Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..500d&link_type=abstract
Meteoritics, vol. 30, no. 5, page 500-501
Other
5
Calcium-Aluminum-Rich Inclusions, Evaporation, Fractionation, Perovskite, Rare Earth Elements, Volatility
Scientific paper
Volatility-fractionated REE patterns were first recognized in calcium-, aluminum-rich inclusions (CAIs) in meteorites over 20 years ago [1]. Such REE patterns can be modeled quite well by equilibrium thermodynamic calculations [2,3,4], but kinetic effects on fractionation of trace elements between gas and solids or liquids have remained less well-known. We have previously reported on experiments in which melts of initial REE-doped chondritic composition [5] and perovskite composition [6] were evaporated in vacuo. In both series of experiments, residues were formed that had large negative Ce anomalies and small negative Pr and Eu anomalies, but no other significant REE fractionations were observed. In an effort to produce more extensive fractionation of REE and other refractory elements, we made a new, more refractory, starting material (stoichiometric CaTiO3 doped with a number of refractory trace elements in chondritic proportions to a total "trace" element level of 5 wt%) and evaporated it under vacuum at a higher temperature (2150 degrees C) than the earlier experiments (1800-2000 degrees C). The results are shown in Fig. 1. Despite the high temperature, the two most extreme evaporation residues reached the solidus during evaporation; the other residues with lower degrees of evaporation remained liquid until cooled at the end of the experiment. The most extreme residue consists largely of mixed Sc, Y and Zr oxides; most of the Ti and nearly all of the Ca have evaporated. In this residue, there are large depletions in Sr, Nb, Ba, Ce and Eu and smaller depletions of La, Pr, Nd, Sm and Yb compared to Sc, Zr and most heavy REE. There is, however, little fractionation of the heavy REE from one another. The chemical composition and even the physical state of the residues change with degree of evaporation, undoubtedly affecting evaporation rates of trace elements. For example, even though the experiments are all carried out at a single temperature and both Ce and Eu are quite depleted in the most extreme residue, Ce evaporates early and Eu evaporates late (Fig. 1). Ultrarefractory inclusions show substantial fractionations among the heavy REE that have been attributed to volatility fractionation [7]. Kinetically controlled evaporation produces large isotopic mass fractionation effects in Ca and Ti [6], whereas there is minimal isotopic fractionation between gas and condensed phases at high temperatures. The lack of large isotopic fractionation effects in ultrarefractory inclusions [7] suggests that these objects did not form by kinetically controlled evaporation. The residues we have produced have bulk chemical compositions consistent with ultrarefractory inclusions, as they are composed of mixed Sc, Zr and Y oxides, but they do not have as much heavy to light REE fractionation nor do they show the substantial fractionation among the heavy REE that is characteristic of ultrarefractory inclusions. This difference supports the idea that the volatility fractionations of trace elements observed CAIs are thermodynamically rather than kinetically controlled. References: [1] Tanaka T. and Masuda A. (1973) Icarus, 19, 523-530. [2] Boynton W. V. (1975) GCA, 39, 569-584. [3] Davis A. M. and Grossman L. (1979) GCA, 43, 1611-1632. [4] MacPherson G. J. and Davis A. M. (1994) GCA, 58, 5599-5625. [5] Wang J. et al. (1993) Meteoritics, 28, 454-455. [6] Davis A. M. et al. (1995) LPS XXVI, 317-318. [7] Davis A. M. (1991) Meteoritics, 26, 330. Fig. 1. Elemental enrichment factors in residues of initial stoichiometric CaTiO3 composition doped with 5% refractory trace elements in chondritic relative proportions, plotted in order of atomic number.
Davis Aileen M.
Hashimoto Akikazu
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