Cadmium isotope fractionation and neutron capture effects in lunar samples

Abstract

Early pioneering studies of Apollo lunar soils revealed a geochemical dichotomy reflecting a dominance of mare and highland lithologies, with variable additions of Procellarum KREEP Terrane material. Here, we use the moderately volatile element cadmium to identify the sources and processes responsible for mass-dependent Cd stable isotope variations in the lunar regolith. In addition, capture of thermal neutrons by ¹¹³Cd, resulting from galactic cosmic rays (GCR) impacting the lunar surface, provides a means of reconstructing the exposure history of the regolith.

We report TIMS double spike Cd stable isotope data on samples from the Apollo 12, 16 and 17 missions, consisting of twelve soils of varying maturity, two ferroan anorthosites, and orange glass 74220. Cadmium abundances are generally lower in mare (12 to 79 ng/g) than highland soils (∼70 to 95 ng/g). Cadmium stable isotope compositions, expressed as ε^112/110Cd, display a larger range in mare (∼0 to + 106) than in highland (+60 to + 97) soils. The two anorthosites exhibit contrasting ε^112/110Cd values (−107 vs. + 47) and Cd concentrations similar to those of highland soils. Orange glass 74220 is Cd-rich (290 ng/g) and has a light Cd isotopic composition (ε^112/110Cd = -27) due to condensation of Cd vaporized during lava fountaining.

A broad trend of decreasing Cd abundance and increasing heavy isotope enrichment with increasing maturity is observed for mare soils but is not apparent for the highland soils. These characteristics might arise from space weathering, including micrometeorite bombardment, but simple mass balance indicates that meteoritic addition has a negligible effect on the lunar regolith Cd. Likewise, neutron capture on ¹¹³Cd tends to increase with maturity in mare soils while being greater and relatively uniform in highland soils, reflecting a longer exposure history and more extensive reworking of the highland regolith. Neutron capture effects were not resolved for immature mare soils, orange glass and one anorthosite, indicating these samples experienced only short near-surface exposure to GCR.

The relationships between Cd abundances and isotope effects reveal three distinct correlations for the highland soils and the mature and immature mare soils, respectively. These are best explained by simple binary mixing between isotopically distinct components. The “heavy” Cd components of mare and highland soils have variable but overall low Cd contents while the cadmium-rich component is always isotopically “light”, and common, at least, to all mare soils. The strong correlation between Cd stable isotopic composition and neutron capture effects in mare soils constrains the ε^112/110Cd of the neutron capture-free component to be −4.9 ± 2.3, that is marginally lighter than that of the Bulk Silicate Earth (0.01 ± 0.94). This component is predominantly found in immature, KREEP-rich soils that were not exposed to GCR. This supports an origin as exhumed material, possibly from the relatively recent Copernicus Crater, and/or as vapor re-distributed over the lunar surface. The ubiquitous presence on the Moon of a cadmium-rich reservoir and its apparent isotopic similarity with the BSE requires further scrutiny for a critical evaluation of its significance and implications for the bulk Moon composition.