Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.456c&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 456-457
Other
Carbon, Chemical Composition, Iron Meteorites, Meteoritic Composition, Minerals, Chondrites, Melting, Mineralogy
Scientific paper
The low-Ni meteorites in IAB-IIICD have high C contents. All large sections show abundant C in the form of cohenite and graphite, the latter commonly mixed with FeS. By point counting we determined that C in the form of cohenite is present in Canyon Diablo and Rifle speciments at levels of about 7 mg/g. We were not able to obtain a precise estimate of the fraction present as graphite, but it is only about 2x smaller than that present as cohenite, thus giving a total of about 10 mg/g C. In his description of the Ni-rich (250 mg/g) IAB iron San Cristobal Buchwald notes that it contains large amounts of cohenite and graphite. Based on his description it appears that the C content is somewhat less than that in Canyon Diablo; in the Ni-rich IIICD iron Freda the amount of C observed by Buchwald seems to be similar to that in San Cristobal. In another Ni-rich IIICD iron Dayton Buchwald observed no cohenite, but noted that the C content of the metal is unusually high, 510 mg/g. We infer that the bulk C content of Dayton was lower than that of San Cristobal, perhaps around 1-2 mg/g. Dayton falls along the low-Ir edge of IIICD whereas Dayton plots well above the low-Ir envelope. These observations suggest that irons forming the low-Ir envelopes of IAB-IIICD have lower contents of C than are found in the IAB portions of this complex group, and that there is also a general tendency for C content to decrease with increasing Ni content. According to the impact model of IAB-IIICD formation, high-Ni irons formed as a result of low-degree impact melting of a chondritic precursor. Irons with high Ni contents formed when impact melting was confined to FeS and the adjacent Ni-rich metal. The C contents of IAB-IIICD irons are much higher than those in other iron meteorite groups. This indicates that the chondritic precursor was also C rich. The two chondrite groups having the highest C contents are CI and CM with about 40 and about 20 mg/g C, respectively, corresponding to C/Fe ratios of about 0.2 and about 0.1. Because these materials are highly oxidized, with vanishingly small contents of metallic Fe-Ni, they cannot be representative of the IAB precursor materials.
Choi Byung Goo
Wasson John T.
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