The H2O concentration and H2O/Ce ratio in olivine-hosted melt inclusions are high in lunar pyroclastic sample 74220 (H2O up to 1410 ppmw; H2O/Ce up to 77) but lower (H2O 10 to 430 ppmw; H2O/Ce 0.3 to 9.4) in all other lunar samples studied before this work. The difference in H2O concentration and in H2O/Ce ratio is absent for other volatile elements (F, S, and Cl) in melt inclusions in 74220 and other lunar samples. Because H2O (or H) is a critical volatile component with significant ramifications on the origin and evolution of the Moon, it is important to understand what causes such a large gap in H2O/Ce ratio between 74220 and other lunar samples. Two explanations have been advanced. One is that volcanic product in sample 74220 has the highest cooling rate and thus best preserved H2O in melt inclusions compared to melt inclusions in other samples. The other explanation is that sample 74220 comes from a localized heterogeneity enriched in some volatiles. To distinguish these two possibilities, here we present new data from two rapidly cooled lunar samples with glassy melt inclusions: olivine-hosted melt inclusions (OHMIs) in 79135 regolith breccia (unknown cooling rate but with glassy MIs similar in texture with those in 74220), and pyroxene-hosted melt inclusions (PHMIs) in 15597 pigeonite basalts (known high cooling rate, second only to 74220 and 15421). In addition, we also investigated new OHMIs in sample 74220. If the gap is due to the difference in cooling rates, samples with cooling rates between those of 74220 and other studied lunar samples should have preserved intermediate H2O concentrations and H2O/Ce ratios. Our results show that melt inclusions in 79135 and 15597 contain high H2O concentrations (up to 969 ppmw in 79135 and up to 793 ppmw in 15597) and high H2O/Ce ratios (up to 21 in 79135 and up to 13 in 15997), bridging the big gap in H2O/Ce ratio among 74220 and other lunar samples. Combined with literature data, we confirm that H2O/Ce ratios of different lunar samples are positively correlated to the cooling rates and independent of the type of mare basalts. We hence reinforce the interpretation that the lunar sample with the highest cooling rate best represents pre-eruptive volatiles in lunar basalts due to the least degassing. Based on Ce concentration in the primitive lunar mantle, we estimate that H2O concentration in the primitive lunar mantle (meaning bulk silicate Moon) is 121 +/- 15 ppmw. Our new data also further constrain F/P, S/Dy and Cl/Ba ratios in lunar basalts and the lunar mantle. Estimated F, P, and S concentrations in the lunar primitive mantle are 4.4 +/- 1.1 ppmw, 22 +/- 8 ppmw, and 67+67 33 ppmw, respectively.

Eucrite meteorites are early-formed (>4.5 Ga) basaltic rocks that are likely to derive from the asteroid 4 Vesta, or a similarly differentiated planetesimal. To understand trace element and moderately volatile element (MVE) behavior more fully within and between eucrites, a laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study is reported for plagioclase and pyroxene, as well as fusion crust and vitrophyric materials for ten eucrites. These eucrites span from a cumulate eucrite (Northwest Africa [NWA] 1923) to samples corresponding to Main Group (Queen Alexandra Range 97053, Pecora Escarpment 91245, Cumulus Hills 04049, Bates Nunatak 00300, Lewis Cliff 85305, Graves Nunataks 98098) and Stannern Group (Allan Hills 81001, NWA 1000) compositions, in addition to Elephant Moraine 90020. Along with a range of refractory trace elements, focus was given to abundances of five MVE (K, Zn, Rb, Cs, Pb) to interrogate the volatile abundance distributions in eucrite mineral phases. Modal recombination analyses of the eucrites reveals the important role of accessory phases (zircon, apatite) in some of the incompatible trace element (ITE) distributions, but not for the MVE which, for the phases that were analyzed, are mostly sited within plagioclase (Cs, Rb, K) and pyroxene (Zn, Pb), and are in equilibrium with a parental melt composition for Main Group eucrites. The new data reveal a possible relationship with total refractory ITE enrichment and texture, with the most ITE enriched Stannern Group eucrites examined (NWA 1000, ALHA 81001) having acicular textures and, in the case of ALHA 81001 a young degassing age (-3.7 Ga). Collectively the results suggest that Stannern Group eucrites may be related to anatexis of the eucritic crust by thermal metamorphism, with the heat source possibly coming from impacts. Impact processes do not have a pronounced effect on the abundances of the MVE, where plagioclase, pyroxene, fusion crust, and whole rock compositions of eucrites are all significantly depleted in the MVE, with Zn/Fe, Rb/Ba and K/U similar to lunar rocks. Assessment of eucrite compositions, however, suggests that Vesta has a more heterogeneous distribution of volatile elements and is similarly to slightly less volatile-depleted than the Moon. Phase dependence of the MVE (e.g., Cl in apatite, Zn primarily into spinel and early formed phases, including pyroxene) is likely to influence comparison diagrams where MVE stable isotopes are shown. In the case of delta Cl-37 versus delta Zn-66, metamorphism and impact processes may lead to a decrease in the delta Cl-37 value for a given delta Zn-66 value in eucrites, raising the possibility that late-stage impact and metamorphism had a profound effect on volatile distributions in early planetesimal crusts.(C) 2021 Elsevier Ltd. All rights reserved.

We have simulated solar wind-based space weathering on airless bodies in our Solar System by implanting hydrogen and helium into orthopyroxene at solar wind energies ( 1 keV/amu). Here we present the results of the first scanning transmission electron microscope (STEM) study of one of these simulants. It has been demonstrated that the visible/near infrared (VNIR) reflectance spectra of airless bodies are dependent on the size and abundance of nanophase iron (npFe(0)) particles in the outer rims of regolith grains. However, the mechanism of formation of npFe(0) in the patina on lunar regolith grains and in lunar agglutinates remains debated. As the lattice is disrupted by hydrogen and helium implantation, broken bonds are created. These dangling bonds are free to react with hydrogen, creating OH and/or H2O molecules within the grain. These molecules may diffuse out through the damaged lattice and migrate toward the cold traps identified at the lunar poles. This mechanism would leave the iron in a reduced state and able to form npFe(0). This work illustrates that npFe(0) can be nucleated in orthopyroxene under implantation of solar wind hydrogen and helium. Our data suggest that the solar wind provides a mechanism by which iron is reduced in the grain and npFe(0) is nucleated in the outer surfaces of regolith grains. This formation mechanism should also operate on other airless bodies in the Solar System. (C) 2015 Elsevier Ltd. All rights reserved.