⁴⁰Ar/³⁹Ar and cosmic ray exposure ages of plagioclase-rich lithic fragments from Apollo 17 regolith, 78461 7. Planetary science

Argon isotopic data is used to assess the potential of low-mass samples collected by sample return missions on planetary objects (e.g., Moon, Mars, asteroids), to reveal planetary surface processes. We report the first ⁴⁰Ar/³⁹Ar ages and ³⁸Ar cosmic ray exposure (CRE) ages, determined for eleven submillimeter-sized (ranging from 0.06 to 1.2 mg) plagioclase-rich lithic fragments from Apollo 17 regolith sample 78461 collected at the base of the Sculptured Hills. Total fusion analysis was used to outgas argon from the lithic fragments. Three different approaches were used to determine ⁴⁰Ar/³⁹Ar ages and illustrate the sensitivity of age determination to the choice of trapped (⁴⁰Ar/³⁶Ar)_t. ⁴⁰Ar/³⁹Ar ages range from ∼4.0 to 4.4 Ga with one exception (Plag#10). Surface CRE ages, based on ³⁸Ar, range from ∼1 to 24 Ma. The relatively young CRE ages suggest recent re-working of the upper few centimeters of the regolith. The CRE ages may result from the effect of downslope movement of materials to the base of the Sculptured Hills from higher elevations. The apparent ⁴⁰Ar/³⁹Ar age for Plag#10 is >5 Ga and yielded the oldest CRE age (i.e., ∼24 Ma). We interpret this data to indicate the presence of parentless ⁴⁰Ar in Plag#10, originating in the lunar atmosphere and implanted in lunar regolith by solar wind. Based on a chemical mixing model, plagioclase compositions, and ⁴⁰Ar/³⁹Ar ages, we conclude that lithic fragments originated from Mg-suite of highland rocks, and none were derived from the mare region.