Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.523h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 523
Other
1
Interstellar Dust, Isotopes, Meteorites, Murchison, Orgueil, Qingzhen, Semarkona, Nitrogen, Silicon Carbide
Scientific paper
Nitrogen in presolar SiC varies over a wide range of concentrations and is typically 14N-rich relative to solar N, consistent with 15N being consumed during CNO processing in stellar envelopes [e.g. 1]. Although C is also heavily processed in the envelopes [1], no clear isotopic correlation exists between C and N [e.g. 2], making N-compositions difficult to interpret. Although the same general N features are seen in SiC from many meteorites, clear differences between meteorites have also been observed. In particular, Murchison SiC appears to have systematically higher 15N/14N ratios than Orgueil SiC [2, 3]. Among 15N-poor SiC grains for both meteorites, 15N/14N and 28Si/14N exhibit a positive correlation (Fig. 1). This indicates that a 15N-rich component has been added in roughly constant abundance (relative to Si), affecting the low-N grains most. The slope of the line bounding the field of Murchison SiC data is ~10x greater than that for Orgueil SiC (Fig. 1), implying that Murchison grains carry on average ~10x more of the 15N-rich component. If both meteorites sampled the same presolar SiC reservoir, as indicated by isotope systematics of other elements [3], then the 15N-rich component could not have been acquired at the stellar source or in interstellar space. Spallation in the early solar system can also be excluded. Although the pre-compaction exposure age for Murchison (>= 145 Ma [4]) is considerably longer than the total exposure history of Orgueil (~25 Ma [5]), 15N production rates [6] are 500_2000x too low to explain the data. A more likely scenario is that terrestrial N (15N/14N~0.0037) has been acquired by the grains during sample processing. Measured 15N/14N ratios seldom exceed that of terrestrial N (Fig. 1). A minimum of ~2.5 x 10^-5 atoms of 14N(sub)terr per 28Si atom (0.01%_0.1% of the 14N in a grain) is required to produce the correlation observed for Orgueil; >=10x more is required for Murchison. Systematic differences between SiC grain-size fractions in Murchison SiC (Fig. 1) may reflect different processing histories [2]. Also, aggregates of tiny SiC grains, which have very large surface areas, exhibit higher 15N/14N ratios than single grains. Do measured 15N/14N ratios reveal anything about N in the stellar sources? The lowest measured 15N/14N ratios in SiC from Orgueil, Murchison, Semarkona, and Qingzhen lie between 0.0001 and 0.0002, and the lines bounding the data fields on Fig. 1 intersect the ordinate at ~0.0001. This suggets that the intrinsic 15N/14N ratios in the most 15N-poor grains is ~0.0001, very close to the predicted ratio for the stellar envelope [e.g., 1]. Grains with 15N/14N ratios higher than solar and grains with moderate 15N depletions, but very high N abundance, probably also reflect the stellar source. Acknowledgments: Supported by NASA NAG-3040. Div. Contrib. #5560 (904). References: [1] El Eid M. F. (1994) Astron. Astrophys., 285, 915-944. [2] Hoppe P. et al. (1994) Astrophys. J., 430, 870-890. [3] Huss G. R. et al. (1993) Meteoritics, 28, 368-369. [4] Hohenberg C. M. et al. (1990) GCA, 54, 2133-2140. [5] Nichols R. H. Jr. et al. (1992) LPS XXIII, 987-988. [6] Reedy R. C., personal communication.
Huss Gary R.
Nichols Robert Hill Jr.
Wasserburg Gerald J.
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