Understanding the Chlorine Isotopic Composition of Apatites in Lunar Basalts

Introduction: On Earth δCl = ~0 ± 5 ‰ [1]. For lunar rocks elevated δCl values of up to 35 ‰ have been reported in apatites [1,2]. The initial study interpreted the elevated δCl in lunar rocks as a result of metal chlorides degassing in low-H2O environments [1]. Additional data [3,4] have shown high-δCl values in samples with apatite containing appreciable amounts of H2O (>2000 ppm). Metasomatism is thought to have fractionated Cl isotopes toward heavy δCl values in sample 79215 [2]. This process, however, is not thought to be widespread on the Moon and cannot account for the fractionation seen in other samples for which data are available, particularly mare basalts. Here we report preliminary measurements of δCl in apatites from lunar impact “melt rock” sample 14073,9. Results are compared with literature δCl data on lunar apatites to evaluate the effects impact processes may have had on δCl values. We then explore a potential scenario of the elevated δCl signature of the Moon.