The Cl isotope composition and halogen contents of Apollo-return samples

Abstract

Lunar mare basalts are depleted in F and Cl by approximately an order of magnitude relative to mid-ocean ridge basalts and contain two Cl-bearing components with elevated isotopic compositions relative to the bulk-Earth value of ∼0‰. The first is a water-soluble chloride constituting 65 ± 10% of total Cl with δ37Cl values averaging 3.0 ± 4.3‰. The second is structurally bound chloride with δ37Cl values averaging 7.3 ± 3.5‰. These high and distinctly different isotopic values are inconsistent with equilibrium fractionation processes and instead suggest early and extensive degassing of an isotopically light vapor. No relationship is observed between F/Cl ratios and δ37Cl values, which suggests that lunar halogen depletion largely resulted from the Moon-forming Giant Impact. The δ37Cl values of apatite are generally higher than the structurally bound Cl, and ubiquitously higher than the calculated bulk δ37Cl values of 4.1 ± 4.0‰. The apatite grains are not representative of the bulk rock, and instead record localized degassing during the final stages of lunar magma ocean (LMO) or later melt crystallization. The large variability in the δ37Cl values of apatite within individual thin sections further supports this conclusion. While urKREEP (primeval KREEP [potassium/rare-earth elements/phosphorus]) has been proposed to be the source of the Moon’s high Cl isotope values, the ferroan anorthosites (FANs) have the highest δ37Cl values and have a positive correlation with Cl content, and yet do not contain apatite, nor evidence of a KREEP component. The high δ37Cl values in this lithology are explained by the incorporation of a >30‰ HCl vapor from a highly evolved LMO.

Tidal pull of the Earth strips the proto-Moon of its volatiles

“…Cl (Sharp et al 2007;Boyce et al 2015;Gargano et al 2020), Ga (Kato and Moynier 2017), and Rb (Pringle and Moynier 2017), indicative of an evaporative origin for volatile element loss from the Moon. Conversely, the enrichment of the lighter isotopes of Cr in lunar magmatic rocks compared to Earth's mantle suggests devolatilisation occurred near equilibrium and under low temperatures (≤ 1800 K) , far below those expected during a giant impact event.…”

Experimental Investigation of Apollo 16 “Rusty Rock” Alteration by a Lunar Fumarolic Gas

“…Apart from lunar apatites (Ustunisik et al., 2015; Wang et al., 2012), these are the heaviest Cl isotopic compositions measured in lunar samples (Shearer et al., 2014). As the overall Cl isotopic composition of the Moon was inferred to be similar to that of the Earth, the occurrence of much heavier isotopic compositions was attributed to the volatilization of metal halides (Gargano et al., 2020; Sharp et al., 2010; Shearer et al., 2014). These metal halides were deposited from the gas phase on pyroclastic glass beads, and in altered regolith and breccia such as 66095 (Shearer et al., 2014).…”

“…Because the light S isotopes preferentially partition into a gas phase, isotopically light δ 34 S in 66095 sulfides indicate that they deposited from a volcanic or fumarolic gas (Shearer et al, 2012(Shearer et al, , 2014. The Cl isotopic composition of 66095, on the other hand, is heavy with respect to lunar igneous rocks, with δ 37 Cl ranging from +14.0% to +15.6% (Gargano et al, 2020;Sharp et al, 2010;Shearer et al, 2014). Apart from lunar apatites (Ustunisik et al, 2015;Wang et al, 2012), these are the heaviest Cl isotopic compositions measured in lunar samples (Shearer et al, 2014).…”

The Zn, S, and Cl isotope compositions of mare basalts: Implications for the effects of eruption style and pressure on volatile element stable isotope fractionation on the Moon

“…Degassing of mare lavas only became important in the final stages of crystallization recorded in apatite -potentially facilitated by cracks/fractures in Revision 1 4 impacts (Barnes et al, 2016;McCubbin and Barnes, 2020), volcanic degassing (Sharp et al, 2010) and devolatilization associated with the Giant Impact (Gargano et al, 2020). The importance of early, global degassing must take into account the rapid solidification of the LMO surface layer 'lid' which would presumably reduce vapor-loss (Barnes et al, 2016;Gargano et al, 2020), and occurred rapidly within thousands of years (Elkins-Tanton et al, 2011). In addition, recent isotopic modeling has shown that the silicate-vapor above the LMO would be in isotopic equilibrium with the magma ocean, resulting in limited isotopic fractionation (Tang and Young, 2020).…”