Physics – Nuclear Physics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.422r&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 422
Physics
Nuclear Physics
Acid Etching, Lunar Soil 75081, Nitrogen, Pyroxenes, Solar Wind
Scientific paper
We report initial results from a series of experiments designed to measure recently implanted solar wind (SW) ions in lunar soil mineral grains [1]. An acid-etching technique similar to the CSSE method developed at ETH Zurich was used to make abundance and isotope measurements of the SW noble gas and nitrogen compositions. Among the samples examined was a pyroxene separate from soil 75081. It was first washed with H2O to remove contamination from the sample finger walls and grain surfaces. H2O also acted as a weak acid, releasing gases from near-surface sites. Treatment with H2SO3 followed the water washes. Acid pH (~1.8 to ~1.0) and temperature (~23 degrees C to ~90 degrees C) and duration of acid attack (several minutes to several days) were varied from step to step. Finally, the sample was pyrolyzed in several steps to remove the remaining gases, culminating with a high-temperature pyrolysis at 1200 degrees C. Measurements of the light noble gases were mostly consistent with those from previous CSSE experiments performed on pyroxene [2,3]. It should be noted, however, that the Zurich SEP component was not easily distinguishable in the steps where it was expected to be observed. We suspect our experimental protocol masked the SEP reservoir, preventing us from seeing its distinctive signature. The most interesting results from this sample are its Kr and Xe isotopic and elemental compositions. Pyroxene apparently retains heavy noble gases as well as ilmenite (and plagioclase [4]). The heavy noble gas element ratios from this sample along with those previously reported [5,6] are, however, considerably heavier than the theoretically determined "solar system" values [7,8]. Explanations for the difference include the possibility that the derivations are incorrect, that there is another component of lunar origin mixing with the solar component, or that some type of loss mechanism is altering the noble gas reservoirs of the grains. The Kr and Xe isotopic compositions for the two acid-etch steps most likely to have released SW gases were identical to the "solar" values reported by the Zurich group [5]. The krypton from both steps appeared to be mixtures of "solar" krypton, an isotopically heavier component (perhaps the Zurich SEP component [5]), and a spallation component. There was, however, no evidence for such a mixture in the xenon. The compositions of the two acid-etch steps were clearly combinations of a solar Xe component and a Xe spallation component. They were also identical to that of U-Xe [9] for isotopes up to 132Xe, with the exception of an ~300 per mil enhancement of the 126Xe/132Xe ratio. This anomaly does not appear to be an artifact of spallation correction. These measurements constitute the first experimental verification of the U-Xe composition for isotopes lighter than 134Xe. Persistent contamination problems and the possibility of nitrogen being held back in the acid residue during the etching process make interpretation of the nitrogen data uncertain. However, results from the steps not obviously affected by contamination show an enhancement of N over Ar by 2x to 12x the "solar" value (from [8]). References: [1] Rider P. E. and Pepin R. O. (1993) GCA, submitted. [2] Wieler R. et al. (1986) GCA, 50, 1997-2017. [3] Benkert J.-P. (1989) Ph.D. thesis, ETH Zurich (No. 8812). [4] Wieler R. (1993) personal communication. [5] Wieler R. et al. (1992) LPS XXIII, 1525-1526. [6] Wieler R. et al. (1993) LPS XXIV, 1519-1520. [7] Anders E. and Grevesse N. (1989) GCA, 53, 197-214. [8] Cameron A. G. W. (1982) In Essays in Nuclear Physics (Barnes et al., eds.), 23-43. [9] Pepin R. O.(1991) Icarus, 92, 2-79.
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