Origin and chronology of chondritic components: A review
Date
2009
Authors
Krot, Alexander
Amelin, Yuri
Bland, P
Ciesla, F.J.
Connelly, J.
Davis, Andrew M
Huss, G R
Hutcheon, Ian D
Makide, K
Nagashima, Kazuhide
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Volume Title
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Pergamon Press Ltd.
Abstract
Mineralogical observations, chemical and oxygen-isotope compositions, absolute207Pb-206Pb ages and short-lived isotope systematics (7Be-7Li,10Be-10B,26Al-26Mg,36Cl-36S,41Ca-41K,53Mn-53Cr,60Fe-60Ni,182Hf-182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides (7Be,10Be, and36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of26Al,41Ca, and53Mn.10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology.41Ca-41K, and60Fe-60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an16O-rich (Δ17O ∼ -24 ± 2‰) nebular gas. The26Al-poor [(26Al/27Al)0 < 1 × 10-5],16O-rich (Δ17O ∼ -24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites-may have formed prior to injection and/or homogenization of26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are16O-depleted (Δ17O > -10‰) and have (26Al/27Al)0 similar to those in chondrules (<1 × 10-5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of16O-poor (Δ17O > -5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.
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Keywords: astrophysics; chondrite; chondrule; chronology; hibonite; irradiation; isotopic composition; melting; mineralogy; olivine; oxygen isotope; radionuclide; solar system; Spherules
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Geochimica et Cosmochimica Acta
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Journal article
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2037-12-31
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