An intrinsic volatility scale relevant to the Earth and Moon and the status of water in the Moon

Abstract

The notion of a dry Moon has recently been challenged by the discovery of high water contents in lunar apatites and in melt inclusions within olivine crystals from two pyroclastic glasses. The highest and most compelling water contents were found in pyroclastic glasses that are not very common on the lunar surface. To obtain more representative constraints on the volatile content of the lunar interior, we measured the Zn content, a moderately volatile element, of mineral and rock fragments in lunar soils collected during Apollo missions. We here confirm that the Moon is significantly more depleted in Zn than the Earth. Combining Zn with existing K and Rb data on similar rocks allows us to anchor a new volatility scale based on the bond energy of nonsiderophile elements in their condensed phases. Extrapolating the volatility curve to H shows that the bulk of the lunar interior must be dry (≤1 ppm). This contrasts with the water content of the mantle sources of pyroclastic glasses, inferred to contain up to approximately 40 ppm water based on H2O/Ce ratios. These observations are best reconciled if the pyroclastic glasses derive from localized water‐rich heterogeneities in a dominantly dry lunar interior. We argue that, although late addition of 0.015% of a chondritic veneer to the Moon seems required to explain the abundance of platinum group elements (Day et al. 2007), the volatile content of the added material was clearly heterogeneous.

Water, fluorine, and sulfur concentrations in the lunar mantle

“…The lower H 2 O contents in the crystalline lunar MIs are not surprising considering the easiness of H 2 O loss during degassing, and the high mobility of H through olivine at igneous temperatures (Chen et al, 2011(Chen et al, , 2013Bucholz et al, 2013;Le Voyer et al, 2014;Newcombe et al, 2014) as evidenced by the loss of up to 85% of the pre-degassing H 2 O from glassy Kilauea MIs, which were degassed at 0.85 bar whereas lunar basalts were degassed at essentially zero pressure. Thus, it is remarkable that H 2 O contents in even these homogenized crystalline lunar MIs are up to 131 ppm, much higher than 0.09 ppm or 1 ppm that Sharp et al (2010) and Albarede et al (2015) have inferred. These results demonstrate that elevated volatile contents of lunar magmas are a more widespread phenomenon than simply being a feature of the pyroclastic deposits of sample 74220.…”

“…3 but the differences are less obvious. Albarede et al (2015) suggested that the high H 2 O concentration in 74220 melt inclusions was due to addition of CI chondrite. However, concentrations of H 2 O, Cl, F, and S in CI chondrites (Palme and O'Neil, 2014) are respectively 208, 230, 1.0, and 78 times the average concentrations in 74220 MIs, meaning that Cl and H 2 O would be elevated similarly.…”

“…Xu et al (2014) used a density argument to place constraints on H 2 O content in pre-eruptive lunar magma to be ≤1000 ppm. Albarede et al (2015) showed that K/U, Rb/Ba, and Zn/Fe ratios in lunar basalts are lower than those in terrestrial basalts by factors of ∼6, ∼7, and 10-200 respectively. Using these moderately volatile elements to construct a volatility scale, Albarede et al (2015) inferred a concentration of ∼1 ppm H 2 O for the bulk moon.…”

Thoughts and Reminiscences on Experimental Trace Element Partitioning

“…This was probably one of those instances where more heat than light illuminated the basic issue. Still, the establishment of the Moon's abundances of volatile alkali elements should, in principle, be an important constraint on the origin of the Moon (e.g., Humayun and Clayton, 1994;Albarède et al, 2015). For example, the Humayun and Clayton paper emphasised that the K isotopic compositions of almost all bodies in the inner solar system were nearly identical.…”

The Moon

“…Thus the Moon seems essentially devoid of water, unless one regards the presence of a few parts per million as significant and the Moon seems drier than the Earth by large factors. The great publicity about traces of water, either on the lunar surface or in the interior of the Moon, seems to be due to the sacred-cow status that has been given to water (see also Albarede et al 2015).…”