Abstract
Chlorine, an extremely hydrophilic volatile element, provides important information regarding the origin of intrinsic volatiles in the Moon. Lunar apatite was found to have a wider spread of δ37Cl (from −1 to +40‰ versus standard mean ocean chloride) than most terrestrial and chondritic ones (0 ± 0.5‰). However, the provenance of the elevated lunar δ37Cl is still enigmatic. Here we report new isotopic data for H and Cl in apatite from three lunar meteorites and discuss possible mechanisms for Cl isotopic fractionation of the Moon. The apatite grain in Dhofar 458 has an average δ37Cl value of +76‰, indicative of an extremely heavy Cl reservoir in the Moon. Volatile loss associated with the Moon-forming Giant Impact and the formation of lunar magma ocean could account for the large Cl isotopic fractionation of the Moon. The observed H2O contents (220–5200 ppm), δD (−100 to +550‰) and δ37Cl values (+3.8 − +81.1‰) in lunar apatite could be understood if late accretion of hydrous components were added to the Moon after the fractionation of Cl isotopes. The heterogeneous distribution of lunar Cl isotopes is probably resulted from complex lunar formation and differentiation processes.
Highlights
Chlorine, an extremely hydrophilic volatile element, provides important information regarding the origin of intrinsic volatiles in the Moon
Volatile loss associated with the Moonforming Giant Impact and the formation of lunar magma ocean could account for the large Cl isotopic fractionation of the Moon
Cl/(Cl + F) ratios (Fig. S2), which is in agreement with the incompatibility of REEs and Cl over F during magmatic evolution
Summary
An extremely hydrophilic volatile element, provides important information regarding the origin of intrinsic volatiles in the Moon. Volatile loss associated with the Moonforming Giant Impact and the formation of lunar magma ocean could account for the large Cl isotopic fractionation of the Moon. The observed H2O contents (220–5200 ppm), δD (−100 to +550‰) and δ37Cl values (+3.8 − +81.1‰) in lunar apatite could be understood if late accretion of hydrous components were added to the Moon after the fractionation of Cl isotopes. A large variation of δ37Cl (from −1‰ to +40‰)[8,9,10,11,12,13,14] was observed in the lunar samples This appears to contradict the elevated volatile contents present in the lunar samples as the large Cl isotopic fractionation was ascribed to metal chlorides degassing from anhydrous magmas[8]. To better understand the origin and fractionation mechanism of lunar Cl, we conducted SIMS (secondary ion mass spectrometry) analyses on apatite [Ca5(PO4)3(F,Cl,OH)] from three lunar meteorites on the basis of detailed mineralogical and petrographic studies, and assess possible fractionation mechanisms and potential reservoirs of lunar Cl isotopes
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