Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.239j&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 239
Computer Science
Scientific paper
To better understand the Pd-Ag and Pb-Tl chronometer systems of iron meteorites, we have performed a series of experiments to evaluate the distribution of Pb, Ag, Pd, and Tl in iron meteorites. Much of the data summarized here has been presented before (Jones and Hart, 1984; Jones et al., 1986; Jones, 1989). However, we have expanded our partitioning data for schreibersite (Fe(sub)3P) and have added Ni, Au, and Mo data for comparison to Ag, Pb, Pd, and Tl. Our experimental and analytical techniques have been described in detail elsewhere (Jones and Drake, 1983; Jones and Hart, 1984; Jones et al., 1986). Table 1 summarizes the results of our experiments near the Fe-FeS euctectic (1000- 900 degrees C). We find that the chalcophile elements Pb, Ag, and Tl are not only extremely incompatible in iron metal, they are also very incompatible in troilite (FeS) and schreibersite. In many cases, we can only give upper limits for the partition coefficients (D). We have compared our experimental D+s for (troilite/Fe-Ni metal) and (schreibersite/Fe-Ni metal) to those inferred for separated phases from iron meteorites. We find that metal- compatible elements (Au, Mo, Ni, Pd) in our experiments generally behave much as would be expected from the analysis of iron meteorites, whereas incompatible elements (Ag, Pb, Tl) do not. We have speculated previously that this discordance between experimentally determined D+s and those inferred from iron meteorites is due to incompatible-element-rich impurities in "metal" and "sulfide" separates (e.g., Jones, 1989). The general concordance of experimentally determined D+s and D+s inferred from natural assemblages further reinforces this view. Bulk "metal" analyses of Ag and Pb indicate that ~0.1-1 wt% of sulfide inclusions within the metal could account for the bulk "metal" Ag and Pb concentrations. If this trace impurity model is correct, we believe that we have a natural explanation for the discrepancy in (i) cooling rates calculated using Ni diffusion profiles (~1-10 degrees C/my; Wood, 1967) and (ii) those calculated using the constraint that Ag isotopic equilibration not occur (>>100 degrees C/my; Kaiser and Wasserburg, 1983). Ag solubility in Fe-Ni metal is very low. If radiogenic Ag in metal encounters small (<100 micrometer) troilite inclusions and finds a phase in which Ag is compatible, then the low solubility of Ag in metal will effectively prohibit further isotopic equilibration. In this model, diffusion rates could be very rapid, but isotopic equilibration could still be very slow. Upon later analysis of inclusion-bearing metal, the radiogenic Ag will be assumed to have been in the metal, the Pd/Ag ratio will still be high, and fast cooling rates will be invoked. Thus, it may be that the isotopic cooling rates of Kaiser and Wasserburg (1983) should be viewed as upper limits to the true cooling rate. References: Jones J.H. (1989) Lunar Planet. Sci. (abstract) 20, 478. Jones J.H. and Drake M.J. (1983) Geochim. Cosmochim. Acta 47, 1199-1209. Jones J.H. and Hart S.R. (1984) Meteoritics (abstract) 19, 248. Jones J.H. et al. (1986) Lunar Planet. Sci. (abstract) 17, 400. Kaiser T. and Wasserburg G.J. (1983) Geochim. Cosmochim. Acta 47, 43-58. Wood J.A. (1967) Icarus 6, 1-49. Table 1, which in the hard copy appears here, shows analytical results.
Benjamin T. M.
Hart Stan R.
Houston Jones Jane
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