EVIDENCE FOR THE PRESENCE OF PLANETESIMAL MATERIAL AMONG THE PRECURSORS OF MAGNESIAN CHONDRULES OF NEBULAR ORIGIN

Abstract

Chondrules are the major high-temperature components of chondritic meteorites, which are conventionally viewed as the samples from the very first generation of undifferentiated planetesimals. Growing evidences from long- and short-lived radionuclide chronologies indicate however that chondrite parent asteroids accreted after or contemporaneously with igneous activities on differentiated asteroids, questioning the pristine nature of chondrites. Here we report a discovery of metal-bearing olivine aggregates with granoblastic textures inside magnesian porphyritic (Type I) chondrules from the CV carbonaceous chondrite Vigarano. Formation of the granoblastic textures requires sintering and prolonged, high-temperature (> 1000 °C) annealing - conditions which are not expected in the solar nebula during chondrule formation, but could have been achieved on parent bodies of olivine-rich differentiated or thermally metamorphosed meteorites. The mineralogy and petrography of the metal-olivine aggregates thus indicate that they are relict, dunite-like lithic fragments which resulted from fragmentation of such bodies. The very old Pb-Pb absolute ages and Al-Mg relative model ages of bulk CV chondrules suggest that such planetesimals may have formed as early as the currently accepted age of the Solar System (4567.2 ± 0.6 Ma). © 2007 Elsevier B.V. All rights reserved.