Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.355g&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 355
Computer Science
3
Acapulco, Acapulcoites, Isotopes, Carbon, Nitrogen, Lodran, Lodranites
Scientific paper
Recent studies on the mineralogy, petrology, and oxygen isotopic composition of lodranites and acapulcoites indicate that these meteorites are probably derived from a common parent body, but experienced different degrees of partial melting [1,2]. Ar-Ar chronometry implies that lodranites were heated ca. 100 degrees C higher than acapulcoites, and cooled more slowly [3], however measurement of nitrogen and xenon in Acapulco [4,5] shows that volatiles are not equilibrated between different phases within the meteorite, hence its thermal history has been complex. The aim of this study is to determine the carbon and nitrogen chemistry of lodranites, for comparison with Acapulco, to indicate the effect that differing thermal histories might have had on the volatile inventories of these meteorites. The carbon chemistry of Acapulco has been described previously [6]. The meteorite contains ca. 400 ppm indigenous carbon, distributed between two major phases: graphite and carbides. Graphite has been identified petrographically in Acapulco [7], where it is intimately associated with metal. In contrast, both Lodran and MAC 88177 contain much lower quantities of indigenous carbon: approximately 100 ppm and 38 ppm respectively, released in decreasing amounts up to 1200 degrees C. In Lodran, delta^13C rises almost monotonically, from -25 per mil at 600 degrees C to -12 per mil at 1200 degrees C; total delta^13C is ca. -23 per mil. Neither meteorite shows evidence for the occurrence of graphite. Nitrogen released by pyrolysis of Acapulco totals ca. 2.8 ppm [4,5], and is resolvable into two components, with delta^15N ca. +10 per mil and -120 per mil [8]. The first component is, as yet, unidentified, but the second is believed to be associated with the metal fraction [8]. The procedure used herein, of several combustion steps below 500 degrees C to remove contaminants, followed by high resolution combustion up to 1200 degrees C, would also resolve discrete nitrogen-bearing components, if present. Analysis of whole-rock Lodran yielded 17.0 ppm nitrogen, with delta^15N ca. +4 per mil. A prominent release of nitrogen occurred between 650 degrees C and 900 degrees C, 50% of the total, with delta^15N varying between ca. +3 per mil and +9 per mil. Lodran is very different from Acapulco in both its carbon and nitrogen chemistry. There is little evidence for the presence of graphite in the former meteorite and it does not appear to contain the component of isotopically light nitrogen that is so abundant in Acapulco. However, Lodran does manifest a nitrogen- bearing component with intermediate isotopic composition, the location of which, in the metal or silicate portion of the meteorite is, as yet, unknown. Since Lodran has apparently experienced an elevated temperature regime compared with Acapulco, it might be possible that nitrogen has been remobilized and mixed in the former meteorite, leading to erasure of the characteristic isotopically light signature. Lodran has a higher whole-rock nitrogen abundance than Acapulco, thus it is unlikely that the presence of heavier nitrogen in Lodran is simply a result of fractionation of a reservoir during open system heating. References: [1] McCoy T. J. et al. (1993) LPS XXIV, 945-946. [2] Clayton R. N. et al. (1992) LPS XXIII, 231-232. [3] Bogard D. D. et al. (1993) LPS XXIV, 141-142. [4] Sturgeon G. and Marti K. (1990) LPS XXI, 1220-1221. [5] Becker R. H. (1991) LPS XXII, 69- 70. [6] Grady M. M. and Pillinger C. T. (1986) GCA, 50, 255-263, [7] Palme H. et al. (1981) GCA, 45, 727-752. [8] Kim Y. et al. (1992) LPS XXIII, 691-692.
Franchi Ian A.
Grady Michael
Pillinger Colin T.
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