Al-26-Mg-26 and Pb-207-Pb-206 systematics of Allende CAIs: Canonical solar initial Al-26/Al-27 ratio reinstated

摘要

The precise knowledge of the initial Al-26/Al-27 ratio [(Al-26/Al-27)(o)] is crucial if we are to use the very first solid objects formed in our Solar System, calcium-aluminum-rich inclusions (CAIs) as the "time zero" age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of Al-27/Mg-24 ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined Al-26-Mg-26 fossil isochron With an (Al-26/Al-27)(o) of (5.23 +/- 0.13)x10(-5). Internal mineral isochrons obtained for three of these CAIs (A44A. AJEF. and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (Al-26/Al-27)(0)=(4.96 +/- 0.25) x 10(-5), anchored to our precisely determined absolute Pb-207-Pb-206 age of 4567.60 +/- 0.36 Ma for the same mineral separates, reinstate the "canonical, (Al-26/Al-27), of 5x10(-5) for the early Solar System. The uncertainty in (Al-26/Al-27)(o) corresponds to a maximum time span of 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the Al-26-Mg-26 and U-Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not Support the supra-canonical Al-26/Al-27 ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supracanonical Al-26/Al-27 ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS. The radiogenic Pb-208*/Pb-206* ratio obtained as a by-product from the Pb-Pb age dating is used to estimate time-integrated Th-232/U-238 ratio (K value) of CAIs. Limited K variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the kappa value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.