V017-07
Hydrogen isotopic ratios in core 73002 and sources of hydrogen in the lunar regolith.
Wednesday, 9 December 2020: 07:24
Virtual
Michael Cato, University of New Mexico Main Campus, Earth and Planetary Sciences, Albuquerque, NM, United States, Zachary D Sharp, Dept. of Earth and Space Sciences, University of New Mexico, Albuquerque, NM, United States, Charles K Shearer, Institute of Meteoritics, University of New Mexico, Albuquerque, NM, United States and The ANGSA Science Team
Abstract:
We utilize a step-heated continuous-flow mass spectrometry method to measure the hydrogen isotope concentrations of samples from the sealed Apollo 17 double drive tube 73001/73002 through the ANGSA program. 73001/73002 was sealed after collection on the moon, stored in a dry nitrogen atmosphere, and all processing of material has been done in a dry-nitrogen glovebox. No sample has ever been exposed to terrestrial atmosphere. The bulk hydrogen isotope value of lunar soil has been interpreted as light (-1000‰) hydrogen mixing with at least one of many possible heavier sources, including comets (Greenwood et al., 2011), chondrites (Tartèse & Anand, 2013; Saal et al., 2913) or degassing of H2 (Sharp et al., 2013). The bulk value can be further subdivided into individual soil components, including glass, individual mineral crystals, different breccia types, and agglutinates. Each component has been shown to have distinct concentrations of hydrogen ranging from 4ppm plagioclase crystals to over 100ppm agglutinates (DeMaris et al, 1974), and each may derive their heavy hydrogen component from different sources.
Initial measurements have proven the efficacy of our method and shown a detection limit of roughly 100 nanograms of H2 with an error of ±20‰ decreasing to less than ±5‰ at higher concentrations. To date, one 102.4mg aliquot of unsieved rind from the top 1cm of 73002 has been measured, with a total H2 content of 18.4ppm and a δD of -142‰ spallation-corrected to -521‰. Of particular interest was our 0-200°C step, which released no hydrogen. The absence of hydrogen in the lowest step indicates that our processing procedures were successful, and no terrestrial water adsorption occurred in this sample. Our measured H2 content is on the extreme low end for measured lunar soils (8.85 to 35.7ppm; Epstein et al., 1971-1974), but not out of range. According to hydrogen content measurements reported by DeMaris et al in 1974, this would indicate the measured 73002 sample contained a larger fraction of glass, plagioclase crystals, and light breccia, than many other measured samples. An uncorrected bulk H2 δD of -142‰ is on the extreme heavy end of measured lunar soils (-144 to -690‰; Epstein et al., 1971-1974). A heavier δD could also be explained by a larger fraction of breccia fragments and glass than most previously measured soils.
