Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.206b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 206
Computer Science
5
Scientific paper
On the basis of trace element studies the lunar regolith is known to contain on average about 1 to 2 wt% of carbonaceous chondrite debris (Ganapathy et al., 1970; Wasson and Baedecker, 1970). Such a component might be expected to be highly modified as a result of impacts at undiminished cosmic velocity. Nevertheless it could be supposed that resilient species such as diamond and silicon carbide, presolar grains recognised in all primitive meteorites, could survive to be distributed within the regolith. Interestingly both the presolar species mentioned are the carriers of characteristic isotopically light nitrogen. A well- known, but so far inadequately explained phenomenon concerning lunar ancient regolith materials is the high abundance of isotopically light nitrogen seen most prominently on soil breccias (Thiemens and Clayton, 1980). Since most of the nitrogen in the lunar soil is believed to have been derived from solar wind implantation, in the absence of any more plausible explanation, a secular change of 30-40% in ^14N/^15N of the sun's corona has been proposed (Kerridge, 1975; Becker and Clayton, 1975). It long ago occurred to us that we could investigate the occurrence of primitive meteorite debris at the lunar surface and possibly shed some light on the problem of light lunar nitrogen by stepped combusting the acid residue of an ancient lunar breccia. In order to avoid the use of excessive amounts of material, the experiment has been postponed until the advent of a new nitrogen isotope mass spectrometer and gas extraction system capable of measuring picomole quantities of the gas to a precision of +-0.1o/oo. Using such an instrument we have measured ca. 0.5-mg samples of 79035 and a residue after destruction of 99.9% of the parent in HF/HCl. Stepped combustion of whole rock 79035 liberated a total of 46 ng nitrogen (82 ppm, SIGMA delta^15N = -l60o/oo isotopic minimum -217o/oo at 850 degrees C) in the now well-known heavy-light-heavy pattern characteristic of ancient breccias. In contrast, the acid residue evolved 90% of its nitrogen 52 ng (103 ppm, SIGMA delta^15N -54o/oo isotopic minimum -74o/oo at 500 degrees C) as a single peak. Additionally delta^15N values of -72 and -45o/oo have been measured at temperatures above 800 degrees C corresponding with small peaks in the release profile, but the amounts of gas were within a factor of two of the blank. The nitrogen release and isotope profile of the major component correspond exactly to that encountered when interstellar diamonds from primitive meteorites are analysed before removal of contaminating organic material. Therefore, we tentatively interpret our data as the first recognition of interstellar grains in the lunar regolith. The appropriate investigation to confirm this conclusion is in progress. Using limiting values for the N abundance in presolar diamonds, we calculate that 79035 contains about 2 ppm diamond, about a factor of 5 less than the theoretical limit, assuming 2wt% CM2 equivalent carbonaceous chondrite material in the sample. The low delta^15N values measured at temperatures greater than 850 degrees would be consistent with about a factor of 50-100 less SiC carbide than diamond in 79035, a figure that corresponds well with the known relative abundance of diamond and SiC in CM2 meteorites. Clearly the abundances of diamond and SiC proposed here are totally incapable of accounting for any significant amount of the light lunar nitrogen. We cannot, however, discount that degassing of an abundant source of interstellar material early in the history of the lunar surface did not provide a source of the light lunar nitrogen by some indirect route. References. Becker, R.H. and Clayton, R.N. Proc. LPSC, Vol. 6, 2131-2149 (1975). Ganapathy, R., Keays, R.R., Laul, J.C. and Anders, Proc. Apollo 11 Lunar Sci. Conf., Vol. 2, 1117-1142 (1970). Kerridge, J.F., Science, 188, 162-164 (1975). Thiemens, M.H. and Clayton, R.N. Earth Plan Sci. Lett., 47, 34-42 (1980). Wasson, J.T. and Baedecker P.A. Proc. Apollo 11 Lunar. Sci. Conf. Vol. 2, 1741-1750.
Arden John W.
Brilliant D. R.
Franchi Ian A.
Pillinger Colin T.
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