Chronology of the Solar System's Oldest Solids

Abstract

Determining the origins of our solar system and, by proxy, other planetary systems, depends on knowing accurately and precisely the timing and tempo of the transformation of the disk of gas and dust to the solids that formed the planets. Relative ages based on the short-lived nuclide 26Al indicate that high-temperature calcium-aluminum inclusions (CAIs) formed before lower temperature chondrules but these ages are heavily dependant on a model of homogeneous distribution of 26Al within the protoplanetary disk. The competing X-wind model argues for heterogeneous distribution of 26Al due to its formation by intra-solar system irradiation such that this system would have no chronological significance. We report a 207Pb- 206Pb isochron age of 4565.45 ± 0.45 Myr for chondrules from the CV chondrite Allende, an age that is 1.66 ± 0.48 Myr younger than the accepted Pb-Pb age for CAIs from this chondrite group. This age offset is in excellent agreement with the relative ages determined using the 26Al-26Mg system, an observation that supports a supernova origin for 26Al and, importantly, the chronological significance of the 26Al-26Mg system in general. This is consistent with an early and brief CAI-forming event followed by recurrent chondrule formation throughout the life span of the protoplanetary disk. The paucity of old chondrules in chondrite meteorites may reflect their early incorporation into the parent bodies of differentiated meteorites after CAIs were effectively removed from the innermost regions of the protoplanetary disk. Lastly, the agreement between the absolute and relative chronology of CAIs and chondrules requires a solar system age younger than ∼4567.5 Myr. © 2008. The American Astronomical Society. All rights reserved.