Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..520h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 520
Computer Science
Instrumental Neutron Activation Analysis, Metal, Cobalt, Nickel, Meteorites, Bishunpur, Redox
Scientific paper
In an earlier electron microprobe (EMP) study [1] of the composition of kamacite in Bishunpur (L3.1) metal grains, a clear trend was found in that Co-rich grains were generally Cr-poor and contained graphite inclusions, while Co-poor grains were generally Cr-rich and devoid of graphite. Moreover, both metal grains in chondrules and large grains outside chondrules (but not those in chondrule rims) followed the same trend. These were interpreted as the results of redox reactions during chondrule formation. In this context, the variation of the Co concentration in kamacite would result from dilution by metallic Fe from partial reduction (by carbon) of the silicates. As Co concentration in Bishunpur kamacite varies by a factor ~5, this suggests that a large fraction of the metal could be produced by reduction during chondrule formation. This rather unorthodox conclusion needs to be verified. One obvious way is to look at Ni concentration, which should reflect dilution by Fe in the same way as Co concentration. However, while kamacite is probably an adequate indicator of bulk Co concentration in metal grains (taenite is much less abundant than kamacite and contains ~5 times less Co than kamacite), this is not true for Ni (taenite is ~10 times more Ni-rich than kamacite, and the Ni concentration in kamacite and taenite reflects equilibration temperature, not bulk concentration). We have therefore measured bulk Ni, Co and Cr concentration in individual metal grains, by neutron activation. Metal grains from Bishunpur were magnetically separated after gentle crushing, and cleaned from adhering silicates by concentrated NaOH etching. Initially, non-chondrule grains were chosen, as they are easier to separate and handle. After neutron irradiation at Pierre-S e Laboratory, each grain was g-counted separately. We presently have data for 14 metal grains, which essentially agree with the previous EMP results for Co and Cr [1]. This shows that EMP broad beam analysis of kamacite gave data representative of whole grains for Co and for Cr, in spite of the latter being in tiny chromite inclusions in the metal. We have at present no information on the graphite content of the analyzed grains. The data for Ni and Co are presented in Fig. 1. The points are randomly distributed, and there is no indication of a correlation. This seems to rule out any strong dilution by reduced Fe, in contradiction to the Co-Cr relation. In addition, the Co/Ni ratio is highly variable, and substantially different from (generally higher than) the solar ratio (Fig. 1), unlike bulk chondrules in Semarkona [2]. Finally, we note that Ni concentration is <10wt% for all grains, while the average Ni concentration in Bishunpur metal calculated from bulk chondrite analysis is 13.6wt% [3]. A satisfactory explanation of these contradictions may include experimental artefacts, oversimplification in assuming that non-chondrule metal grains are pure products of chondrule formation, metamorphic modifications of metal chemical composition, etc... References. [1] Mostefaoui S. and Perron C. (1994) Meteoritics, 29, 506. [2]Grossman J. F. and Wasson J. T. (1985) GCA, 49, 925. [3] Jarosewich E. (1990) Meteoritics, 25, 323.
Hill Hugh G. M.
Mostefaoui Smail
Perron Claude
Robin Eric
Rocchia Robert
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