Equilibrium condensation of solar gas is often invoked to explain the abundance of refractory elements in planets and meteorites. This is partly motivated, by the observation that the depletions in ...both the least and most refractory rare earth elements (REEs) in meteoritic group II calcium-aluminum-rich inclusions (CAIs) can be reproduced by thermodynamic models of solar nebula condensation. We measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in eight CAIs to test this scenario. Contrary to expectation for equilibrium condensation, we find light isotope enrichment for the most refractory REEs and more subdued isotopic variations for the least refractory REEs. This suggests that group II CAIs formed by a two-stage process involving fast evaporation of preexisting materials, followed by near-equilibrium recondensation. The calculated time scales are consistent with heating in events akin to FU Orionis- or EX Lupi-type outbursts of eruptive pre-main-sequence stars.
Carbon is an essential element for life, but its behavior during Earth's accretion is not well understood. Carbonaceous grains in meteoritic and cometary materials suggest that irreversible ...sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling, we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 astronomical unit after the first million years. On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of Earth thus implies carbon loss in its precursor material through sublimation within the first million years.
Mineralogical observations, chemical and oxygen–isotope compositions, absolute
207Pb–
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, and
36Cl 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 of
26Al,
41Ca, and
53Mn.
10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology.
41Ca–
41K, and
60Fe–
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 an
16O-rich (Δ
17O
∼
−24
±
2‰) nebular gas. The
26Al-poor (
26Al/
27Al)
0
<
1
×
10
−5,
16O-rich (Δ
17O
∼
−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 of
26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are
16O-depleted (Δ
17O
>
−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 of
16O-poor (Δ
17O
>
−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.
ABSTRACT Solar cosmic-ray (SCR) interactions with a protoplanetary disk have been invoked to explain several observations of primitive planetary materials. In our own Solar System, the presence of ...short-lived radionuclides (SLRs) in the oldest materials has been attributed to spallation reactions induced in phases that were irradiated by energetic particles in the solar nebula. Furthermore, observations of other protoplanetary disks show a mixture of crystalline and amorphous grains, though no correlation between grain crystallinity and disk or stellar properties have been identified. As most models for the origin of crystalline grains would predict such correlations, it was suggested that amorphization by stellar cosmic-rays may be masking or erasing such correlations. Here we quantitatively investigate these possibilities by modeling the interaction of energetic particles emitted by a young star with the surrounding protoplanetary disk. We do this by tracing the energy evolution of SCRs emitted from the young star through the disk and model the amount of time that dust grains would spend in regions where they would be exposed to these particles. We find that this irradiation scenario cannot explain the total SLR content of the solar nebula; however, this scenario could play a role in the amorphization of crystalline material at different locations or epochs of the disk over the course of its evolution.
–
We review recent results on O‐ and Mg‐isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum‐rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous ...chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in 16O to varying degrees relative to the standard mean ocean water value: the Δ17OSMOW value ranges from approximately −16‰ to −35‰, and recorded heterogeneous distribution of 26Al in their formation region: the inferred (26Al/27Al)0 ranges from approximately 6.5 × 10−5 to <2 × 10−6. There is no correlation between O‐ and Mg‐isotope compositions of the refractory objects: 26Al‐rich and 26Al‐poor refractory objects have similar O‐isotope compositions. We suggest that 26Al was injected into the 26Al‐poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of 26Al/27Al ratio (approximately 5 × 10−5). The 26Al‐rich and 26Al‐poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of 26Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their 26Al‐26Mg systematics, and the canonical (26Al/27Al)0 does not represent the initial abundance of 26Al in the solar system; instead, it may or may not represent the average abundance of 26Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical 26Al/27Al ratio relative to the timing of complete delivery of stellar 26Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing 26Al resulted in no observable changes in O‐isotope composition of the solar system. Instead, the variations in O‐isotope compositions between individual CAIs indicate that O‐isotope composition of the CAI‐forming region varied, because of coexisting of 16O‐rich and 16O‐poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O‐isotope compositions of these reservoirs with time, e.g., due to CO self‐shielding in the disk.
The CV3 Allende is one of the most extensively studied meteorites in worldwide collections. It is currently classified as S1—essentially unshocked—using the classification scheme of Stöffler et al. ...(1991), however recent modelling suggests the low porosity observed in Allende indicates the body should have undergone compaction-related deformation. In this study, we detail previously undetected evidence of impact through use of Electron Backscatter Diffraction mapping to identify deformation microstructures in chondrules, AOAs and matrix grains. Our results demonstrate that forsterite-rich chondrules commonly preserve crystal-plastic microstructures (particularly at their margins); that low-angle boundaries in deformed matrix grains of olivine have a preferred orientation; and that disparities in deformation occur between chondrules, surrounding and non-adjacent matrix grains. We find heterogeneous compaction effects present throughout the matrix, consistent with a highly porous initial material. Given the spatial distribution of these crystal-plastic deformation microstructures, we suggest that this is evidence that Allende has undergone impact-induced compaction from an initially heterogeneous and porous parent body. We suggest that current shock classifications (Stöffler et al., 1991) relying upon data from chondrule interiors do not constrain the complete shock history of a sample.
•EBSD analyses used to image microstructural deformation within the CV3 Allende.•Matrix grains show higher degrees of deformation than chondrule grains.•Chondrules present concentration of crystal-plastic deformation at grain edges.•Localised heating results from constructive interference of shock waves.•Impact-induced compaction models are supported by evidence within Allende.
We present numerical simulations of the thermal and dynamical histories of solid particles (chondrules and their precursors—treated as 1-mm silicate spheres) during passage of an adiabatic shock wave ...through a particle–gas suspension in a minimum-mass solar nebula. The steady-state equations of energy, momentum, and mass conservation are derived and integrated for both solids and gas under a variety of shock conditions and particle number densities using the free-molecular-flow approximation. These simulations allow us to investigate both the heating and cooling of particles in a shock wave and to compare the time and distance scales associated with their processing to those expected for natural chondrules. The interactions with the particles cause the gas to achieve higher temperatures and pressures both upstream and downstream of the shock than would be reached otherwise. The cooling rates of the particles are found to be nonlinear but agree approximately with the cooling rates inferred for chondrules by laboratory simulations. The initial concentration of solids upstream of the shock controls the cooling rates and the distances over which they are processed: Lower concentrations cool more slowly and over longer distances. These simulations are consistent with the hypothesis that large-scale shocks, e.g., those due to density waves or gravitational instabilities, were the dominant mechanism for chondrule formation in the nebula.
The supernova injection model for the origin of the short-lived radionuclides (SLRs) in the early solar system is reviewed. First, the meteoritic evidence supporting the model is discussed. Based on ...the presence of
60Fe it is argued that a supernova must have been in close proximity to the nascent Solar System. Then, two models of supernova injection, the supernova trigger model and the aerogel model, are described in detail. Both these injection model provide a mechanism for incorporating SLRs into the early solar system. Following this, the mechanisms present in the disk to homogenize the freshly injected radionuclides, and the timescales associated with these mechanisms, are described. It is shown that the SLRs can be homogenized on very short timescales, from a thousand years up to ∼1 million years. Finally, the SLR ratios expected from a supernova injection are compared to the ratios measured in meteorites. A single supernova can inject enough radionuclides to explain the radionuclide abundances present in the early solar system.
Chemical models of solar nebula chemistry are presented which show the influence of progressive water depletion from the inner solar nebula. The main focus of this work is the equilibrium ...distribution of S resulting from this process. Under canonical solar nebula conditions, H
2S is the dominant S-bearing species in the gas phase and troilite (FeS) is the primary reservoir for S after condensation. As water vapor diffuses out to its condensation front, the equilibrium distribution of S changes significantly. With the removal of water vapor, SiS becomes the most abundant S-bearing gas and MgS and CaS compete with FeS as the main sulfide reservoir. These results allow us to argue that some of the minerals in the enstatite chondrites formed through the heterogeneities associated with the nebular ice condensation front, and that the sulfur abundance in Jupiter reflects a depletion in H
2S that is the result of inner nebula sulfur chemistry under varying oxygen abundance.
We review two models for the origin of the calcium-, aluminum-rich inclusion (CAI) oxygen isotope mixing line in the solar nebula: (1) CO self-shielding, and (2) chemical mass-independent ...fractionation (MIF). We consider the timescales associated with formation of an isotopically anomalous water reservoir derived from CO self-shielding, and also the vertical and radial transport timescales of gas and solids in the nebula. The timescales for chemical MIF are very rapid. CO self-shielding models predict that the Sun has Δ
17O
SMOW ∼ −20‰ (Clayton, 2002), and chemical mass-independent fractionation models predict Δ
17O
SMOW ∼0‰. Preliminary Genesis results have been reported by McKeegan et al. (McKeegan K. D., Coath C. D., Heber, V., Jarzebinski G., Kallio A. P., Kunihiro T., Mao P. H. and Burnett D. S. (2008b) The oxygen isotopic composition of captured solar wind: first results from the Genesis.
EOS Trans. AGU 89(53),
Fall Meet. Suppl., P42A-07 (abstr)) and yield a Δ
17O
SMOW of ∼ −25‰, consistent with a CO self-shielding scenario. Assuming that subsequent Genesis analyses support the preliminary results, it then remains to determine the relative contributions of CO self-shielding from the X-point, the surface of the solar nebula and the parent molecular cloud.
The relative formation ages of chondritic components can be related to several timescales in the self-shielding theories. Most importantly the age difference of ∼1–3
My between CAIs and chondrules is consistent with radial transport from the outer solar nebula (>10
AU) to the meteorite-forming region, which supports both the nebular surface and parent cloud self-shielding scenarios. An elevated radiation field intensity is predicted by the surface shielding model, and yields substantial CO photolysis (∼50%) on timescales of 0.1–1
My. An elevated radiation field is also consistent with the parent cloud model. The elevated radiation intensities may indicate solar nebula birth in a medium to large cluster, and may be consistent with the injection of
60Fe from a nearby supernova and with the photoevaporative truncation of the solar nebula at KBO orbital distances (∼47
AU). CO self-shielding is operative at the X-point even when H
2 absorption is included, but it is not yet clear whether the self-shielding signature can be imparted to silicates. A simple analysis of diffusion times shows that oxygen isotope exchange between
16O-depleted nebular H
2O and chondrules during chondrule formation events is rapid (∼minutes), but is also expected to be rapid for most components of CAIs, with the exception of spinel. This is consistent with the observation that spinel grains are often the most
16O-rich component of CAIs, but is only broadly consistent with the greater degree of exchange in other CAI components. Preliminary disk model calculations of self-shielding by N
2 demonstrate that large δ
15N enrichments (∼ +800‰) are possible in HCN formed by reaction of N atoms with organic radicals (e.g., CH
2), which may account for
15N-rich hotspots observed in lithic clasts in some carbonaceous chondrites and which lends support to the CO self-shielding model for oxygen isotopes.
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