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Early Solar System chronology and isotopic linkage of non-carbonaceous chondrules, clasts and achondrites

Philip M. Reger, The University of Western Ontario

Abstract

Chondrules are ubiquitous igneous spherules, the main constituents of ordinary chondrites, and are considered to be critical building blocks of planetesimals, yet their age and formation mechanism(s) remain debated. Differences between ages determined from the long-lived Pb-Pb and the short-lived 26Al-26Mg chronometers have been attributed to the heterogeneous distribution of 26Al in the solar nebula, a radionuclide that was responsible for early differentiation of planetesimals.

To evaluate this hypothesis, the 26Al-26Mg and the 207Pb-206Pb isotopic compositions were both measured by multi-collector ICP-MS in individual chondrules and igneous clasts in unequilibrated ordinary chondrites, and three achondrites. A subset of the chondrules were also analyzed by in situ 26Al-26Mg secondary ionization mass spectrometry (SIMS), to compare with their bulk Mg model ages and Pb-Pb ages, and for 54Cr isotopic anomalies. A comparison between bulk Mg model ages and Pb-Pb ages in the same chondrules suggests that the absolute ages for CAIs may be too young, and that Pb-Pb ages date precursor formation rather than time of crystallization. Precursors may have been previous generations of chondrules that were recycled by transient shock waves in the protoplanetary disk.

Chronological results of EC 002 date its crystallization at 4566 Ma which is the oldest evidence of planetary crust formation. Analysis of two angrites NWA 10463 and NWA 8535 indicate a more diverse suite of processes that shaped the evolution of the angrite parent body and differentiation of early formed planetesimals.

All together, the presented results support a homogeneous distribution of 26Al in the solar nebula.