Silicon Isotopic Fractionation of CAI-like Vacuum Evaporation Residues

Journal Article · Thu Jun 18 04:00:00 EDT 2009 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2009.07.008· OSTI ID:965072

Knight, K; Kita, N; Mendybaev, R; Richter, F; Davis, A; Valley, J

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 et al., 2002, 2007a). 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, {alpha}{sub Si}, 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 {alpha}{sub Si} = 0.98985 {+-} 0.00044 (2{sigma}) for {sup 29}Si/{sup 28}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 Mg{sub 2}SiO{sub 4} (Davis et al., 1990) and of a melt with CI chondritic proportions of the major elements (Wang et al., 2001). There appears to be some compositional control on {alpha}{sub Si}, whereas no compositional effects have been reported for {alpha}{sub Mg}. We use the values of {alpha}Si and {alpha}Mg, 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.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 965072
• Report Number(s): LLNL-JRNL-414068
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALCIUM
CHEMICAL COMPOSITION
CHONDRITES
CONDENSATES
EVAPORATION
FRACTIONATION
GLASS
KINETICS
MAGNESIUM
RESIDUES
SILICATES
SILICON
SOLAR SYSTEM
VACUUM EVAPORATION