Si and Mg Isotope Fractionations in Melilite in Type B Cais

Introduction: Si and Mg isotopic compositions are indicators of evaporation processes experienced by CAIs [1]. Bulk Si and Mg isotopic compositions (up to ~5 permil/amu for Si and ~11 permil/amu for Mg) have been reported by [2]. Isotopic heterogeneity within CAIs is of interest because it may reveal how evaporation processes occurred. It may also reveal if secondary processes such as recondensation and equilibration have occurred. Fairly large Mg isotopic fractionations within individual CAIs have been reported [3,4] but it is not known if such Mg isotopic heterogeneity is common among CAIs. Si isotope measurements of CAIs by SIMS are scarce [5]. A recent report [6] suggested that precise and accurate Si isotope measurements may be difficult. Here we report Si and Mg isotope measurements of CAIs by SIMS. Samples: Type B CAIs, two (AL-2 and AL-9) from Allende (CV3) and one (SH-1) from Sahara 98044 (CV3) were examined. AL-2 and SH-1 have well developed melilite mantle whereas AL-9 has a very irregular shape and the melilite mantle is seen only locally. Fassaite is present in the mantle (and/or core) of these CAIs. Anorthite is present in the core (and/or mantle) of AL-9 and SH-1. In AL-2, it is present only near the rim. Melilite compositions in AL-2 and SH-1 are normal, ranging from Ak15 to Ak60 and from Ak20 to Ak43, respectively. Melilite compositions in AL-9 are more gehlenitic (from Ak5 to Ak27) suggesting that this CAI may be akin to type A CAIs. No systematic change in the melilite compositions from the core to the rim was observed in these CAIs. Crystals of synthetic akermanite and gehlenite and glass samples of Ak50 and Ak30 compositions were used for calibration purposes. Ion microprobe measurements: A Cameca IMS-6F ion probe at Univ. of Tokyo was used for the measurements. Since instrumental isotopic fractionation could results from many factors, exactly the same measurement conditions are required for standards and samples. Therefore, polished sections were prepared in which both a standard sample (glass of Ak50 composition) and a CAI were set. Intrinsic isotope fractionations were obtained from the difference in isotopic compositions between a standard sample and a CAI. Both Si and Mg isotopic fractionations were measured with the same ion probe setting except for the entrance slit width which was adjusted to give