Silicon isotopic fractionation of CAI-like vacuum evaporation residues

Calcium-, aluminum-rich inclusions (CAIs) are often enriched in the heavy isotopes of magnesium and silicon relative to bulk solar system materials. It is likely that these isotopic enrichments resulted from evaporative mass loss of magnesium and silicon from early solar system condensates while they were molten during one or more high-temperature reheating events. Quantitative interpretation of these enrichments requires laboratory determinations of the evaporation kinetics and associated isotopic fractionation effects for these elements. The experimental data for the kinetics of evaporation of magnesium and silicon and the evaporative isotopic fractionation of magnesium is reasonably complete for Type B CAI liquids (Richter F. M., Davis A. M., Ebel D. S., and Hashimoto A. (2002) Elemental and isotopic fractionation of Type B CAIs: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta 66, 521–540; Richter F. M., Janney P. E., Mendybaev R. A., Davis A. M., and Wadhwa M. (2007a) Elemental and isotopic fractionation of Type B CAIlike liquids by evaporation. Geochim. Cosmochim. Acta 71, 5544–5564.). However, the isotopic fractionation factor for silicon evaporating from such liquids has not been as extensively studied. Here we report new ion microprobe silicon isotopic measurements of residual glass from partial evaporation of Type B CAI liquids into vacuum. The silicon isotopic fractionation is reported as a kinetic fractionation factor, aSi, corresponding to the ratio of the silicon isotopic composition of the evaporation flux to that of the residual silicate liquid. For CAI-like melts, we find that aSi = 0.98985 ± 0.00044 (2r) for Si/Si with no resolvable variation with temperature over the temperature range of the experiments, 1600–1900 C. This value is different from what has been reported for evaporation of liquid Mg2SiO4 (Davis A. M., Hashimoto A., Clayton R. N., and Mayeda T. K. (1990) Isotope mass fractionation during evaporation of Mg2SiO4. Nature 347, 655–658.) and of a melt with CI chondritic proportions of the major elements (Wang J., Davis A. M., Clayton R. N., Mayeda T. K., and Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO–MgO–SiO2–CaO–Al2O3–TiO2–REE melt system. Geochim. Cosmochim. Acta 65, 479–494.). There appears to be some compositional control on aSi, whereas no compositional effects have been reported for aMg. We use the values of aSi and aMg, to calculate the chemical compositions of the unevaporated precursors of a number of isotopically fractionated CAIs from CV chondrites whose chemical compositions and magnesium and silicon isotopic compositions have been previously measured. 2009 Elsevier Ltd. All rights reserved. 0016-7037/$ see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.07.008 * Corresponding author. Present address: Lawrence Livermore National Laboratory, Livermore, CA 94550, United States. Tel.: +1 925 422 9396. E-mail address: [email protected] (K.B. Knight).