Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

Abstract

Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar magmatic differentiation. Mare basalts sample the largest lunar mantle volume, from olivine-and pyroxene-rich cumulates, whereas ferroan anorthosites and magnesian-suite rocks represent early crustal materials. Incompatible element enriched rocks, known as 'KREEP,' probably preserve evidence for the last highly differentiated melts. Here we show that mare basalts, including Apollo samples and meteorites, have remarkably consistent δ 66 Zn values (+1.4 ± 0.2) and Zn abundances (1.5 ± 0.4 ppm). Analyses of magnesian-suite rocks show them to be characterized by even heavier δ 66 Zn values (2.5 to 9.3) and low Zn concentrations. KREEP-rich impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts (δ 66 Zn = 1.3) and a low Zn abundance (0.5 ppm). Much of this variation can be explained through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light Zn can be explained by either direct condensation or mixing and contamination processes at the lunar surface. The δ 66 Zn of Sayh al Uhaymir 169 is probably compromised by mixing processes of KREEP with mafic components. Correlations of Zn with Cl isotopes suggest that the urKREEP reservoir should be isotopically heavy with respect to Zn, like magnesian-suite rocks. Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to lunar formation, and planetary processes, either during giant impact, or magmatic differentiation. Our results provide unambiguous evidence for the latter process. Notwithstanding, with the currently available volatile stable isotope datasets, it is difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or exclusively from later magmatic differentiation. If the Moon did begin initially volatile-depleted, then the mare basalt δ 66 Zn value likely preserves the signature, and the Moon lost 96% of its Zn inventory relative to Earth and was also characterized by isotopically heavy Cl (δ 37 Cl = ≥8). Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories.

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

“…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).…”

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

“…These data suggest localized surface-related processes that produced anomalously low or high isotope values for Zn, S, and Cl consistent with vacuum degassing. Bulk-rock low and high-Ti mare basalts have elevated, yet restricted ranges of Zn, S, and Cl isotope compositions (relative to the silicate Earth) that have been interpreted to reflect devolatilization during the Giant Impact (Day et al, 2020;Gargano et al, 2020;Moynier et al, 2006;Paniello et al, 2012;Wing and Farquhar, 2015). In contrast, the wide ranges in H, Cl, and S isotope compositions of lunar apatite are interpreted to reflect isotope fractionation during magmatic degassing (Barnes et al, 2014;Barnes et al, 2016;Boyce et al, 2015;Faircloth et al, 2020).…”

“…Figure 2:  66 Zn (‰ vs. JMC-Lyon) values of terrestrial and planetary materials. Data from (Creech and Moynier, 2019;Day et al, 2020;Herzog et al, 2009;Moynier et al, 2006;Moynier et al, 2010;Moynier et al, 2007;Paniello et al, 2012;Sossi et al, 2018). Legend is same as Fig.…”

“…Most bulk-rock mare basalts have generally elevated, yet limited, ranges in  66 Zn,  34 S, and  37 ClSBC+WSC values (averages 1.31 ± 0.44, 0.59 ± 0.22, and 4.1 ± 4.0‰, respectively) compared to the silicate Earth (0.15, -1.28, and 0‰) (Labidi et al, 2013;Sharp et al, 2013b;Sossi et al, 2018). There are several anomalous samples that have low  66 Zn values (10017, 12018, 15016, and 14053) explained by degassing and subsequent vapor deposition (Day et al, 2017;Day et al, 2020). This idea is also seen in Cl, with the particularly high  37 ClSBC, WSC values in 10017-405 (9.2 and 12.6‰), 12018 (10.1 and 5.0‰), and 14053 (11.2 and 6.1‰) (Gargano et al, 2020).…”

Conditions and extent of volatile loss from the Moon during formation of the Procellarum basin

“…We chose Zn because its abundance in lunar rocks is well constrained (e.g., ref. 47), it remains lithophile under the conditions of core formation on the Moon (48)(49)(50), and it adheres to a simple and wellcharacterized vaporization behavior from silicate melts (35,51). Moreover, because it exists as Zn 2+ in silicate melts, its vaporization stoichiometry differs from those of the alkalis (A + ), such that precise constraints on fO 2 are best evaluated by fixing Zn abundances, via exchange reactions of the following type, ZnO l…”