Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.526k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 526
Computer Science
3
Deposits, Vapor, Irradiation, Lunar Samples, 10084, 78221, Rims, Amorphous, Wind, Solar
Scientific paper
Debate has been renewed regarding the origin of amorphous rims on lunar soil grains[1-3]. In our previous work, we documented that large compositional differences existed between rims and their host grains, which suggested that a major component of the amorphous rims was deposited material [4,5]. Recent work on amorphous rims on interplanetary dust particles (IDPs) suggest that a characterisitic chemical signature of irradiation is cation deficiency which manifests itself as a stoichiometric excess of oxygen [6]. In light of these results, we have re-examined the compositions of amorphous rims on lunar soil grains with emphasis on the quantitative analysis of both rims and substrates for oxygen. In this way, we evaluate the relative contributions of vapor deposition and solar-wind irradiation to the formation of amorphous rims on lunar soil grains. We analyzed ultramicrotome thin sections of aliquots of the <10 micrometer size fraction of soils 10084 and 78221 using a JEOL 2010 TEM equipped with a thin-window energy-dispersive x-ray spectrometer. Using a probe size of ~20-50 nm, we analyzed the bulk rim on grains along with the crystalline substrate immediately below the rim. We only used data where absorption effects were not evident in the substrate analysis. Preliminary data have been obtained from 13 rims and their hosts. Rims were analyzed on plagioclase, pyroxene, and cristobalite. Apparent rims thicknesses ranged from 50- to 200-nm, and most rims contained 5- to 20-nm-sized inclusions of Fe metal. As in our earlier work, all analyzed rims showed compositional differences from their hosts. Of the 13 analyzed rims, 5 were reduced; for these rims, assuming that the Fe and S are present in their reduced forms, there is still insufficient oxygen to balance the cations, thus, some of the Si must also be in a reduced form. Three of the 13 rims were stoichiometric; again, assuming that Fe and S are present in reduced forms, the analyzed cations balance the analytical oxygen. The remaining 5 rims were cation deficient and contained excess oxygen in the analyses; for these rims, more oxygen was present than could be accounted for with the analyzed cations (it is suggested that the "excess oxygen" in IDPs is balanced by H, probably in the form of hydroxyl [3]). Based upon our earlier arguments [3-5], we conclude that the reduced and stoichiometric rims consist largely of deposited material; their chemical systematics are consistent with the deposition of impact-generated vapors, although we cannot rule out a sputter-deposited component. The composition of the cation-deficient rims has been largely determined by interaction of the grains with ionizing radiation (i.e. the solar wind). However, all of the cation deficient rims contain elements that are not indigenous to the host grains, thus, even these rims contain a significant deposited component. References: [1] Bernatowicz T. et al. (1994) Science, 264, 1779. [2] Hapke B. et al. (1994) Science, 264, 1779. [3] Keller L. P. and McKay D. S. (1994) Science, 264, 1780. [4] Keller L. P. and McKay D. S. (1993) Science, 261, 1305. [5] Keller L. P. and McKay D. S. (1994) Meteoritics, 29, 480. [6] Bradley J. P. (1994) Science, 265, 925.
Keller Lindsay P.
McKay David S.
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