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Volatile distributions in and on the Moon revealed by Cu and Fe isotopes in the 'Rusty Rock' 66095

TitleVolatile distributions in and on the Moon revealed by Cu and Fe isotopes in the 'Rusty Rock' 66095
Publication TypeJournal Article
Year of Publication2019
AuthorsDay JMD, Sossi P.A, Shearer C.K, Moynier F.
Date Published2019/12
Type of ArticleArticle
ISBN Number0016-7037
Accession NumberWOS:000491872000008
KeywordsCondensates; constraints; Copper isotopes; differentiation; earth; elements; evaporation; fractionation; Geochemistry & Geophysics; Iron; iron isotopes; lunar; Moon; origin; Rusty Rock; siderophile; Volatile; water; zinc

The Apollo 16 'Rusty Rock' impact melt breccia 66095 is a volatile-rich sample, with the volatiles inherited through vapor condensation from an internal lunar source formed during thermo-magmatic evolution of the Moon. We report Cu and Fe isotope data for 66095 and find that bulk-rocks, residues and acid leaches span a relatively limited range of compositions (3.0 +/- 1.3 wt.% FeO [range = 2.0-4.8 wt.%], 5.4 +/- 3.1 ppm Cu [range = 3-12 ppm], average delta Fe-56 of 0.15 +/- 0.05 parts per thousand [weighted mean = 0.156 parts per thousand] and delta Cu-65 of 0.72 +/- 0.14 parts per thousand[weighted mean = 0.78 parts per thousand]). In contrast to the extreme enrichment of the light isotopes of Zn and heavy isotopes of Cl in 66095, delta Fe-56 and delta Fe-56 in the sample lie within the previously reported range for lunar mare basalts (0.92 +/- 0.16 parts per thousand and 0.12 +/- 0.02 parts per thousand, respectively). The lack of extreme isotopic fractionation for Cu and Fe isotopes reflects compositions inherent to 66095, with condensation of a cooling gas from impact-generated fumarolic activity at temperatures too low to lead to the condensation of Cu and Fe in the sample, but higher than required to condense Zn. Together with thermodynamic models, these constraints suggest that the gas condensed within 66095 between 700 and 900 degrees C (assuming a pressure of 10(-6) and an fO(2) of IW-2). That the Cu and Fe isotopic compositions of sample 66095 are within the range of mare basalts removes the need for an exotic, volatile-enriched source. The enrichment in Tl, Br, Cd, Sn, Zn, Pb, Rb, Cs, Ga, B, Cl, Li relative to Bi, Se, Te, Ge, Cu, Ag, Sb, Mn, P, Cr and Fe in the 'Rusty Rock' is consistent with volcanic outgassing models and indicates that 66095 likely formed distal from the original source of the gas. The volatile-rich character of 66095 is consistent with impact-generated fumarolic activity in the region of the Cayley Plains, demonstrating that volatile-rich rocks can occur on the lunar surface from outgassing of a volatile-poor lunar interior. The 'Rusty Rock' indicates that the lunar interior is significantly depleted in volatile elements and compounds and that volatile-rich surface rocks likely formed through vapor condensation. Remote sensing studies have detected volatiles on the lunar surface, attributing them dominantly to solar wind. Based on the 'Rusty Rock', some of these surface volatiles may also originate from the Moon's interior. (C) 2019 Elsevier Ltd. All rights reserved.

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