The extent of moderately volatile elements (MVE) depletion and its effects on the Moon's evolutionary history remain contentious, partly due to unintentionally biased sampling by the Apollo missions from the Procellarum KREEP Terrane. In this study, we analyzed the Zn and K isotope compositions of a series of lunar basaltic meteorites, which vary in Th content and are likely to represent a broader sampling range than previous studies, including samples from the far side of the Moon. Our findings indicate remarkably consistent Zn and K isotope compositions across all lunar basalt types, despite significant variations in Th content. This consistency suggests a relatively homogeneous isotopic composition of volatile elements within the Moon, unaffected by subsequent impact events that formed major basins. Our results suggest that the estimates of MVE abundance and isotopic compositions from the Apollo returned samples are likely representative of the bulk Moon, supporting a globally volatile-depleted lunar interior.

Preferential enrichment of the heavier isotopes of moderately volatile elements (MVE) in samples from asteroids and the Moon have been attributed to volatile loss during the formation and differentiation of their parent bodies. Analogs for planetary feedstocks include the howardite-eucrite-diogenite meteorites, which originate from a differentiated planetesimal or planetesimals, likely including (4) Vesta. Complications arise in the interpretation of volatile depletion in these meteorites, however, due to post-crystallization processes including metamorphism and later impacts that acted upon them. We present new coupled Cu and Zn isotope data for a suite of eucrites that, when combined with published data, show significant ranges (delta 65Cu = -1.6 to +0.9%o; delta 66Zn = -7.8 to +13.5%o). Exclusion of eucrites that have been affected by metamorphism, impact contamination or surface condensation of isotopically light Zn and Cu leads to a range of 'pristine' compositions (delta 66Zn = +1.1 +/- 2.3%o; delta 65Cu = +0.5 +/- 0.5%o; 2 St. Dev.), implying inherent MVE variability within the eucrite parent body. As low-mass differentiated bodies, Vesta and the Moon represent endmembers in planet evolution. For the Moon, extensive volatile loss can be explained by a cataclysmic giant impact origin and later magma ocean crystallization. In contrast the parent body of eucrite meteorites likely heterogeneously lost volatile elements and compounds during differentiation. Vesta as the potential source of eucrite meteorites offers an important endmember composition for likely feedstocks to planets, representing the remaining vestige of what was likely to have been a larger population of differentiated objects in the inner Solar System shortly after nebula accretion. Mixing contributions of non-carbonaceous and carbonaceous chondrites constrained by nucleosynthetic Zn isotope anomalies suggests a significant fraction of Earth's accretion could have come from volatile-poor and differentiated planetary feedstocks that would have had limited effects on the bulk silicate Earth (BSE) Zn isotope composition. Furthermore, volatile-poor feedstocks cannot explain the BSE Cu isotope composition, which instead may have been modified by terrestrial core formation. Pristine eucrites offer key insights into early planetesimal differentiation and the role of volatile loss on small mass bodies within nascent solar systems.

In the 50 years since the first lunar sample return, the investigation of H2O in the Moon has experienced several stages of developments and paradigms. In the early years since Apollo sample return, only bulk soil and bulk rock samples were analyzed for H2O as well as other volatiles. From 1970 to 2007, it was thought that the Moon is essentially devoid of innate H2O, containing probably less than 1 ppb. New technologies gradually enabled the measurements of H2O in lunar glass beads, soil glass, minerals such as apatite and anorthite, and olivine-hosted melt inclusions. The advancements in measurement techniques led to improved data and new insights. Starting from 2008, significant H2O in deep-sourced lunar rocks has been reported, resulting in a paradigm shift from a bone-dry Moon to a fairly wet Moon, although there is still debate about whether the bulk silicate Moon contains similar to 100 ppm of H2O (similar to that in the Earth's MORB mantle) or only a few ppm H2O. The advances on our knowledge of H2O in the Moon is accompanied by increased understanding of other volatiles in the Moon. Gradually, the degrees of depletion of various volatiles in the Moon relative to the Earth were inferred. Using assessed data from available lunar samples, mostly the melt inclusions, and also bulk rock analyses, it is found that the inferred degrees of depletion for volatile elements in the Moon relative to the Earth do not vary much and are independent of the condensation temperature. It is proposed that an early veneer delivered the volatiles to both the Earth and the Moon, but the Moon received proportionally less of the early veneer planetesimals. In addition to H2O in the interior of the Moon, significant surface H2O in the form of ice in lunar polar regions and structural OH in agglutinate glass in lunar regolith originating from solar wind implantation has also been gradually quantified.

Moderately volatile elements (MVE) are key tracers of volatile depletion in planetary bodies. Zinc is an especially useful MVE because of its generally elevated abundances in planetary basalts, relative to other MVE, and limited evidence for mass-dependent isotopic fractionation under high-temperature igneous processes. Compared with terrestrial basalts, which have delta Zn-66 values (per mille deviation of the Zn-66/Zn-64 ratio from the JMC-Lyon standard) similar to some chondrite meteorites (similar to+0.3 parts per thousand), lunar mare basalts yield a mean delta Zn-66 value of +1.4 +/- 0.5 parts per thousand (2 st. dev.). Furthermore, mare basalts have average Zn concentrations similar to 50 times lower than in typical terrestrial basaltic rocks. Late-stage lunar magmatic products, including ferroan anorthosite, Mg- and Alkali-suite rocks have even higher delta Zn-66 values (+3 to +6 parts per thousand). Differences in Zn abundance and isotopic compositions between lunar and terrestrial rocks have previously been interpreted to reflect evaporative loss of Zn, either during the Earth-Moon forming Giant Impact, or in a lunar magma ocean (LMO) phase. To explore the mechanisms and processes under which volatile element loss may have occurred during a LMO phase, we developed models of Zn isotopic fractionation that are generally applicable to planetary magma oceans. Our objective was to identify conditions that would yield a delta Zn-66 signature of similar to+1.4%0 within the lunar mantle. For the sake of simplicity, we neglect possible Zn isotopic fractionation during the Giant Impact, and assumed a starting composition equal to the composition of the present-day terrestrial mantle, assuming both the Earth and Moon had zinc 'consanguinity' following their formation. We developed two models: the first simulates evaporative fractionation of Zn only prior to LMO mixing and crystallization; the second simulates continued evaporative fractionation of Zn that persists until similar to 75% LMO crystallization. The first model yields a relatively homogenous bulk solid LMO delta Zn-66 value, while the second results in a stratification of delta Zn-66 values within the LMO sequence. Loss and/or isolation mechanisms for volatiles are critical to these models; hydrodynamic escape was not a dominant process, but loss of a nascent lunar atmosphere or separation of condensates into a proto-lunar crust are possible mechanisms by which volatiles could be separated from the lunar interior. The results do not preclude models that suggest a lunar volatile depletion episode related to the Giant Impact. Conversely, LMO models for volatile loss do not require loss of volatiles prior to lunar formation. Outgassing during planetary magma ocean phases likely played a profound role in setting the volatile inventories of planets, particularly for low mass bodies that experienced the greatest volatile loss. In turn, our results suggest that the initial compositions of planets that accreted from smaller, highly differentiated planetesimals were likely to be severely volatile depleted. (C) 2017 Elsevier Inc. All rights reserved.