An ancient reservoir of volatiles in the Moon sampled by lunar meteorite Northwest Africa 10989

A Stephant,M Anand,H.O Ashcroft,X Zhao,S Hu,R.L Korotev,S Strekopytov,R.C Greenwood,E Humphreys-Williams,Y Liu,G Tang,Q Li,I.A Franchi

doi:10.1016/j.gca.2019.07.045

Abstract

Northwest Africa (NWA) 10989 is a recently found lunar meteorite we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates. The petrology, bulk geochemistry and mineralogy of NWA 10989 are consistent with it being a lunar meteorite with intermediate-iron bulk composition, composed of 40% of mare basaltic material and ∼60% non-mare material, but with no obvious KREEP-rich basaltic components. It is probable that the source region for this meteorite resides near a mare–highlands boundary, possibly on the farside of the Moon. Analyses of chlorine and hydrogen abundances and isotopic composition in apatite and merrillite grains from NWA 10989 indicate sampling of at least two distinct reservoirs of volatiles, one being similar to those for known mare basalts from the Apollo collections, while the other potentially represents a yet unrecognized reservoir. In situ Th-U-Pb dating of phosphates reveal two distinct age clusters with one ranging from 3.98 ± 0.04 to 4.20 ± 0.02 Ga, similar to the ages of cryptomare material, and the other ranging from 3.32 ± 0.01 to 3.96 ± 0.03 Ga, closer to the ages of mare basalts known from the Apollo collections. This lunar breccia features mixing of material, among which a basaltic D-poor volatile reservoir which doesn’t appear to have been recorded by Apollo samples.

Highlights

Northwest Africa (NWA) 10989 is a recently found lunar meteorite we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates
The discovery and categorization of more lunar meteorites, meteorites with intermediate-iron bulk composition, suggests that the canonical ‘Apollo’ model of the Moon, in which all rocks are explained as a mixture of Feldspathic Highland Terrane (FHT), Procellarum KREEP Terrane (PKT), where KREEP stands for potassium (K), rare earth elements (REE), and phosphorous (P) (Warren and Wasson, 1979), and Mare (Jolliff et al, 2000), may not be applicable to the entire lunar surface (Korotev et al, 2009)
NWA 10989 is a fragmental polymict breccia composed of minerals and mineral fragments, various lithic and impact-melt clasts, embedded in a dark-brown glassy matrix (Fig. 1)

Summary

Introduction

Northwest Africa (NWA) 10989 is a recently found lunar meteorite we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates. This study presents a comprehensive mineralogical and geochemical data on mineral, lithic and impact-melt clasts in this meteorite including its bulk-rock major- and traceelement composition, abundance and isotopic composition of chlorine and hydrogen, and U-Th-Pb dates in apatites and a merrillite. This new dataset is utilized to perform comparisons with other lunar samples in order to gain insights into lunar petrogenetic processes and evaluate potential mantle source region(s) for lithologies contained in NWA 10989. The X-ray maps were used to calculate modal mineral abundances in different lithologies/clasts using the ImageJ software

Results

Discussion

Conclusion