Chondrites are extraordinary mixtures of materials with diverse origins that formed around other stars, in the solar nebula, and in their parent asteroids. Most chondrites were so severely altered by aqueous fluids, thermal metamorphism, and impacts that the original characteristics of their components have been largely erased. But a few pristine chondrites have preserved an exquisite mineralogical, chemical, isotopic, and chronological record of the first few million years of solar system history. The properties of diverse types of carbonaceous, ordinary, and enstatite chondrites focusing on the most pristine samples are reviewed to establish the chemical, isotopic, and mineralogical properties and origins of their components and to elucidate the asteroidal processes that modified them. Refractory inclusions – amoeboid olivine aggregates and Ca–Al-rich inclusions – were the first solids to form in the solar nebula near to the protosun. Chondrules and associated metallic Fe–Ni grains were still forming several million years later when the earliest planetesimals, which melted due to heat from 26 Al decay, were colliding. In the least-altered chondrites, matrix material, which coats chondrules and other components, is largely composed of micrometer-sized silicates and amorphous materials, which formed at high temperatures, plus small amounts (up to 200 ppm) of presolar oxides and silicates.

Chondrites and Their Components

Calcium–aluminum-rich inclusions (CAIs) are sub-mm- to cm-sized clasts in chondritic meteorites. They are composed almost entirely of CaO-Al 2 O 3 -MgO-SiO 2 -TiO 2 , and they contain the same minerals predicted by thermodynamic calculations to condense out of a gas of solar composition during cooling from very high temperatures. These features, together with ages of 4.567 Ga, suggest that CAIs are the oldest and most primitive solid objects formed at the time our solar system was born. CAIs possess endemic nuclear anomalies of nucleosynthetic origin, enrichment in 16 O relative to other solar system materials, and also radiogenic anomalies from the in situ decay of short-lived nuclides such as 26 Al and 10 Be that existed when the CAIs (and solar system) formed. CAIs are complex objects whose petrologic and isotopic properties give clues to the events – and chronology of those events – that occurred during the first 1–2 million years of the solar system's existence. Reading that ancient record has been greatly enabled by recent and continuing advances in analytical laboratory instrumentation, and thus interest in CAIs remains very high.

Calcium-Aluminum-rich Inclusions in Chondritic Meteorites

High-precision bulk aluminum-magnesium isotope measurements of calcium-aluminum-rich inclusions (CAIs) from CV carbonaceous chondrites in several laboratories define a bulk 26Al-26Mg isochron with an inferred initial 26Al/27Al ratio of approximately 5.25 × 10−5, named the canonical ratio. Nonigneous CV CAIs yield well-defined internal 26Al-26Mg isochrons consistent with the canonical value. These observations indicate that the canonical 26Al/27Al ratio records initial Al/Mg fractionation by evaporation and condensation in the CV CAI-forming region. The internal isochrons of igneous CV CAIs show a range of inferred initial 26Al/27Al ratios, (4.2–5.2) × 10−5, indicating that CAI melting continued for at least 0.2 Ma after formation of their precursors. A similar range of initial 26Al/27Al ratios is also obtained from the internal isochrons of many CAIs (igneous and nonigneous) in other groups of carbonaceous chondrites. Some CAIs and refractory grains (corundum and hibonite) from unmetamorphosed or weakly metamorphosed chondrites, including CVs, are significantly depleted in 26Al. At least some of these refractory objects may have formed prior to injection of 26Al into the protosolar molecular cloud and its subsequent homogenization in the protoplanetary disk. Bulk aluminum and magnesium-isotope measurements of various types of chondrites plot along the bulk CV CAI isochron, suggesting homogeneous distribution of 26Al and magnesium isotopes in the protoplanetary disk after an epoch of CAI formation. The inferred initial 26Al/27Al ratios of chondrules indicate that most chondrules formed 1–3 Ma after CAIs with the canonical 26Al/27Al ratio.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12141

26Al-26Mg isotope systematics of the first solids in the early solar system

Volatility-dependent fractionation of the rock-forming elements at high temperatures is an early, widespread process during formation of the earliest solids in protoplanetary disks. Equilibrium condensation calculations allow prediction of the identities and compositions of mineral and liquid phases coexisting with gas under presumed bulk chemical, pressure, and temperature conditions. A graphical survey of such results is presented for systems of solar and nonsolar bulk composition. Chemical equilibrium was approached to varying degrees in the local regions where meteoritic chondrules, Ca-Al-rich inclusions, matrix, and other components formed. Early, repeated vapor-solid cycling and homogenization, followed by hierarchical accretion in dust-rich regions, is hypothesized for meteoritic inclusions. Disequilibrium chemical effects appear to have been common at all temperatures, but increasingly so in lessrefractory meteoritic components. Work is needed to better model high-temperature solid solutions, indicators of these processes.

/pdf/condensation-of-rocky-material-in-astrophysical-environments-18ks1qkqoh.pdf

Condensation of Rocky Material in Astrophysical Environments

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CBb chondrite Hammadah al Hamra 237 and none were observed in the CBa chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in 16O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in 16O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with 16O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in 16O-poor gaseous reservoirs or coated by 16O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

— Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Leoville and Vigarano are irregularly-shaped objects, up to 5 mm in size, composed of forsteritic olivine (Fa 50%) melting. We infer that AOAs are aggregates of high-temperature nebular condensates, which formed in CAI-forming regions, and that they were absent from chondrule-forming regions at the time of chondrule formation. The absence of low-Ca pyroxene and depletion in moderately volatile elements (Mn, Cr, Na, K) suggest that AOAs were either removed from CAI-forming regions prior to condensation of these elements and low-Ca pyroxene or gas-solid condensation of low-Ca-pyroxene was kinetically inhibited.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1945-5100.2001.tb01905.x

Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, Leoville and Vigarano: Products of nebular condensation, accretion and annealing

Dmitryivanovite (CaAl 2 O 4 ) is a newly described, calcium aluminum oxide from the Northwest Africa 470 (NWA470) CH3 chondrite (Ivanova et al. 2002). NWA470 contains abundant small Ca,Al-rich inclusions (CAIs), and dmitryivanovite, whose composition is close to stoichiometric CaAl 2 O 4 [Ca 1.000 (Al 1.993 Si 0.003 Ti 0.002 ) 1.998 O 4 ], was found in one of these CAIs. It occurs as ~10 μm subhedral grains intergrown with grossite (CaAl 4 O 7 ), perovskite, and melilite. Electron backscatter diffraction (EBSD) analysis revealed that dmitryivanovite is a high-pressure polymorph of CaAl 2 O 4 ( a = 7.95, b = 8.62, c = 10.25 A, β = 93.1°, space group P 2 1 / c , and Z = 12). Dmitryivanovite is the third phase to be described from nature in the binary system of CaO–Al 2 O 3 , the other two being hibonite (CaAl 12 O 19 ) and grossite (CaAl 4 O 7 )—all are found in CAIs. The presence of CaAl 2 O 4 in NWA470 suggests a local elevated dust/gas ratio in the solar nebula. The phase diagram of CaAl 2 O 4 shows that ~2 GPa is required to stabilize the high-pressure CaAl 2 O 4 polymorph at 1327 °C, above which CaAl 2 O 4 condenses from the solar nebula. Because it is unlikely that the solar nebula ever had such a high total gas pressure, it appears more probable that condensation of the low-pressure polymorph occurred in the solar nebula with an enhanced dust-to-gas ratio and that subsequently the high-pressure polymorph was produced by shock metamorphism, most likely after the CaAl 2 O 4 -bearing CAI was incorporated into the NWA470 parent asteroid.

http://www.minsocam.org/MSA/AmMin/TOC/Abstracts/2009_Abstracts/MJ09_Abstracts/Mikouchi_p746_09.pdf

Dmitryivanovite: A new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis

High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine–anorthite mixtures

Evaluation of a curve-fitting method for diffuse reflectance spectra in the UV–Visible–NIR wavelength region

C class asteroids frequently exhibit reflectance spectra consistent with thermally metamorphosed carbonaceous chondrites, or a mixture of phyllosilicate-rich material along with regions where they are absent. One particularly important example appears to be near-Earth asteroid 1999 JU3, the target of the Hayabusa II sample return mission [1], although not all spectra indicate this. In fact most spectra of 1999 JU3 are featureless, suggesting a heterogeneous regolith. Here we explore an alternative cause of dehydration of regolith of C class asteroids - impact shock melting. Impact shock melting has been proposed to explain some mineralogical characteristics of CB chondrites, but has not been considered a major process for hydrous carbonaceous chondrites. What evidence is there for significant shock melting in the very abundant CMs, or less abundant but still important CI chondrites?

/pdf/evidence-for-impact-shock-melting-in-cm-and-ci-chondrite-35hz2geteu.pdf

M. Komatsu

Papers

Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, Leoville and Vigarano: Products of nebular condensation, accretion and annealing

Dmitryivanovite: A new high-pressure calcium aluminum oxide from the Northwest Africa 470 CH3 chondrite characterized using electron backscatter diffraction analysis

High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine–anorthite mixtures

Evaluation of a curve-fitting method for diffuse reflectance spectra in the UV–Visible–NIR wavelength region

Evidence for Impact Shock Melting in CM and CI Chondrite Regolith Samples