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

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 []
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.


Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s