¹J.I.Simon,^1,2R.Christoffersen,³J.Wang,¹M.D.Mouser,⁴R.D.Mills,^1,2,5D.K.Ross,²Z.Rahman,³C.M.O’D.Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.02.008]
¹Center for Isotope Cosmochemistry and Geochronology, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA
²Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, USA
³Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015-1305, USA
⁴Department of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599, USA
⁵University of Texas at El Paso/Jacobs-JETS, Houston, TX 77058, USA
Copyright Elsevier

In this detailed geochemical, petrological, and microstructural study of felsite clast materials contained in Apollo breccia samples 12013, 14321, and 15405, little evidence was found for relatively enriched reservoirs of endogenic lunar volatiles. NanoSIMS measurements have revealed very low volatile abundances (≤2 to 18 ppm hydrogen) in nominally anhydrous minerals (NAMS) plagioclase, potassic alkali feldspar, and SiO₂ that make up a majority of these felsic lithologies. Yet these mineral assemblages and clast geochemistries on Earth would normally yield relatively high volatiles contents in their NAMS (∼20 to ≥80 ppm hydrogen). This difference is particularly notable in felsite 14321,1062 that exhibits extremely low volatile abundances (≤2 ppm hydrogen) and a relatively low amount of microstructural evidence for shock metamorphism given that it is a clast of the most evolved (∼74 wt. % SiO₂) rock-type returned from the Moon. If taken at face value, ‘wet’ felsic magmas (∼1.2 to 1.7 wt. % water) are implied by the relatively high hydrogen contents of feldspar in felsite clasts in Apollo samples 12013 and 15405, but these results are likely misleading. These felsic clasts have microstructural features indicative of significantly higher shock stress than 14321,1062. These crustal lithologies likely obtained no more water from the lunar interior than the magma body producing 14321,1062. Rather, we suggest hydrogen was enriched in samples 12013 and 15405 by impact induced exchange, and/or partial assimilation of volatiles added to the surface of the Moon by a hydrated impactor (asteroid or comet) or the solar wind. Thus, the best estimate for magmatic water contents of felsic lunar magmas comes from 14321,1062 that leads to a calculated magmatic water content of ≤0.2 wt.%. This dry felsic magma has a slightly greater, but comparable water content to the ancient mafic magmas implied by the other lithologies that we have studied. Based on this and expanding evidence for a significantly dry ancient or early degassed Moon it is likely that some recent estimates (100’s ppm) of the water abundances in the lunar parental magma ocean have been overestimated.

¹G. Avice,¹M. Moreira,²J. D. Gilmour
The Astrophysical Journal 889, 68 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab5f0c]
¹Unversité de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France
²Department of Earth and Environmental Science, School of Natural Sciences, University of Manchester, Manchester, M13 9PL, UK

Nucleosynthetic isotopic anomalies in meteorites and planetary objects contribute to our understanding of the formation of the solar system. Isotope systematics of chondrites demonstrate the existence of a physical separation between isotopic reservoirs in the solar system. The isotopic composition of atmospheric xenon (Xe) indicates that its progenitor, U-Xe, is depleted in ¹³⁴Xe and ¹³⁶Xe isotopes relative to solar or chondritic end-members. This deficit supports the view that nucleosynthetic heterogeneities persisted during the solar system formation. Measurements of xenon emitted from comet 67P/Churyumov–Gerasimenko (67P) identified a similar, but more extreme, deficit of cometary gas in these isotopes relative to solar gas. Here we show that the data from 67P demonstrate that two distinct sources contributed xenon isotopes associated with the r-process to the solar system. The h-process contributed at least 29% (2σ) of solar system ¹³⁶Xe. Mixtures of these r-process components and the s-process that match the heavy isotope signature of cometary Xe lead to depletions of the precursor of atmospheric Xe in p-only isotopes. Only the addition of pure p-process Xe to the isotopic mixture brings ¹²⁴Xe/¹³²Xe and ¹²⁶Xe/¹³²Xe ratios back to solar-like values. No pure p-process Xe has been detected in solar system material, and variation in p-process Xe isotopes is always correlated with variation in r-process Xe isotopes. In the solar system, p-process incorporation from the interstellar medium happened before incorporation of r-process nuclides or material in the outer edge of the solar system carries a different mixture of presolar sources as have been preserved in parent bodies.