Abstract
Exposure of rocks and regolith to solar (SCR) and galactic cosmic rays (GCR) at the Moon's surface results in the production of ‘cosmogenic’ deuterium and noble gas nuclides at a rate that depends on a complex set of parameters, such as the energy spectrum and intensity of the cosmic ray flux, the chemical composition, size, and shape of the target as well as the shielding depth. As the effects of cosmic rays on the D production in lunar samples remain poorly understood, we determine here the D content and noble gas (He-Ne-Ar) characteristics of nominally anhydrous mineral (olivine and pyroxene) grains and rock fragments, respectively, from different documented depths (0 to ≥4.8 cm) within Apollo olivine basalt 12018. Deuterium concentrations, determined by secondary ion mass spectrometry, and cosmogenic 3He, 21Ne, and 38Ar abundances, measured by CO2 laser extraction static mass spectrometry, are constant over the depth range investigated. Neon isotope ratios (20Ne/22Ne ≈0.86 and 21Ne/22Ne ≈0.85) of the cosmogenic endmember are comparable to the theoretical signature of GCR-produced neon. These observations indicate that the presence of significant amounts of SCR nuclides in the studied sub-samples can be ruled out. Hence, D within the olivines and pyroxenes must have been predominantly produced in situ by GCR-induced spallation reactions during exposure at the lunar surface. Comparison of the amount of D with the 21Ne (184 ± 26 Ma) or 38Ar (193 ± 25 Ma) exposure ages yields a D production rate that is in good agreement with the value of (2.17±0.11)×10−12 mol(grock)−1Ma−1 from Füri et al. (2017). These results confirm that cosmic ray effects can substantially alter the hydrogen isotope (D/H) ratio of indigenous ‘water’ in returned extraterrestrial samples and meteorites with long exposure ages.
•Abundances of D, 3He, 21Ne, and 38Ar in Apollo 12018 are constant with depth.•Isotope ratios of cosmogenic Ne agree with the signature of Ne produced by GCRs.•SCRs had a negligible effect on the D content and He-Ne-Ar characteristics of 12018.•D in 12018 olivines and pyroxenes was produced by GCR-induced spallation reactions.•GCR (and SCR) effects must be taken into account when interpreting lunar D/H ratios.