Oxygen isotope abundance measurements in fine size fractions of Luna 16 and 20 soil

Abstract

Determination of 8 18 O values in fine size fractions of Luna 16 and 20 soil appears to indicate that a simple monotonic increase in 18 O abundance with decreasing grain size does not exist. The weighted average 8 18 O values for Luna 16 and 20 soil are approximately + 6.33% and + 6.65% relative to the s.m.o.w. standard. These averages (within errors of about ± 0.2 %0) fall inside the range of 8 18 O values found by other workers for lunar soils. Three possible interpretations of the patterns of 18 O-enrichment with grain size are suggested. (1) The generally accepted explanation is that the heavy isotope enrichments reflect fractional evaporation and/or fractional condensation processes resulting from bombardment of the lunar soil by nuclear particles in the solar wind and/or micrometorites. Thus the fines at each locality could be a mixture of fractions with a specific range of grain sizes each with a different exposure history. The grain size fractions could each have been derived from impact events at specific distances from the Luna sample localities. (2) The low 8 18 O values ( ca , + 5.1%0, s.m.o.w.) obtained for two fractions are difficult to explain, but could indicate concentrations of minerals with low 18 O abundances, such as ilmenite or olivine, in these grain sizes. Similarly the relatively enriched values obtained for other fractions could indicate concentrations of phases enriched in 18 O and/or susceptible to 18 O-enrichment in these grain sizes. (3) At least some of the results obtained could be artefacts resulting from the exposure of the samples to the atmosphere of various laboratories.