Chondritic Meteorites and the High-Temperature Nebular Origins of Their Components


Paper:	Chondritic Meteorites and the High-Temperature Nebular Origins of Their Components
Volume:	341, Chondrites and the Protoplanetary Disk
Page:	15
Authors:	Scott, E.R.D.; Krot, A.N.
Abstract:	We present an overview of the chondrite groups focusing on the major constraints that can be derived on thermal history of silicates in the solar nebula from the mineralogical, chemical, and isotopic properties of the components in the least-altered chondrites. Recent advances in developing a chronology for the formation of CAIs and chondrules and plausible models for their oxygen isotopic compositions provide the basis for understanding their origin. Evidence from short-lived and long-lived isotopes, oxygen isotopes, nuclear isotopic effects and petrologic studies all suggest that refractory inclusions and grains were the first solids to form in the protosolar disk, probably within a period of <0.3 Myr, when the protosun was accreting rapidly (possibly as a class 0 or I protostar). Refractory inclusions formed in an ¹⁶O-rich, reducing environment of near-solar composition, <10⁻⁴ bar pressure, and temperatures >1300 K. Most chondrules appear to have formed 1-3 Myr after refractory inclusions, when the protosun was accreting more slowly. Chondrules in a single chondrite group probably formed over a much shorter period. Several types of chondrules formed under diverse conditions that were generally more oxidizing with lower ambient temperatures and higher total pressures or high dust/gas ratios, so that liquids were stable for hours. Formation of type I chondrules involved melting, evaporation, condensation, and accretion of solid, partly melted and completely melted materials. Chondrite matrices are mixtures of materials that probably formed in diverse locations in the solar nebula and traces of presolar materials. Matrices in pristine carbonaceous chondrites are largely composed of crystalline Mg-rich silicates (forsterite and enstatite) and amorphous Fe-Mg silicate. The cooling rates, composition, and structure of the Mg-rich silicates suggest that they probably condensed during heating events that formed chondrules. The near-solar composition of the matrix implies that the amorphous silicates have similar origins and that gas and dust were not separated during condensation. Comets and chondritic, porous interplanetary dust particles have more pre-solar material than chondrites, but also contain abundant forsterite and enstatite crystals resembling those in matrices of primitive chondrite matrices. The associated amorphous silicate is Fe-rich suggesting that the Mg-rich silicates may have formed by nebular condensation rather than by annealing. Since forsterite and enstatite are abundant around many protostars, the processes that heated silicate dust in the solar nebula may be common to other protostellar disks. Due to large images in this paper, this PDF file may take longer than usual to load.