Abstract
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.
Highlights
The regolith of planetary objects (e.g., Moon, Mars, and asteroids) can be defined as the boundary layer between the solid crust and outer space
Solar wind-implanted 36Ar On the lunar surface, solar wind is the most dominant source of 36Ar
We present the first 40Ar/39Ar ages on plagioclase-rich lithic fragments separated from trench regolith collected at station 8 during the Apollo 17 mission
Summary
The regolith of planetary objects (e.g., Moon, Mars, and asteroids) can be defined as the boundary layer between the solid crust and outer space. The regolith evolution on planetary objects is a function of the continuous flux of impactors of various sizes, volcanism, as well as the constant bombardment by solar and galactic energetic particles. Regolith generally consists of fragmental and unconsolidated rock material. The hyper-velocity of the impacts and the continuous bombardment of energetic particles and micrometeorites reduce the initially rocky planetary surface to increasingly finer grain-sized regolith. On the lunar surface, the mean grain size of lunar regolith ranges from 40 to 800 μm and averages between 60 and 80 μm (Lucey et al 2006). A record of impact events and interactions with cosmic rays preserved in
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