, U.S.A.
⁶The Earth Institute, Columbia Climate School, Columbia University, New York, New York 10025, U.S.A.
⁷Earth System Science Interdisciplinary Center, College Park, Maryland 20742, U.S.A.
⁸Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science Division,
The Lyndon B. Johnson Space Center, National Aeronautics and Space Administration, Houston, Texas 77058, U.S.A
Copyright: The Mineralogical Society of America

We compare the stable isotope compositions of Zn, S, and Cl for Apollo mare basalts to better constrain the sources and timescales of lunar volatile loss. Mare basalts have broadly elevated yet limited
ranges in δ66Zn, δ34S, and δ37ClSBC+WSC values of 1.27 ± 0.71, 0.55 ± 0.18, and 4.1 ± 4.0‰, respectively,
compared to the silicate Earth at 0.15, –1.28, and 0‰, respectively. We find that the Zn, S, and Cl
isotope compositions are similar between the low- and high-Ti mare basalts, providing evidence of
a geochemical signature in the mare basalt source region that is inherited from lunar formation and
magma ocean crystallization. The uniformity of these compositions implies mixing following mantle
overturn, as well as minimal changes associated with subsequent mare magmatism. Degassing of
mare magmas and lavas did not contribute to the large variations in Zn, S, and Cl isotope compositions found in some lunar materials (i.e., 15‰ in δ66Zn, 60‰ in δ34S, and 30‰ in δ37Cl). This reflects
magma sources that experienced minimal volatile loss due to high confining pressures that generally
exceeded their equilibrium saturation pressures. Alternatively, these data indicate effective isotopic
fractionation factors were near unity.
Our observations of S isotope compositions in mare basalts contrast to those for picritic glasses
(Saal and 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. Additionally, in highly vesiculated basalts, the δ34S and δ37Cl values of
apatite grains are higher and more variable than the corresponding bulk-rock values. The large isotopic
range in the vesiculated samples is explained by late-stage low-pressure “vacuum” degassing (P/PSat ~ 0)
of mare lavas wherein vesicle formation and apatite crystallization took place post-eruption. Bulk-rock
mare basalts were seemingly unaffected by vacuum degassing. Degassing of mare lavas only became
important in the final stages of crystallization recorded in apatite—potentially facilitated by cracks/
fractures in the crystallizing flow. We conclude that samples with wide-ranging volatile element isotope compositions are likely explained by localized processes, which do not represent the bulk Moon.