To compare to the Earth, we have also measured the Rb isotopic compositions of terrestrial basalts and granites (Table 1). We have measured the isotopic compositions of Rb in six Apollo lunar samples comprising five mare basalts and one norite (Table 1) to better constrain the bulk Rb isotopic composition of the Moon, which is expressed in δ 87Rb notation (the permil departure of the 87Rb/ 85Rb ratio relative to reference material NIST SRM984). In Section 4, we show that this saturation can be explained in the context of viscous drainage of vapor in the protolunar disk onto Earth if the α-viscosity of the vapor layer was 10 −3–10 −2, which is consistent with viscosities predicted if the magnetorotational instability (MRI) was active. In Section 3, we calculate the saturation of Rb (∼99%) in the vapor medium needed to explain the heavy Rb isotope enrichment of lunar rocks by evaporation, and show that the same degree of saturation can account for previously documented heavy isotope enrichments in other MVEs K, Ga, Cu, and Zn. In Section 2, we report the results of Rb isotopic analyses of Apollo lunar samples (primarily basalts) indicating that the Moon is enriched in the heavy isotopes of Rb relative to Earth. We report here new and high-precision Rb isotopic analyses of lunar and terrestrial samples that we use to understand why the Moon is depleted in MVEs relative to Earth. 2019), but the extents of those fractionations are uncertain, and a quantitative understanding of how they relate to the conditions of the Moon formation is still missing. 2015 Wang & Jacobsen 2016 Kato & Moynier 2017 Pringle & Moynier 2017 Wimpenny et al. Isotopic analyses of several MVEs have revealed heavy isotope enrichments of lunar rocks compared to Earth (Herzog et al. 2018) involved in the formation of the Moon. One difficulty in assessing these scenarios is the lack of thermodynamic data constraining the condensation/evaporation behaviors of MVEs at the very high temperatures (presumably >3500 K Canup et al. 2019), and (iv) loss to space from the lunar magma ocean (Day & Moynier 2014 Saxena et al. 2015), (iii) removal of volatile elements from a stratified protolunar disk by accretion of the vapor layer onto Earth (Charnoz & Michaut 2015 Wang et al. 2018), (ii) accretion of the Moon from a partially condensed, volatile-depleted silicate melt carried across the Roche limit by viscous spreading (Canup et al. The settings considered to explain these depletions are (i) incomplete condensation of a vapor cloud surrounding the Earth (synestia) in the aftermath of a very energetic impact event (Lock et al. Among these is the lunar depletion in moderately volatile elements (MVEs) such as Rb, K, and Zn (Ringwood & Kesson 1977 O'Neill 1991 Albarède et al. 2012 Nakajima & Stevenson 2015 Lock et al. Simulations of this giant impact can explain the angular momentum of the Earth–Moon system and the small mass of the lunar core, but other chemical and isotopic constraints are more challenging to reproduce (Canup & Asphaug 2001 Pahlevan & Stevenson 2007 Canup 2012 Cuk & Stewart 2012 Reufer et al. The prevailing view of the origin of the Moon is that it formed by a collision between the proto-Earth and a large impactor that has been named Theia, which produced a disk of liquid and vapor from which the Moon grew (Hartmann & Davis 1975 Cameron & Ward 1976). In the framework of an α-disk model, the α-viscosity needed to explain the ∼99% saturation calculated here is 10 −3–10 −2, which is consistent with a vapor disk where viscosity is controlled by magnetorotational instability. 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. Evaporation under this saturation can also explain the previously documented isotopic fractionations of K, Ga, Cu, and Zn of lunar rocks relative to Earth. This isotopic composition is consistent with evaporation of Rb into a vapor medium that was ∼99% saturated. Combining our new measurements with previous data reveals that the Moon has an 87Rb/ 85Rb ratio higher than Earth by +0.16 ± 0.04‰. We measured the Rb isotopic compositions of lunar and terrestrial rocks to understand why MVEs are depleted in the Moon.

Identifying the cause of this depletion is important for understanding how the Earth–Moon system evolved in the aftermath of the Moon-forming giant impact. Lunar rocks are severely depleted in moderately volatile elements (MVEs) such as Rb, K, and Zn relative to Earth.

Lunar disk graph