Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.601w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 601
Other
Chondrites, Cm, Noble Gas, Noble Gases, Planetary, Solar, Rims, Solar Nebula
Scientific paper
Introduction. Identifiable components of certain carbonaceous chondrites have been interpreted as nebula products. MacPherson et al. [1] described some rims around refractory inclusions in Allende (C3V) as accretionary, and Metzler et al. [2] attributed a nebular origin to fine-grained dust mantles (rims) in CM meteorites. In nebula formation scenarios such as that evoked by Metzler et al., we would expect rim material to be largely devoid of solar-implanted noble gases while the brecciated matrix material would be relatively solar gas-rich. Of particular interest in this study, therefore, is examining the microdistribution of solar noble gases in carbonaceous chondrites to attempt to assess the validity of current models for rim formation. Procedure. A polished thick section of Murchison (CM) (~300 micrometers thick) was prepared without epoxy impregnation since epoxy is not compatible with the high-vacuum, noble gas mass spectrometer. Noble gases were extracted from targeted regions in situ by pulsed-laser excavation and measured by an ion-counting noble gas mass spectrometer [3]. Typical excavation volumes were about 150 micrometers x 150 micrometers x 100 micrometers. Previous Results. Neon isotopic data from these measurements were reported at LPSC XXVI [3]. All data extracted from the section's rim areas plot within the triangle defined by a three component mixture between 'planetary' (Ne-A), solar wind, and spallation. However, as pointed out by R. Wieler, the data could also be interpreted as a three component mixture between Ne-A, spallation and Q-Ne [4]. Current Results and Discussion. Figure 1 plots the noble gas elemental ratios for representative (other rim and matrix data, though in agreement, are not plotted in order to reduce clutter) rim and matrix analyses along with Q, referenced to 36Ar and normalized to solar wind (ilmenite 12001 [5]). As expected, the heavier noble gases are dominated by Q. In fact, the Xe and Kr isotopic data are indistinguishable from Q within error (not plotted). In contrast, the lighter gases are overabundant relative to their Q complement. While this enrichment might be interpreted as a contribution from an A-type planetary component (Ne,He-A), similarity between rims and matrix, as shown in Fig. 1, suggests the presence of solar gas since the matrix is certainly a solar gas carrier for this 'gas-rich' meteorite. An identical solar-enriched signature was measured in the inner as well as outer portion of a single ~300 micrometers thick rim, suggesting that all the material in this rim was similarly exposed. Thus, solar wind irradiation appears to be pervasive in this section of Murchison, present in both matrix and throughout all rims measured in this study. Therefore, modification of the simple one-stage model for nebula accretion of rimmed objects [2] may be required. References: [1] MacPherson G. J. et al. (1985) GCA, 49, 2267-2279. [2] Metzler K. et al., GCA, 56, 2873-2897. [3] Hohenberg C. M. (1980) Rev. Sci. Instrum., 51, 1075-1082. [4] Wieler R. et al. (1992) GCA, 56, 2907-2921. [5] Eberhardt P. et al. (1972) Proc. LSC Conf. 3rd, 1821-1856.
Guntalilib E.
Hohenberg Charles M.
Kehm Karl
Poelstra Karen
Woolum Dorothy S.
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