Chronology and short-lived radionuclide inventory of the earliest solar system solids: Isotope investigation of calcium-aluminum-rich inclusions from the Allende meteorite

Ca-Al-rich inclusions (CAIs), found in chondritic meteorites are the oldest known solids to have formed within the solar system 4567.6 Ma ago, playing a fundamental role in early solar system chronology. The 26 Al--26Mg (t1/2 &sim 0.7 Ma) isotope systematics of bulk CAIs and their mineral separates was determined by high precision MC-ICPMS to establish a precise initial 26Al abundance and constrain the timescale of CAI formation. A well-defined initial 26Al/27Al ratio in the CAIs reinstates the "canonical" 26Al/27Al ratio of &sim5x10-5 as the initial solar 26Al abundance. The uncertainty in the determined initial 26Al/27Al ratio corresponds to a maximum time span of +/-20 Ka suggesting that the main formation of Allende CAIs culminated within this short timescale.Most Allende CAIs experienced various degrees of secondary alteration on the meteorite parent body. The 26Al--26Mg systematics in CAI spinel and pyroxene determined by in situ MC-SIMS is in good agreement with the high-precision data for bulk rock and mineral separates for the same CAIs obtained by MC-ICPMS. However, the in situ 26Al-- 26Mg systematics of melilite is not. This observed discrepancy is consistent with petrographic evidence for some secondary alteration in the CAIs and suggests either open-system exchange or internal redistribution of Mg isotopes in melilite. Mg isotopes redistribution in the Allende CAIs must, however, be volumetrically small in order to satisfy the mass balance of the precisely defined bulk and mineral CAI data.Wadalite a Cl-rich secondary mineral in Allende Type B CAIs shows evidence for a very high abundance of 36Cl (t1/2 &sim 0.3 Ma), four times higher than the previously reported values in sodalites from CV chondrites. Together with the lack of 26Al (t1/2 &sim 0.7 Ma) in the paragenetic mineral assemblage, the high abundance of 36Cl suggests that 36Cl was produced by late energetic solar particle irradiation >4 Ma after the formation of the solar system in an optically thin disk. This is consistent with Spitzer surveys of young stellar objects that suggest protoplanetary disks are largely cleared of gas and dust after 3-4 Ma, enabling efficient production of certain short-lived radionuclides by high-energy irradiation.