O¹⁸/O¹⁶, Si³⁰/Si²⁸, C¹³/C¹² and D/H studies of Apollo 14 and 15 samples

Abstract

New δO¹⁸ and δSi³⁰ data on Apollo 14 and 15 samples further confirm the remarkable isotopic similarity of lunar rocks from different sites. As in the case of Apollo 11 and 12 samples, the lunar fines are enriched in O¹⁸ and Si³⁰ relative to the crystalline rocks from which they are derived. Certain Apollo 14 breccias (F-2) are also enriched in O¹⁸, and thus may in large part be derived from lunar soil. The Apollo 14 bottom trench samples are depleted in O¹⁸ relative to surface soils. The anorthosite 15415 has a δO¹⁸ very similar to that obtained for plagioclases in the lunar basalts. Utilizing successive partial fluorinations, large O¹⁸ and Si³⁰ enrichments (up to + 55 and + 30‰, respectively) were observed on the surfaces of grains of all Apollo 14 and 15 fines, but no such enrichment exists for 14321, a rock breccia (class F-4). These large O¹⁸- and Si³⁰-enrichments are directly related to the amount of solar wind hydrogen present in the sample. These isotopic effects must be a result of exposure of the lunar fines to bombardment by meteorites, micrometeorites, and/or nuclear particles, with preferential loss of Si²⁸ and O¹⁶ by fractional vaporization, or preferential gain of Si³⁰ and O¹⁸ by fractional condensation. These effects are accompanied by a reduction in the overall oxygen/silicon ratio, indicating that the O₂/Si ratio of material carried away during vaporization is > 2. Further confirmation has been obtained that the deuterium concentration in hydrogen gas extracted from the lunar fines is less than about 3 ppm. Even this small amount of deuterium could be due to spallation because an abnormally deuterium-rich fraction of gas was extracted during the melting of the lunar samples. The bulk of the hydrogen in the lunar fines is clearly of solar wind origin; the latter apparently has a δD value close to -1000 (0 ppm deuterium). The small amounts of H₂O (~10 μmole/g) extracted from one lunar soil and one rock breccia have δO¹⁸ values of -5.9 and -18.2, well within the δO¹⁸ range of terrestrial atmospheric H₂O. Taken together with other considerations, this strongly indicates that most of this water is a terrestrial contaminant taken to the moon by the Apollo spacecraft or added to the samples after their return to earth. The carbon concentrations and the δC¹³ values in lunar samples are both highly variable, about 10 to 260 ppm and about - 30 to +20, respectively. Nonetheless, there is a progressive enrichment in δC¹³ with increasing carbon content, and all the data can be explained by mixing solar wind carbon having a variable δC¹³ of +10 to +25 with lunar basalts containing about 25 ppm carbon and having a δC¹³ = -30 to -20. However, because of possible addition of meteoritic carbon, and because of possible C¹³ enrichment due to particle bombardment, we cannot at this time definitely determine whether the bulk of the lunar carbon is of solar wind or meteoritic origin.

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