ION MICROPROBE STUDIES OF EFREMOVKA CAIS .2. POTASSIUM ISOTOPE COMPOSITION AND CA-41 IN THE EARLY SOLAR-SYSTEM

The potassium isotope composition of refractory phases in Efremovka CA Is has been measured by an ion microprobe. Extensive studies of terres trial standards were carried out to ensure that the experimental proce dures adopted for potassium isotope studies provide accurate K-41/K-39 ratios of the analysed phases. Excess K-41 is found to be present in refractory phases with high Ca/K ratios (>3 x 10(5)) in all the four E fremovka CAIs analysed in this study. The excess K-41 in these phases correlates well with their Ca-40/K-39 ratios. Several possible causes for this excess have been considered and it is concluded that in situ decay of Ca-41 can best explain our observations. The results obtained in this work substantiate the conclusion drawn from our initial study (Srinivasan et al., 1994) and confirm the presence of the short-lived nuclide Ca-41 (tau similar to 0.15 Ma) in the early Solar System with an initial Ca-41/Ca-40 value of (1.41 +/- 0.14) x 10(-8) at the time of formation of the Efremovka CAIs. We have considered several process es that may lead to the presence of Ca-41 in the early Solar System. T hese include, production by energetic particles from an active early S un, low energy particle induced reactions in a molecular cloud complex of which the proto-solar cloud was a part and freshly synthesized mat erial from suitable stellar source(s). The last alternative turns out to be the best one from plausibility considerations. The Efremovka CAI s with excess K-41 also have excess Mg-26 that can be attributed to th e decay of the short-lived nuclide Al-26 (tau similar to 1 Ma) within these objects. The presence of both Al-26 and Ca-41 in Efremovka CAIs is used to infer the most probable stellar site(s) for the synthesis o f these nuclides in a self consistent manner. Our observations coupled with predicted stellar production rates suggest an asymptotic giant b ranch (AGE) star to be a plausible source. However, we cannot complete ly rule but a supernova or a Wolf-Rayet star as being responsible for the synthesis and subsequent injection of these nuclides to the solar nebula. In spite of this inability to pinpoint the exact stellar sourc e, our result constrains the time interval between the injection of fr eshly synthesized Ca-41 and Al-26 to the solar nebula and the formatio n of first Solar System solids (CAIs) to less than a million year.