The isotopic composition and concentration of water, hydrogen, and carbon in some Apollo 15 and 16 soils and in the Apollo 17 orange soil

Abstract

Samples 15021, 64421, 65513, 61221, and 74220 were analyzed for their water, carbon, and hydrogen contents and their isotopic composition. The H₂O contents of the Apollo 16 samples are on the average significantly higher than for the samples from other sites, whereas the H₂O content of 74220 (orange soil) is the lowest yet observed on a lunar soil sample. By considering lunar soils from all the Apollo sites which exhibit a wide range in H₂O/H₂ ratios, it is possible to show that the δD of the "lunar water" approaches an asymptotic value of about - 125‰ at high H₂O/H₂ ratios. A δD of about - 125 is thus considered to be typical of the "lunar water" in all lunar soils analyzed by us to date. Such a δD value is characteristic of terrestrial meteoric waters, and suggests that these small amounts of H₂O represent terrestrial contamination. The H₂ concentrations and isotopic compositions in 15021, 64421, and 65513 (14 to 31 μmoles H₂/g and δD = - 710 to - 805) are very similar to data obtained on previously analyzed lunar soils. Sample 61221, however, has a low H₂ content (3.9 μmoles/g), compatible with the fact that it is a trench sample that has had a short exposure time at the lunar surface. Sample 7 4220 is unlike any other lunar soil yet analyzed in that it contains only 0.1 μmoles H₂/g. The hydrogen in all these samples appears to be dominantly of solar wind origin. The H₂/rare gas ratios in the Apollo 16 samples are roughly a factor of three higher than that for Apollo 11 and 12 soils. Except for samples 61221 and 74220 the new data on the isotopic composition and concentration of carbon lie within the range of the previously analyzed lunar soils and breccias. Apollo 16 sample 61221, 8 is unusual in that it has a remarkably low δC¹³ value (- 13.9) for a sample containing as much as 97 ppm carbon. This carbon may be of meteoritic origin. Contrary to data reported by Gibson and Moore (1973) we did not find any HCN gas in 61221. Sample 7 4220 is also very unusual in that our carbon analysis is the lowest yet reported in any lunar soil or basalt (~3.5 ppm). The δC¹³ value in 74220 (- 7.2) is unusually high for a lunar sample containing so little carbon.