Vapor Drainage in the Protolunar Disk as the Cause for the Depletion in Volatile Elements of the Moon

Abstract

Lunar rocks are severely depleted in moderately volatile elements such as Rb, K, and Zn relative to Earth. Identifying the cause of this depletion is important for understanding how the Earth-Moon system evolved in the aftermath of the Moonforming giant impact. We measured the Rb isotopic compositions of lunar and terrestrial rocks to understand why moderately volatile elements are depleted in the Moon. Combining our new measurements with previous data reveals that the Moon has an 87 Rb/ 85 Rb ratio higher than Earth by +0.16±0.04 ‰. This isotopic composition is consistent with evaporation of Rb into a vapor medium that was ~99% saturated. Evaporation under this saturation can also explain the previously documented isotopic fractionations of K, Ga, Cu and Zn of lunar rocks relative to Earth. We show that a possible setting for achieving the same saturation upon evaporation of elements with such diverse volatilities is through viscous drainage of a partially vaporized protolunar disk onto Earth. In the framework of an a-disk model, the a-viscosity needed to explain the ~99% saturation calculated here is 10 −3 to 10 −2 , which is consistent with a vapor disk where viscosity is controlled by magnetorotational instability.

The Fate of Sulfur and Chalcophile Elements During Crystallization of the Lunar Magma Ocean

“…As inferred for other volatile elements including Zn, Ga, K, and Rb (Day and Moynier, 2014; Day et al., 2020; Kato et al., 2017; Nie et al., 2019; Paniello et al., 2012; Wang and Jacobsen, 2016), we consider it more likely that the heavy Cu isotopic composition of the lunar mantle instead represents (an) episode of extensive volatile loss, consistent with the recent model of lunar volatile element depletions of Nie et al. (2019).…”

“…Variable degrees of cumulate assimilation or cumulate source incorporation to KREEP basalts therefore do not affect the observation that measured Cu contents of KREEP are much higher than the Cu content predicted for sulfide liquid-saturated urKREEP liquid. Due to the similarity in calculated D i (Figure 7) is consistent with loss of Cd during the Moon-forming event, as proposed for other similarly volatile elements (e.g., Day and Moynier, 2014;Nie et al, 2019).…”

“…Under the assumption of more realistic lower bulk Moon S estimates (i.e., BM ≤ BSE), sulfide liquid segregation cannot explain the observed heavy Cu isotopic composition of the Moon (Xia et al, 2019). As inferred for other volatile elements including Zn, Ga, K, and Rb (Day and Moynier, 2014;Day et al, 2020;Kato et al, 2017;Nie et al, 2019;Paniello et al, 2012;Wang and Jacobsen, 2016), we consider it more likely that the heavy Cu isotopic composition of the lunar mantle instead represents (an) episode of extensive volatile loss, consistent with the recent model of lunar volatile element depletions of Nie et al (2019).…”

Geochemical Constraints on the Origin of the Moon and Preservation of Ancient Terrestrial Heterogeneities

“…Further insight on the mechanism of MVE depletion of the Moon relative to Earth is provided by isotopic studies. It has been found that lunar samples exhibit massdependent isotopic fractionation relative to Earth for various MVEs (K, Rb, Zn, Cl, Sn, Cr, Ga; Herzog et al, 2009;Sharp et al, 2010;Paniello et al, 2012;Boyce et al, 2015Boyce et al, , 2018Kato et al, 2015;Wang and Jacobsen, 2016;Day et al, 2017;Kato and Moynier, 2017;Pringle and Moynier, 2017;Dhaliwal et al, 2018;Sossi et al, 2018;Wang et al, 2019a,b;Stephant et al, 2019;Nie and Dauphas, 2019;Wimpenny et al, 2019, see Figure 4). Different elements show different directions and magnitudes of fractionation and there is considerable variability in the fractionation of different lunar samples for some elements.…”

Isotope Fractionation during Condensation and Evaporation during Planet Formation Processes

“…Notably it has become clear that volatile depletion took place under conditions rather distinct from vacuum evaporation. Similarly, volatile element isotope signatures have provided new insights into the conditions of lunar formation (e.g., ref 48,49 ) or into the volatile loss during the existence of magma oceans [14][15][16] . These studies have highlighted that the gas speciation can be inferred from the pattern of isotope fractionation and the degree of fractionation can be intrinsically linked to the rate of transport away from an evaporating surface.…”