Role of C During the Formation of IAB-IIICD Iron Meteorites

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 [1] 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 [1] 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 [1] observed no cohenite, but noted that the C content of the metal [2] 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 [3] 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 [6]. References: [1] Buchwald (1975) Handbook of Iron Meteorites, Univ. of California. [2] Moore C. B. et al. (1969) in Meteorite Research (P. M. Millman, ed.), 738. [3] Choi B.-G. et al. (1994) GCA, submitted. [4] Moore C. B. (1971) in Handbook Elem. Abund. Met., 81. [5] Otting W. and Zahringer J. (1967) GCA, 31, 1949-1960. [6] Robert F. and Epstein S. (1982) GCA, 46, 81. [7] Clayton R. N. (1993) Ann. Rev. Earth Planet Sci., 21, 115.