G. Srinivasan

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

The potassium isotope composition of refractory phases in Efremovka CAIs has been measured by an ion microprobe. Extensive studies of terrestrial standards were carried out to ensure that the experimental procedures adopted for potassium isotope studies provide accurate 41K/39K ratios of the analysed phases. Excess 41K is found to be present in refractory phases with high Ca/K ratios (>3 × 105) in all the four Efremovka CAIs analysed in this study. The excess 41K in these phases correlates well with their 40Ca/39K ratios. Several possible causes for this excess have been considered and it is concluded that in situ decay of 41Ca 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 41Ca (τ ~ 0.15 Ma) in the early Solar System with an initial 41Ca/40Ca value of (1.41 ±0.14) × 10−8 at the time of formation of the Efremovka CAIs. We have considered several processes that may lead to the presence of 41Ca in the early Solar System. These include, production by energetic particles from an active early Sun, low energy particle induced reactions in a molecular cloud complex of which the proto-solar cloud was a part and freshly synthesized material from suitable stellar source(s). The last alternative turns out to be the best one from plausibility considerations. The Efremovka CAIs with excess 41K also have excess 26Mg that can be attributed to the decay of the short-lived nuclide 26Al(τ ~ 1 Ma) within these objects. The presence of both 26Al and 41Ca in Efremovka CAIs is used to infer the most probable stellar site(s) for the synthesis of these nuclides in a self consistent manner. Our observations coupled with predicted stellar production rates suggest an asymptotic giant branch (AGB) star to be a plausible source. However, we cannot completely rule out 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 source, our result constrains the time interval between the injection of freshly synthesized 41Ca and 26Al to the solar nebula and the formation of first Solar System solids (CAIs) to less than a million year.

ION MICROPROBE STUDIES OF EFREMOVKA CAIS. I: MAGNESIUM ISOTOPE COMPOSITION

Five coarse-grained Ca-Al-rich refractory inclusions (CAIs) from the Efremovka CV3 chondrite are analysed for their magnesium isotopic composition using an ion microprobe. The analysed inclusions represent the three common CAI types (A, B1, and B2). Data for magnesium isotopic composition of the major mineral phases (melilite, spinel, fassaite, and anorthite) in these inclusions are used to determine intrinsic magnesium isotopic fractionations and to identify nonlinear effect in 26Mg due to the decay of extinct 26Al. The magnesium isotopic mass fractionation, F(Mg), generally favours the heavier isotopes, as is expected in the case of coarse-grained CAIs. The F(Mg) data for two of the inclusions, however, show contrasting trends. Sympathetic behaviour found in both isotopic and petrographic data in one type B1 inclusion suggests that the parent melt of this inclusion experienced a short-duration volatilization event prior to crystallization. The isotopic data also suggest possible presence of relict spinel in this inclusion that constrains the cooling rate of the parent melt to >70°C/h during crystallization. Hot and dense nebular settings that allow for such fast cooling can only be localized in nature. The pristine nature of the Efremovka CAIs, as evident from the lack of secondary alteration products in them, is well supported by the isotopic data. Three of the five inclusions yield well-behaved Mg-Al isochrons with (26Al27Al)0 close to the canonical value of 5 × 10−5. The anorthite data for another inclusion suggest minor disturbance in the isotopic systematics and yield a nearly well-behaved isochron with a slightly lower value of (26Al27Al)0. Data for melilite and anorthite in the fifth inclusion define apparent isochrons that suggest partial re-equilibration and exchange of magnesium isotopes between these two phases. The Efremovka data are suggestive of a relatively homogeneous distribution of 26Al in the solar nebula.