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 specimens at levels of ~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 ~10 mg/g C. Buchwald  also reports 10 mg/g in Canyon Diablo. The ratio of C to Fe in Canyon Diablo (70 mg/g Ni) is thus about 0.01. Odessa (72 mg/g Ni) has similar carbon content in the forms of cohenite and graphite. 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, perhaps 2X, 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. He also reports that a large interior cavity seems to have contained graphite. 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 (and low-Ga and -Ge) 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. We are gathering more data to test these generalizations. 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. In our  mixing model we interpret the IIICD irons to be individual primary impact melts, and the IAB irons to have formed by mixing of these primary melts. The observations of low C content of Dayton but higher contents in high-Ni Freda, and San Cristobal are consistent with the view that low-temperature primary melts formed from precursors with low C/Fe ratios. 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 ~40 and ~20 mg/g C, respectively, corresponding to C/Fe ratios of ~0.2 and ~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. The C content of Renazzo is exceptionally high, 14 mg/g [5,6], giving it a C/Fe ratio of 0.06. Since Renazzo's delta-17O of -0.7 permil is quite close to that of IAB irons (-0.45 permil) and Renazzo is metal rich, it may be similar to the precursor materials melted to for the IAB irons. Studies of the form of the C in Renazzo offer evidence regarding its form in IAB precursors. We speculate that refractory H in IAB will have high D enrichments similar to those observed in Renazzo . References:  Buchwald (1975) Handbook of Iron Meteorites, Univ. of California.  Moore C. B. et al. (1969) in Meteorite Research (P. M. Millman, ed.), 738.  Choi B.-G. et al. (1994) GCA, submitted.  Moore C. B. (1971) in Handbook Elem. Abund. Met., 81.  Otting W. and Zahringer J. (1967) GCA, 31, 1949-1960.  Robert F. and Epstein S. (1982) GCA, 46, 81.  Clayton R. N. (1993) Ann. Rev. Earth Planet Sci., 21, 115.
- Pub Date:
- July 1994
- Chemical Composition;
- Iron Meteorites;
- Meteoritic Composition;
- Lunar and Planetary Exploration;
- CARBON; IAB; IIICD; METEORITES;