Copper and zinc isotopic variation in Apollo 17 double drive tube 73001/2 reveals space weathering history of lunar regolith

Abstract

As a part of the Apollo Next Generation Sample Analysis Program, we report the Cu and Zn isotopes in the Apollo 17 regolith core, double-drive tube 73001/2. The intervals in the upper core, which sampled the regolith closest to the lunar surface, are enriched in heavy Cu and Zn isotopes compared to the deeper intervals. The top 2 cm have a δ⁶⁵Cu value of 2.85 ± 0.01 ‰ and a δ⁶⁶Zn value of 5.54 ± 0.02 ‰. The intervals become lighter in isotopic composition to a depth of 8 cm. Below this depth, the average δ⁶⁵Cu is 1.02 ± 0.08 ‰, while the average δ⁶⁶Zn is 2.27 ± 0.24 ‰. We find strong correlations between the isotopic fractionations of Cu and Zn and the maturity index I_S/FeO. These correlations in the core result from a binary mixing between highly space-weathered soil at the lunar surface and deeper, shielded soil, with isotopic fractionation occurring at the surface due to space weathering and soil mixing occurring due to impact gardening. Using the K, Fe, Cu, and Zn isotopes measured in 73001/2, we find a strong correlation between the degree of isotope fractionation and volatility. We model the isotopic fractionation of K, Fe, Cu, and Zn by space weathering in lunar soils using mass balance equations between the lunar atmosphere and lunar soil and find agreement with the fractionation observed in 73001/2. Using the fractionation observed in 73001/2, we present a new exposure age model using Cu isotope fractionation in lunar soils.