O¹⁸/O¹⁶ and Si³⁰/Si²⁸ studies of some Apollo 15, 16, and 17 samples

Abstract

The whole-rock δO¹⁸ values from 8 sample sites on the near side of the moon vary only from about +5.4 to +6.8, a remarkably small variation compared to that observed in meteorites and the earth. However, the whole-rock surf ace soil samples from all of these lunar localities are slightly enriched in O¹⁸ and Si³⁰. This is due to very large O¹⁸ and Si³⁰ enrichments that are observed on the outermost 2 to 4 percent of material (the "amorphous" coatings) on the surfaces of the grains and glassy agglutinates that make up the lunar fines. These isotopic effects, which are brought about by fractional vaporization or fractional vapor condensation as a result of particle bombardment of the soils exposed at the lunar surface, are observed experimentally by partial-fluorination "stripping" experiments. The δO¹⁸ "stripping" patterns for the Apollo 16 soil samples are generally somewhat different from those previously observed at other lunar sites. This is attributed to larger amounts of adsorbed water on the outermost grain-surfaces in the Apollo 16 fines. Sevetal experiments were conducted which indicate that the δO¹⁸ value of this adsorbed water is about -20 to -10 per mil, or very similar to ordinary terrestrial meteoric waters. These data correlate well with the common occurrence of goethite in trace amounts in the Apollo 16 rocks. The Apollo 17 orange soil sample is unique among all lunar soils studied so far, in that it shows no surface O¹⁸ or Si³⁰ enrichment whatsoever. Thus the orange soil cannot have been exposed at the lunar surface for any significant length of time. Other experiments were made which show that the O¹⁸ and Si³⁰ enrichment patterns on the grain surfaces are still preserved even after heating at 630°C for 5 hours, at which temperature other indications of a lunar surface exposure history have been largely obliterated (i.e., loss of solar wind H₂, He, etc.). Thus, such O¹⁸ and Si³⁰ enrichment patterns may represent one of the few pieces of evidence that may allow us to "see through" a lunar heating or metamorphic event and thus decipher whether or not a particular breccia sample ever resided on the lunar surface as part of the regolith. There are also indications that the exact shapes of these isotopic patterns may be sensitive to such heating events and to other aspects of the petrologic history of lunar soils and soil-breccias.