Chondrule formation and evolution of the early solar system

Abstract

Chondrules are millimeter to sub millimeter size silicate spherules that formed during localized and transient high temperature events in the early solar system. Although it is not yet understood how chondrules formed, recent studies have provided important clues in understanding the physical conditions of chondrule formation. In this paper I review recent developments in studies of chondrules to provide constraints on high temperature processes during the evolution of protoplanetary disks, including: (1) the timing and duration of the chondrule forming high temperature period, based on long lived and short lived chronometers; (2) the thermal history of chondrule melts during each chondrule forming event, as constrained from laboratory experiments and petrological and isotopic studies of chondrules, including shock wave heating as a plausible heat source for chondrule formation; and (3) sources for shock waves and chondrule forming regions in the protoplanetary disk. Given our current knowledge, I provide the following possible answers to the above four points: (1) the period of chondrule formation began shortly after the formation of calcium aluminum rich inclusions and lasted for a few million years; (2) chondrule precursors were heated at a rate of > 10 4 K/h in the temperature range of ~ 1400 1600 K, and melt droplets were cooled from a peak temperature of ~ 1800 2200 K at a rate of ~ 5 1000 K/h; (3) high velocity shock waves (> 20 km/s) in a low density gas region (< 10 19 particles/m 3 ) may be appropriate for localized transient heating events associated with chondrule formation; and (4) X ray flares from the young Sun in its T Tauri stage might be the source of the shock waves.

Possible Relevance of the Allende Meteorite Conditions in Prebiotic Chemistry: An Insight into the Chondrules and Organic Compounds

“…The local Ni-Fe enrichment of the Allende meteorite provides insights into the diversity of Ni-Fe-based compounds in carbonaceous chondrites. The diversity of the minerals observed in this study and confirmed in other analysis [44] indicates that these compounds may be present in early scenarios of the solar system and in the formation of our planet.…”

Isotopes: From Earth to the Solar System

“…John Wood (1962) discussed the possibility that chondrules formed as molten condensates from a gas, but ruled out this possibility (Wood 1967) on the ground that equilibrium condensation of chondrule melts from a solar gas would require unreasonably high pressures. This view has been almost universally accepted in the meteoritical community, so that all current models of chondrule formation are based on melting of solid precursors, rather than direct condensation as liquids (Tachibana 2006). This approach, with the assumption that chondrule chemical compositions are determined by local thermodynamic equilibrium, leads to proposals of large dust/gas enrichments (to account for the oxidation state of iron) and large nebular volumes (to account for the lack of isotopic fractionation in sulfur and potassium).…”

Critical cooling rates for glass formation in levitated Mg2SiO4-MgSiO3 chondrule melts

“…Chondrules in chondritic meteorites are small silicate spherules of approximately 0.1 -1.0 mm in diameter (Zanda, 2004;Tachibana, 2006). These chondrules are thought to have been formed from "levitated" melt droplets in a space environment some 4.6 billion years ago (Amelin et al, 2002).…”