Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.454w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 454
Other
4
Cais, Evaporation, Fun Inclusions, Rare-Earth Elements, Refractory Inclusions
Scientific paper
Evaporation experiments suggest that enrichments in the heavy isotopes of oxygen, magnesium, and silicon in some CAIs are caused by kinetic effects during evaporation [1]. Volatility-fractionated REE patterns found in some CAIs have been modeled with some success using equilibrium thermodynamics [2,3], but little is known about kinetic effects on REE patterns. We have begun an investigation of REE fractionation under conditions where large isotope effects are produced by the kinetic isotope effect. We synthesized a starting material containing CI chondritic relative proportions of MgO, Al2O3, SiO2, CaO, TiO2, and FeO, and doped it with 100 ppm each of the REE. Samples of this material were evaporated in a vacuum furnace [4] at 10^-6 torr and 1800 or 2000 degrees C for periods of a few seconds to 5 hr. The mass fraction evaporated ranged from 7.6 to 95.4%. Most residues consist of olivine and glass. Chemical compositions of the residues were determined by electron and ion microprobe. Results for selected elements are shown in Fig. 1. There is no significant evaporation of Ca, Al, and Ti up to 95% mass loss; the evaporation behavior of Mg, Si, and Fe is similar to that found by Hashimoto [5]. There is no significant evaporation of most of the REE up to 95% mass loss. Ce is much more volatile than the other REE under these conditions: a tenfold negative Ce anomaly developed between 60 and 70% mass loss and the anomaly reached 5 X 10^-4 at 95% mass loss. A small Pr anomaly (50% Pr loss) also appeared in the highest-mass-loss residue. Thermodynamic calculations show that Ce has approximately the same volatility as other LREE under solar nebular oxygen fugacity, but is much more volatile than the other REE under oxidizing conditions [6]. We suspect that conditions in the residue in our vacuum evaporation experiments became oxidizing because evaporation reactions involving most major element oxides involve release of oxygen. The four known HAL-type hibonite-rich inclusions, which have large negative Ce and small negative Pr anomalies [7], probably formed by extremely rapid evaporation in the solar nebula or by evaporation in an environment much poorer in hydrogen. Normal CAIs do not have Ce anomalies. If they experienced evaporation in the solar nebula, the process must have occurred slowly enough to maintain reducing conditions in the residue. References: [1] Davis A. M. et al. (1990) Nature, 347, 655-658. [2] Boynton W. V. (1975) GCA, 39, 569-584. [3] Davis A. M. and Grossman L. (1979) GCA, 43, 1611-1632. [4] Hashimoto A. (1990) Nature, 347, 53-55. [5] Hashimoto A. (1983) Geochem. J., 17, 111-145. [6] Davis A. M. et al. (1982) GCA, 46, 1627-1651. [7] Ireland T. R. et al. (1992) GCA, 56, 2503-2520. Figure 1, which appears in the hard copy, shows fraction remaining vs. percent evaporated for vacuum evaporation of material of chondritic composition.
Clayton Robert N.
Davis Aileen M.
Wang Jeffrey
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