Water-soluble chlorine with distinct isotopic compositions in lunar materials with application to volatiles in A17 core 73001/73002

Abstract:

Lunar materials are well recognized to be volatile depleted and exhibit a range of volatile stable isotope anomalies. Conversely, surface materials on the Moon such as soils and pyroclastic glasses have been shown to exhibit enrichments in volatiles such as halogens and chalcophiles yet retain the anomalously high δ³⁷Cl values characteristic of lunar materials. The Apollo 17 core 73001 specifically has remained sealed since collection and represents an invaluable study sample regarding volatile element processing on the Moon. Of the two lunar soils measured for chlorine isotopes to date, no relationship is observed in soil maturity and δ³⁷Cl values (Shearer et al. 2014), although the trend of isotopically light and Cl-rich water-soluble chloride (WSC) is consistent in both 64501 and 61220 relative to the structural chloride (SBC)(Sharp et al. 2010)(Fig 1A). These samples have WSC and SBC δ³⁷Cl values of 5.6 and 15.7‰, and 6.1 and 14.3‰, respectively. The ratio of Cl within the WSC component relative to the SBC component is consistent with other samples (both soils and rocks), with the majority of Cl present in the water-soluble fraction for most lunar materials (Fig 1B).

The δ³⁷Cl values of Apollo 16 soils have been explained by fumarolic deposition akin to 66095, which exhibits similarly high δ³⁷Cl values in both the WSC and SBC (Shearer et al. 2014). The Cl isotope values that are independent of soil maturity, however, are in stark contrast to the δ³⁴S values of lunar soils which reflect the open system behavior of S during micrometeorite bombardment and ion sputtering (Shearer et al. 2014 and references therein). The occurrence of water-soluble chlorine (WSC) with low δ³⁷Cl values on the other hand suggest that if chlorine volatilization and subsequent isotope fraction occurred, the system apparently remained closed without efficient Cl-loss. The Apollo 73002 drive tube with variations in soil maturity (Is/FeO = 18-45) intermediate between those of 61220 and 64501 represents the first chance of testing whether an active cycle of Cl volatilization and isotope fractionation occurred within lunar soils and if the fumarolic deposition of chlorides was widespread across the surface of the Moon or localized/more extensive at the A16 site.