Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.217e&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 217
Other
6
Scientific paper
The refractory siderophile elements (RSEs; W, Mo, Os, Re, Ir, Ru, Pt, and Rh) are predicted to be among the first solids to condense from a cooling gas of solar composition (Grossman 1973). In Ca-, Al-rich inclusions (CAIs), where they are enriched by a factor of 20 to 30 times CI abundances, these elements are concentrated in Fremdlinge and refractory metal nuggets (RMNs). Attempts to explain the formation histories of Fremdlinge and RMNs rely heavily on our understanding of how the RSEs initially condensed. One hypothesis assumes condensation of all RSEs into a single homogeneous alloy, and explains the commonly observed multi-phase textures in terms of exsolution and oxidation processes (Palme and Wlotzka 1976). Other models invoke processes such as multi-phase condensation (separate alloys containing hcp, fcc, and bcc metals) (Sylvester et al. 1990, 1992) or condensation from nonsolar gas compositions (Lin et al. 1991). To better understand the process of RSE condensation, we conducted a transmission electron microscopy (TEM) study of RMNs in an Allende "fluffy" Type-A inclusion. We initially characterized RMNs within the Type-A inclusion using EMPA to identify nuggets rich in all of the RSEs. Such RMNs were selected for TEM study because they provide the best opportunity for determining the extent of solid solution between potentially incompatible metals. Five nuggets fitting the desired criteria, all of which were enclosed within spinel grains, were removed from the CAI for TEM analyses. TEM analyses showed that the nuggets range in size from 50 to 100 nm, that all are euhedral, all are single crystals, and all are homogeneous within analytical resolution (~5 nm). The average CI-normalized RSE abundances of these RMNs is plotted in Fig. 1. Error bars represent the range of nugget compositions rather than analytical error, which is less. Rh was below the detection limit of 0.2 wt% in all nuggets. The compositions of the nuggets are strikingly similar to the calculated composition, also plotted in Fig. 1, of an alloy formed by condensation of all RSEs into a single alloy. The calculated composition assumes ideal solid solution and removal from the solar gas at 1535 K. This value was chosen because it is the predicted condensation temperature of spinel (Lattimer et al. 1978), the host phase of the measured RMNs. The slight differences present between measured and calculated compositions may result from nonunity activity coefficients or, for Fe and Mo, oxidation. These observations demonstrate that complete solid solution between the RSEs in approximately solar proportions is possible, suggesting that observed Fremdlinge and RMN compositions inconsistent with single-phase condensation may more appropriately be explained by mechanisms such as secondary processing, rather than by condensation processes. References: Grossman L. (1973) Geochim. Cosmochim. Acta. 37, 1119-1140. Lattimer J. M., Schramm D. N. and Grossman L. (1978) Ap. J. 219, 230-249. Lin Y. T., El Goresy A. and Fang H. (1991) Meteoritics (abstract) 26, 364- 365. Palme H. and Wlotzka F. (1976) Earth Planet. Sci. Lett. 33, 45-60. Sylvester P. J., Simon S. B. and Grossman L. (1992) Lunar Planet. Sci. (abstract) 23, 1397-1398. Sylvester J. P., Ward B. J., Grossman L. and Hutcheon I. D. (1990) Geochim. Cosmochim. Acta. 45, 3491-3508. Figure 1, which in the hard copy appears here, shows CI- normalized refractory siderophile element abundances of five RMNs in an Allende "fluffy" Type A inclusion, AL.017fA. Abundances labeled 1535 K are calculated assuming single-phase condensation (see text).
Buseck Peter R.
Eisenhour Don Dee
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