Lunar bulk chemical composition: a post-Gravity Recovery and Interior Laboratory reassessment

Abstract

New estimates of the thickness of the lunar highlands crust based on data from the Gravity Recovery and Interior Laboratory mission, allow us to reassess the abundances of refractory elements in the Moon. Previous estimates of the Moon fall into two distinct groups: earthlike and a 50% enrichment in the Moon compared with the Earth. Revised crustal thicknesses and compositional information from remote sensing and lunar samples indicate that the crust contributes 1.13–1.85 wt% Al 2 O 3 to the bulk Moon abundance. Mare basalt Al 2 O 3 concentrations (8–10 wt%) and Al 2 O 3 partitioning behaviour between melt and pyroxene during partial melting indicate mantle Al 2 O 3 concentration in the range 1.3–3.1 wt%, depending on the relative amounts of pyroxene and olivine. Using crustal and mantle mass fractions, we show that that the Moon and the Earth most likely have the same (within 20%) concentrations of refractory elements. This allows us to use correlations between pairs of refractory and volatile elements to confirm that lunar abundances of moderately volatile elements such as K, Rb and Cs are depleted by 75% in the Moon compared with the Earth and that highly volatile elements, such as Tl and Cd, are depleted by 99%. The earthlike refractory abundances and depleted volatile abundances are strong constraints on lunar formation processes.

Water in the Moon's interior: Truth and consequences

“…As a result of the above isotopic arguments, we simply posit that the abundances of SiO 2 , TiO 2 , Al 2 O 3 , FeO, MnO, MgO and CaO are the same on the Earth and Moon. This supposition is supported by the more extensive petrologic arguments of Warren (2005), Ringwood (1992) and Taylor and Wieczorek (2014) who reached the same conclusion for major elements and 27 refractory lithophile elements (FeO excepted); in our case, we assume a terrestrial FeO content, but admit that present constraints on the density of the lunar mantle and size of the lunar core would permit a slightly higher FeO and lower corresponding Mg# (Warren and Dauphas, 2014). In the same way, we also assume that the refractory lithophile elements are present at terrestrial levels in the Moon, for elements with 50% condensation temperatures >1300 K. Our BSM composition is given in Table 3 and described in detail in the Supplementary Material, and BSM/BSE ratios are shown in Fig.…”

“…For these two systems, volatile depletion works in opposite directions, resulting in low 87 Sr/ 86 Sr ratios like those observed in lunar anorthosites (Carlson and Lugmair, 1988), while volatile depletion works to increase the U/Pb ratio of mantle sources and results in high ratios of 207 Pb/ 204 Pb and 206 Pb/ 204 Pb as observed in the interiors of volcanic glass beads (Tera and Wasserburg, 1976). The point to be made here is that recent high-precision studies on well-dated lunar rocks, where multiple isotope systems give concordant ages, has revealed the isotopic evolution of initial Sr and Pb isotope ratios among lunar mantle sources at different ages, and these isotopic evolutions are often consistent with Rb/Sr and U/Pb ratios that begin to approach terrestrial ratios.…”

“…Measurement of water in lunar apatite, at levels similar to terrestrial apatite, has added weight to this discovery (Boyce et al, 2010(Boyce et al, , 2014McCubbin et al, 2010;Greenwood et al, 2011;Anand et al, 2014;Barnes et al, 2013Barnes et al, , 2014Tartèse et al, , 2014. These surprising results, the culmination of over four decades of intensive geochemical dowsing, provide a severe connature of the Moon compared with the Earth (Krähenbühl et al, 1973a(Krähenbühl et al, , 1973bTera and Wasserburg, 1976;Ringwood and Kesson, 1977;McDonough et al, 1992). There exists a large body of evidence that the abundances of highly-and moderately-volatile elements in lunar basalts are depleted to levels that are as much as 1000× lower than their abundances in terrestrial mid-ocean ridge and ocean-island basalts.…”

“…Numerous authors have attempted to calculate the composition of the BSM (Wänke et al, 1975;Ringwood and Kesson, 1977;Buck and Toksoz, 1980;Warren, 1985;Rasmussen and Warren, 1985;Ringwood, 1992;Albarede et al, 2014;Taylor and Wieczorek, 2014), driven by various inputs including the analysis of returned Apollo samples, remote mapping of the lunar surface, the recognition of the highlands as being largely feldspathic, and Apollo-era analysis of seismic data suggesting the highlands crustal thickness to be in the range of 60-70 km. Based on these inputs, the major element composition of many BSM estimates have tended to be enriched in FeO, CaO, Al 2 O 3 and refractory trace elements compared with the Earth.…”

Mineral Ecology: Chance and Necessity in the Mineral Diversity of Terrestrial Planets

“…Nonetheless, we have not attempted to use lunar crust composition as a basis for comparison, because it is heterogeneous and regionally variable (e.g., Taylor 2009, Taylor & Wieczorek 2014; thus, limiting the comparison to crustal compositions would not be practical. Instead we have used models of the bulk Moon (Table 3).…”