Magmatic volatiles (H, C, N, F, S, Cl) in the lunar mantle, crust, and regolith: Abundances, distributions, processes, and reservoirs

Abstract

Many studies exist on magmatic volatiles (H, C, N, F, S, Cl) in and on the Moon, within the last several years, that have cast into question the post-Apollo view of lunar formation, the distribution and sources of volatiles in the Earth-Moon system, and the thermal and magmatic evolution of the Moon. However, these recent observations are not the first data on lunar volatiles. When Apollo samples were first returned, substantial efforts were made to understand volatile elements, and a wealth of data regarding volatile elements exists in this older literature. In this review paper, we approach volatiles in and on the Moon using new and old data derived from lunar samples and remote sensing. From combining these data sets, we identified many points of convergence, although numerous questions remain unanswered.The abundances of volatiles in the bulk silicate Moon (BSM), lunar mantle, and urKREEP [last ~1% of the lunar magma ocean (LMO)] were estimated and placed within the context of the LMO model. The lunar mantle is likely heterogeneous with respect to volatiles, and the relative abundances of F, Cl, and H 2 O in the lunar mantle (H 2 O > F >> Cl) do not directly reflect those of BSM or urKREEP (Cl > H 2 O ≈ F). In fact, the abundances of volatiles in the cumulate lunar mantle were likely controlled by partitioning of volatiles between LMO liquid and nominally anhydrous minerals instead of residual liquid trapped in the cumulate pile. An internally consistent model for lunar volatiles in BSM should reproduce the absolute and relative abundances of volatiles in urKREEP, the anorthositic primary crust, and the lunar mantle within the context of processes that occurred during the thermal and magmatic evolution of the Moon. Using this mass-balance constraint, we conducted LMO crystallization calculations with a specific focus on the distributions and abundances of F, Cl, and H 2 O to determine whether or not estimates of F, Cl, and H 2 O in urKREEP are consistent with those of the lunar mantle, estimated independently from the analysis of volatiles in mare volcanic materials. Our estimate of volatiles in the bulk lunar mantle are 0.54-4.5 ppm F, 0.15-5.3 ppm H 2 O, 0.26-2.9 ppm Cl, 0.014-0.57 ppm C, and 78.9 ppm S. Our estimates of H 2 O are depleted compared to independent estimates of H 2

Solubility of water in lunar basalt at low pH2O

“…Several authors have suggested that H 2 dissolution could be important in the eruptive products of planetary bodies, including the Moon, that are more reduced than the Earth (Zhang and Ni 2010;ElkinsTanton and Grove 2011;Zhang 2011;Sharp et al 2013;McCubbin et al 2015). However, extrapolation (from 0.7-3 GPa) to low pressures of recent experimental data on the solubility of H 2 in basaltic melts (Hirschmann et al 2012) suggests that basaltic melt in equilibrium with 1 atm of pure H 2 likely contains only ~0.4 ppm (by weight) dissolved molecular hydrogen.…”

“…1978; Fogel and Rutherford 1995;Nicholis and Rutherford 2009;Rutherford and Papale 2009;Wetzel et al 2015), and others have proposed H 2 (Sharp et al 2013;McCubbin et al 2015). Arguments in support of CO as the primary propellant hinge on the H-depleted nature of lunar magmas compared to most terrestrial magmas (Saal et al 2008;Hauri et al 2011;Wetzel et al 2015;Chen et al 2015) and on the presence of Fe metal blebs in erupted lunar melts, thought to be the result of the reduction of FeO in the melt in response to graphite oxidation to CO during magma ascent (e.g., Nicholis and Rutherford 2009).…”

“…Indirect evidence for the presence of graphite in lunar magmas was presented by Sato (1979), who demonstrated that Apollo 17 orange glass undergoes self-reduction upon heating, from which he inferred the presence of ~50 ppm graphite in the magma, although thus far primary igneous graphite has not been found in lunar samples (McCubbin et al 2015 and references therein).…”

“…, Stolper 1982a;McMillan 1994;Dixon et al 1995;Moore et al 1998;Berndt et al 2002;Benne and Behrens 2003;Zhang et al 2007;Behrens et al 2009;Lesne et al 2010;Morizet et al 2010;Persikov et al 2010;Zhang and Ni 2010;Iacono-Marziano et al 2012;Shishkina et al 2014), water solubility and diffusion have not been studied for melts approaching the high iron contents of lunar magmas, and at the low fO 2 's and relatively low pH 2 O conditions of the Moon. Another source of uncertainty in lunar volcanic degassing models is the role of molecular hydrogen (Elkins-Tanton and Grove 2011;Zhang 2011;Hirschmann et al 2012;Sharp et al 2013;McCubbin et al 2015), with some authors suggesting that molecular hydrogen may be the main propellant of the lunar fire-fountain eruptions (Sharp et al 2013;McCubbin et al 2015), while other authors argue that the low solubility of molecular hydrogen in silicate melts at low pressures and low total H contents precludes a significant role for molecular hydrogen dissolution under low-pressure near-surface lunar conditions (Hirschmann et al 2012). …”

The Moon: An Archive of Small Body Migration in the Solar System

“…chondritic) asteroid sourced impactors (Day and Walker 2015). These impactors could also have contributed to the Moon's volatile and moderately volatile element budget and isotopic signature (McCubbin et al 2015;Tartèse 2016) (although degassing of the magma ocean may have contributed to some later depletion of volatile species Kato et al 2015). Examination of the lunar rock hydrogen and nitrogen isotope records indicate that the sources of impactors that delivered such volatiles to the lunar magma ocean were dominated by water or ice-rich carbonaceous chondrite-like asteroid projectiles (Barnes et al 2016).…”