Chlorine isotopic compositions of apatite in Apollo 14 rocks: Evidence for widespread vapor-phase metasomatism on the lunar nearside ∼4 billion years ago

Abstract

Compared to most other planetary materials in the Solar System, some lunar rocks display high δ 37 Cl signatures. Loss of Cl in a H<<Cl environment has been invoked to explain the heavy signatures observed in lunar samples, either during volcanic eruptions onto the lunar surface or during large scale degassing of the lunar magma ocean. To explore the conditions under which Cl isotope fractionation occurred in lunar basaltic melts, five Apollo 14 crystalline samples were selected (14053,19, 14072,13, 14073,9, 14310,171 along with basaltic clast 14321,1482) for in situ analysis of Cl isotopes using secondary ion mass spectrometry. Cl isotopes were measured within the mineral apatite, with δ 37 Cl values ranging from +14.6 ± 1.6 ‰ to +40.0 ± 2.9 ‰. These values expand the range previously reported for apatite in lunar rocks, and include some of the heaviest Cl isotope compositions measured in lunar samples to date. The data here do not display a trend between increasing rare earth elements contents and δ 37 Cl values, reported in previous studies. Other processes that can explain the wide inter-and intra-sample variability of δ 37 Cl values are explored. Magmatic degassing is suggested to have potentially played a role in fractionating Cl isotope in these samples. Degassing alone, however, could not create the wide variability in isotopic signatures. Our favored hypothesis, to explain small scale heterogeneity, is late-stage interaction with a volatile-rich gas phase, originating from devolatilization of lunar surface regolith rocks ~4 billion years ago. This period coincides with vapor-induced metasomastism recorded in other lunar samples collected at the Apollo 16 and 17 landing sites, pointing to the possibility of widespread volatile-induced metasomatism on the lunar nearside at that time, potentially attributed to the Imbrium formation event.

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

“…; Potts et al. ). Isotopic fractionation from degassing assuming Graham's Law diffusion and Rayleigh Distillation can be modeled such thatwhereand M 1 is the mass of the light isotopologue (i.e., Fe 35 Cl 2 ), and M 2 is the mass of the heavy isotopologue (i.e., Fe 35 Cl 37 Cl).…”

“…If devolatilization occurred, the d 37 Cl values could be raised from a nebular value of À7& to a near-zero value seen on Earth. Chlorine degassing has been shown to produce significant heavy isotope enrichments resulting in high d 37 Cl values >35& measured in lunar samples (Sharp et al 2010b;Boyce et al 2015;Barnes et al 2016;Potts et al 2018). Isotopic fractionation from degassing assuming Graham's Law diffusion and Rayleigh Distillation can be modeled such that Fig.…”

Chlorine Isotope Composition of Apatite from the >3.7 Ga Isua Supracrustal Belt, SW Greenland

“…The formation of perchlorate fractionates Cl isotopes [39,45], but only a few examples of extreme δ 37 Cl values have been found on Earth, e.g., 12% in volcanic fumaroles [46]. This behavior is in contrast to lunar samples, where δ 37 Cl values as high as 81.1% have been reported [36,37], which is possibly due to the large kinetic isotope effects during the rapid diffusion of chlorine species to the vapor phase during degassing [41]. Kusebauch et al [47] suggested that fluid-rock interactions may not only cause elemental mobilization but in some cases also the fractionation of their isotopes.…”

“…The ratio of two stable isotopes 37 Cl and 35 Cl in an unknown sample is usually compared to standard mean ocean chloride (SMOC) using Equation (1) δ 37 Cl =       37 Cl/ 35 Cl sample 37 Cl/ 35 Cl SMOC − 1       × 1000 (1) and is reported in the standard per mil notation (% ). Chlorine isotopes have recently gained attention in studies of lunar [32][33][34][35][36][37] and Martian apatite samples [38,39]. In lunar materials, relatively large natural variations in δ 37 Cl have been detected (from −1% up to 40% [32,36] or even from 3.8% to 81.1% [37]), while the range of δ 37 Cl in terrestrial geological samples is much smaller with the vast majority of values falling between −2% and 2% [40,41].…”

“…Chlorine isotopes have recently gained attention in studies of lunar [32][33][34][35][36][37] and Martian apatite samples [38,39]. In lunar materials, relatively large natural variations in δ 37 Cl have been detected (from −1% up to 40% [32,36] or even from 3.8% to 81.1% [37]), while the range of δ 37 Cl in terrestrial geological samples is much smaller with the vast majority of values falling between −2% and 2% [40,41]. The range of chlorine isotope compositions of Martian apatite (from −5.5% to 8.6% [38,39]) is similar to the δ 37 Cl values of terrestrial samples rather than to the composition of lunar apatite.…”