The chlorine isotopic composition of the Moon: Insights from melt inclusions

Xuchao Zhao,Alice Stephant,Queenie H.S. Chan,Ian A. Franchi,Mahesh Anand,Magali Bonifacie

doi:10.1016/j.epsl.2019.115715

Abstract

The Moon exhibits a heavier chlorine (Cl) isotopic composition compared to the Earth. Several hypotheses have been put forward to explain this difference, based mostly on analyses of apatite in lunar samples complemented by bulk-rock data. The earliest hypothesis argued for Cl isotope fractionation during the degassing of anhydrous basaltic magmas on the Moon. Subsequently, other hypotheses emerged linking Cl isotope fractionation on the Moon with the degassing during the crystallization of the Lunar Magma Ocean (LMO). Currently, a variant of the LMO degassing model involving mixing between two end-member components, defined by early-formed cumulates, from which mare magmas were subsequently derived, and a KREEP component, which formed towards the end of the LMO crystallization, seems to reconcile some existing Cl isotope data on lunar samples. To further ascertain the history of Cl in the Moon and to investigate any evolution of Cl during magma crystallization and emplacement events, which could help resolve the chlorine isotopic variation between the Earth and the Moon, we analysed the Cl abundance and its isotopic composition in 36 olivine- and pyroxene-hosted melt inclusions (MI) in five Apollo basalts (10020, 12004, 12040, 14072 and 15016). Olivine-hosted MI have an average of 3.3±1.4ppm Cl. Higher Cl abundances (11.9 ppm on average) are measured for pyroxene-hosted MI, consistent with their formation at later stages in the crystallization of their parental melt compared to olivines. Chlorine isotopic composition (δ37Cl) of MI in the five Apollo basalts have weighted averages of +12.8±2.4‰ and +10.1±3.2‰ for olivine- and pyroxene-hosted MI, respectively, which are statistically indistinguishable. These isotopic compositions are also similar to those measured in apatite in these lunar basalts, with the exception of sample 14072, which is known to have a distinct petrogenetic history compared to other mare basalts. Based on our dataset, we conclude that, post-MI-entrapment, no significant Cl isotopic fractionation occurred during the crystallization and subsequent eruption of the parent magma and that Cl isotopic composition of MI and apatite primarily reflect the signature of the source region of these lunar basalts. Our findings are compatible with the hypothesis that in the majority of the cases the heavy Cl isotopic signature of the Moon was acquired during the earliest stages of LMO evolution. Interestingly, MI data from 14072 suggests that Apollo 14 lunar basalts might be an exception and may have experienced post-crystallization processes, possibly metasomatism, resulting in additional Cl isotopic fractionation recorded by apatite but not melt inclusions.

Highlights

post-entrapment crystallization (PEC) ranges from 7% to 65% in olivinehosted melt inclusions (MI), with an average of 46% while in pyroxene-hosted MI, PEC ranges from 37% to 72% with an average of 54% (Table 1)
Selected back scattered electron (BSE) images, demonstrating the diversity of analysed MI, are shown in Fig. 1, while BSE images of all MI analysed in study are provided in supplementary Fig. S1
Olivine- and pyroxene-hosted MI from Apollo 10020, 12004, 12040 and 15016 lunar basalts exhibit Cl isotopic composition similar to the range observed for apatite from Apollo mare basalts measured previously

Summary

Introduction

Lunar samples exhibit a wide range of measured Cl isotopic ratio (Barnes et al, 2016; Boyce et al, 2015; Potts et al, 2018; Sharp et al, 2010; Tartèse et al, 2014; Treiman et al, 2014) compared to the Earth (Bonifacie et al, 2008; Manzini et al, 2017; Sharp et al, 2013). Sharp et al (2010) performed the first analyses for Cl isotopes in lunar basalts and suggested that volatilization of metal chlorides (e.g. FeCl2 or ZnCl2) during the eruption of mare magmas was responsible for the observed Cl isotopic fractionation. This hypothesis was founded on the fact that the lowest δ37Cl measured in lunar samples overlapped with the terrestrial values. For the Moon, this scenario goes against numerous studies arguing for a relatively wet Moon (Hauri et al, 2011; Saal et al, 2008), albeit still volatile-depleted compared to the Earth (Albarede et al, 2015)

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