Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.380k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 380
Other
Acapulco, Cape York, Chromite, Dhajala, Forest Vale, Metal, Nitrogen, Ste. Marguerite
Scientific paper
Nitrogen shows a large isotopic variation in bulk meteorites. Extreme isotopic variations in some carbonaceous chondrites probably reflect incomplete mixing in the solar nebula, since carbon-bearing presolar grains were found in meteorites with unusual nitrogen isotopic signatures [1]. As nitrogen isotopic characteristics of solar system materials are studied, the strengths and limitations of a single isotope ratio have to be explored in discussions of origin. Several recent investigations were directed toward understanding the distribution and behavior of nitrogen in different classes of meteorites. We report nitrogen isotopic structures in separated phases of chondrites, a primitive achondrite, and an iron meteorite and implications regarding genetic relationships. The texture of the Acapulco meteorite reveals extensive solid-state recrystallization at higher temperature. Metallic Fe-Ni has a fine structure akin to the Widmanstatten structure. Nitrogen measurements on individual mineral separates of Acapulco reveal that the metal phase is the major carrier of N (13 +/- 3ppm) with a very light N component (delta ^15N <= -150 per mil), while all silicates carry the heavy component (delta ^15N + 10 per mil) [2], demonstrating that N was not equilibrated between metal and silicates. Furthermore, the opaque mineral chromite also shows isotopic disequilibrium with isotopically light N (delta ^15N <= -70 per mil). Is this a distinct feature of the Acapulco parent body, or can we recognize possible relationships with other parent bodies? Possible genetic links with H chondrites were explored in a study of metal separates of three ordinary H chondrites: Dhajala (H3.8), Foreast Vale (H4), and Ste. Marguerite (H4). However, their N concentrations and isotopic signatures are very different from those of Acapulco metal. In the case of Dhajala, which is assumed to be a primitive object but has no "presolar" signatures [3], the N concentration in the metal is very low (1.7ppm) and no isotopic variations are observed between bulk (mean delta ^15N + 1.1 per mil) and metal (0.0 per mil) except for a spallation component in the melt step. For a study of possible relationships of Acapulco metal with iron meteorites, nitrogen was measured on the etched metallic phases of Acapulco and Cape York (IIIAB), which is considered to be a equilibrated meteorite. The results show that nitrogen is enriched in the Ni-rich phase (taenite and/or plessite) with 690 ppm N in Cape York and 87 ppm N in Acapulco. The large nitrogen concentration in these samples may indicate the presence of N phases in etched metals. However, the mean delta ^15N values of N-enriched phases in Cape York (-80 per mil) are different from that of Acapulco taenite (-130 per mil), and Cape York metal separates do not reveal any lighter nitrogen components. Oxygen isotopic signatures have been used to recognize possible genetic relationships between different classes of meteorites. Unfortunately, oxygen systematics are not available in metal phases, and if silicate inclusions are used for this purpose, the question of phase equilibration needs to be considered. We have shown that, based on nitrogen in Acapulco, an association of chromite and metal is indicated. Therefore, chromite may provide a useful link in this approach. Based on oxygen isotopes an association between IIIA and B irons and pallasites was observed [4] and their nitrogen isotopic signatures are very light [5]. Oxygen isotopes suggest a genetic link between IIE iron meteorites and H chondrites, and the measured nitrogen isotopic signatures in metals of three H chondrites are indeed close to those in IIE iron [5]. References: [1] Zinner E. et al. (1989) GCA, 53, 3273-3290. [2] Kim Y. and Marti K. (1993) LPS XXIV, 801-802. [3] Alexander C. M. O' D. et al. (1990) EPSL, 99, 220-229. [4] Clayton R. N. et al. (1986) LPS XVII, 141. [5] Prombo C. A. (1984) Ph.D. thesis, Univ. of Chicago.
Kim Yup
Marti Kurt
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