FORMATION OF CAIs: CONSTRAINS FROM THE CALCIUM ISOTOPE COMPOSITION OF CAI (SJ101) FROM THE ALLENDE CV3 CHONDRITE

Introduction: The calcium-aluminum rich inclusions (CAIs) represent the oldest objects formed in the Solar System [1], and therefore knowledge of their chemical and isotope compositions is critical for a better understanding of the origin and evolution of our Solar System and its various components, including the Earth. Previous studies on CAIs showed large mass-dependent isotope effects attributed to non-equilibrium evaporation/condensation processes [2]. Specifically, non-equilibrium evaporation of silicate melt into space leaves the residual melt isotopically heavy, as light isotopes are preferentially lost during evaporation. Alternatively, recondensation of vapors enriched in light isotopes, could in turn generate CAIs with light isotope signatures [2]. These positive and negative mass-dependent isotope anomalies in CAIs have been observed for isotope systems of elements such as Ca, Mg and Si [2-5]. Here we present new Ca isotope data from a large (~2.5x1.5 cm) CAI inclusion extracted from the Al-lende CV3 chondrite. Based on petrology and chemical composition this inclusion has been classified as a forsterite-bearing Type B inclusion with the Group II REE pattern, and named SJ101 [6]. We measured mass-dependent and non-mass-dependent (i.e. nucleo-synthetic) Ca isotope variations in the bulk CAI, using the thermal ionization mass-spectrometry (TIMS). The results are discussed with respect to trace element and Mg and Si isotope data acquired previously from the same CAI [6]. We also present a compilation of our recent Ca isotope measurements from various objects of the Solar System including terrestrial and martian basalts, and speculate about processes responsible for the anomalous Ca isotope signatures in CAIs. Analytical Method: Powder from the bulk CAI inclusion was dissolved in a mixture of concentrated HF-HNO 3 acid at 120 °C for 48 hours. The sample was dried down and three times treated with concentrated HNO 3 and once with 6N HCl in order to avoid the formation of CaF 2 [7]. Finally, the sample was re-dissolved in 2.5N HCl from which two aliquots have been taken-one for mass-dependent and the other for non-mass-dependent Ca isotope analysis. The former was mixed with a