Silicon Isotope Ratio Analysis of a Cai by Laser Ablation Mc-icpms and Implications for the Astrophysics of Cai Formation

Introduction: While it is well known that igneous CAIs tend to have high Si/Si and Mg/Mg relative to chondritic values, there have been few systematic studies that relate observed isotope fractionations in CAIs to their conditions and timescales of formation. Together with a wealth of information about the physical chemistry associated with evaporation and condensation of CAI-like liquids [1] we apply silicon isotope ratios obtained in situ by UV laser ablation MC-ICPMS, and existing Mg/Mg MC-ICPMS data [2], to construct a PH2-time curve unique to a CAI that constrains the astrophysical setting of CAI melting. Analytical Methods: Silicon isotope ratios were measured using UV laser ablation MC-ICPMS. Mass interferences were eliminated by operating with a mass resolving power (m/Δm) of ~9000. Corrections for instrumental mass bias were perfomed by samplestandard bracketing with our in-house standard (San Carlos olivine). We used a 193 nm excimer laser operated with a pulse repetition rate of 2 to 6 Hz and UV fluence of 21 to 27 J/cm. In situ analyses were acquired from spots measuring 100 μm in diameter and ~20-30 μm deep. Precision of the LA-MC-ICPMS analyses is on the order of +/− 0.2‰ 1σ for both δSi and δSi. The accuracy of our Si isotope ratio results was assessed using a synthetic glass on the CaMgSi2O6-CaAl2Si2O6 join prepared with a 1% spike of Si. Sample Description: For this study we analyzed Leoville 144A, an igneous, compact type A CAI from a CV3 meteorite. It is composed mainly of melilite, magnesian spinel, minor Al-Ti rich diopside and minor perovskite typically associated with diopside. Modeling: Isotopic fractionation in a volatilizing CAI is controlled by the relative rates of evaporation and diffusion of the constituent phases. In order to model the isotopic evolution of the CAIs we obtained numerical solutions to the problem of elemental and isotopic fractionation at the moving surface of an evaporating sphere coupled with diffusional transport within the sphere. The shrinking sphere represents a volatilizing protoCAI where the rate of surface retreat is a function of the average volatility of the object as prescribed by differences between ambient and equilibrium vapor pressures of the constituents of the object. The implicit finite difference scheme for solving this problem, akin to an inverse Stefan problem, was described previously [3] and has been modified to account for the effect of Si and Mg isotope fractionation that includes isotope-specific rates of diffusion within the sphere. In order to validate our calculation scheme we calculated models for the measured changes in Mg isotope ratios and chemical compositions attending evaporation of type B CAI-like liquids in the laboratory reported by Richter et al. [1] and Si isotope fractionation measured for analogous experiments reported by Janney et al. [4]. The calculations show that our models accurately reproduce the experimental results for both Si and Mg isotope fractionation using the Si/Si and Mg/Mg fractionation factors of 0.9898 and 0.9869, respectively (Fig. 1). These fractionation factors are the same as those reported in the original studies [1,4] and verify that diffusion played a negligible role in determining the buk isotopic compositions of the the experimental products.