Investigation of the carbon, hydrogen, oxygen, and silicon isotope and concentration relationships on the grain surfaces of a variety of lunar soils and in some Apollo 15 and 16 core samples
The concentrations and δ13C values of carbon have been measured in several lunar soils. These data are compared with the results of similar experiments on different aliquots of the same soils that were previously partially fluorinated to remove 1-7% of the grain-surf ace oxygen. Only minor decreases ( < 15%) are observed in the carbon concentrations as a result of the partial fluorination procedure, but the δ13C values are markedly lowered (a 10-40%0 decrease), and the solar-wind hydrogen concentrations are reduced by 90-95%, without basically changing the 8 D values. Inasmuch as similar results were obtained for a partially fluorinated soil 3-5 times larger than most of the analyzed samples, this indicates that significant contamination by low-13C carbon does not occur during our analytical procedures. Many other tests were made to insure that the analytical results were not artifacts of our experimental procedure, including various purification procedures, mass spectrometer scans of the gases to check for impurities, and tests to determine the rates of reaction of carbon with F2 and of H2 with partially fluorinated lunar soil. Analogous experiments on a finely ground Hawaiian basalt glass control sample showed essentially no changes in either δ13C or ppm carbon. Our data indicate that the carbon in lunar soils is isotopically very heterogeneous and that there must be a small component of carbon ( < 15% of the total) that is extremely enriched in 13C, up to + 100%0 to + 200%0 or greater. This 13C-rich carbon apparently resides in the amorphous films that coat the surf aces of the soil grains, where it coexists with the solar-wind hydrogen and with silicate material that is correspondingly extremely enriched in the heavy isotopes of silicon and oxygen. All of these heavy isotope enrichments may be due to the same process, namely fractional condensation or fractional vaporization of material vaporized by micrometeorite and meteorite bombardment, but we also cannot rule out addition of 13C-rich carbon from an unknown extralunar source. This 13C-rich grain-surface carbon is not completely incorporated into the glassy agglutinates during their formation; a large fraction must be lost, possibly as CO and CH4 formed by reaction between the carbon and the coexisting solar-wind H2 during impact fusion. Some of the Apollo 15 core samples are apparently contaminated with terrestrial organic carbon and atmospheric water. However, both these samples and the Apollo 16 core samples, when subjected to stepwise partial fluorination, show very systematic 30Si enrichment patterns in the released SiF 4 that show a progressive decrease in δ30Si with depth in the core. Of all the parameters measured by us, the δ30Si enrichment effects seem to give the most consistent and permanent record of stratification in the regolith, indicating the relative degree of exposure of the soil grains at the lunar surface.