Matrix Of Carbonaceous Chondrites Research Articles

Thermal History of Matrix Forsterite Grains from Murchison Based on High-resolution Tomography

Abstract Protoplanetary disks are dust- and gas-rich structures surrounding protostars. Depending on the distance from the protostar, this dust is thermally processed to different degrees and accreted to form bodies of varying chemical compositions. The primordial accretion processes occurring in the early protoplanetary disk such as chondrule formation and metal segregation are not well understood. One way to constrain them is to study the morphology and composition of forsteritic grains from the matrix of carbonaceous chondrites. Here, we present high-resolution ptychographic X-ray nanotomography and multimodal chemical microtomography (X-ray diffraction and X-ray fluorescence) to reveal the early history of forsteritic grains extracted from the matrix of the Murchison CM2.5 chondrite. The 3D electron density maps revealed, at unprecedented resolution (64 nm), spherical inclusions containing Fe–Ni, very little silica-rich glass and void caps (i.e., volumes where the electron density is consistent with conditions close to vacuum) trapped in forsterite. The presence of the voids along with the overall composition, petrological textures, and shrinkage calculations is consistent with the grains experiencing one or more heating events with peak temperatures close to the melting point of forsterite (∼2100 K), and subsequently cooled and contracted, in agreement with chondrule-forming conditions.

Interactions between organic compounds and olivine under aqueous conditions: A potential role for organic distribution in carbonaceous chondrites

AbstractOlivine is a principal anhydrous silicate mineral in chondritic meteorites. The structure of this mineral is composed of independent SiO4 tetrahedra linked by divalent cations (mainly Mg). Under hydrothermal conditions, olivine is transformed into serpentine, which is a major hydrated phyllosilicate in the matrix of carbonaceous chondrites. Although carbonaceous chondrites contain various types of organic matter, the interaction between organic compounds and olivine at low temperature has not been considered in the literature. We performed an experiment to test the adsorption of N‐containing organic compounds (i.e., alkylpyridines and alkylimidazoles) on olivine using liquid chromatography under aqueous conditions (pH = 2.5–10.5) at 20–40 °C. The N‐containing cyclic compounds were interacted with the SiO4 tetrahedra of olivine and their different adsorption abilities depended on the organic structures. Because alkylpyridines often occur at different locations than alkylimidazoles in carbonaceous chondrites, the results of this study suggest that olivine can separate the N‐containing compounds associated with aqueous fluid flows by asteroidal chromatography in the meteorite parent body. Liquid chromatography based on solid phase minerals may hence be a useful technique for simulating the behavior of organic compounds in carbonaceous asteroids under aqueous activity.

The distribution of chiral asymmetry in meteorites: An investigation using asymmetric autocatalytic chiral sensors

We separated and analyzed several organic and inorganic phases of the carbonaceous chondrite matrix to determine whether they contained any inherent asymmetry. Our intent was to determine any possible foci of asymmetry besides the one determined for meteoritic amino acids. As a probe, we employed a very sensitive asymmetric autocatalytic reaction. We were able to determine that asymmetry still resides in powders after extraction with water and solvents as well as in the insoluble organic material (IOM) obtained after demineralization. Asymmetry is not found any longer in the IOM after hydrothermal treatment and in meteorite powders from which all organics had been removed by O 2 plasma at low temperature. The data are interpreted to indicate a diverse molecular asymmetry residing in yet unknown meteorite organics; these organics might have had an inductive effect on organic molecular evolution upon exogenous delivery to the early Earth.

Manganese–chromium isotope systematics of carbonaceous chondrites

In this article we present the results of Cr isotope investigations of different types of carbonaceous chondrites and of the pallasite Eagle Station. The 53Cr/ 52Cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55Mn/ 52Cr ratios. The slope of the correlation line yields a 53Mn/ 55Mn ratio of (8.5 ± 1.5) × 10 − 6 at the time of Mn/Cr fractionation. Mapping this ratio onto an absolute time scale yields a time for this event of 4568.1 + 0.8/− 1.1 Ma ago. This time is very similar to the formation age of Efremovka CAIs of 4567.2 ± 0.6 Ma [Y. Amelin, A. N. Krot, I. D. Hutcheon, A. A. Ulyanov, Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions, Science 297 (2002) 1678–1683], to a time of the chondrule formation of 4568 ± 1 Ma ago [L.E. Nyquist, D. Lindstrom, D. Mittlefehldt, C.-Y. Shih, H. Wiesmann, S. Wentworth, R. Martinez, Manganese-chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites, Meteorit. Planet. Sci. 36 (2001) 911–938], which, most likely, constrains early global high-temperature Mn/Cr fractionation in a nebular setting. The bulk samples of carbonaceous chondrites exhibit clear 54Cr excesses ( 54Cr ⁎) that are correlated with the 53Cr excesses ( 53Cr ⁎) and also with Mn/Cr ratios. One possible explanation of this correlation is that 54Cr ⁎ is also radiogenic, like 53Cr ⁎, and was formed by the decay of the short-lived parent radionuclide 54Mn. The very short half-life of 54Mn of 312 days would require that both short-lived radionuclides 53Mn and 54Mn were generated locally in spallation reactions during the early period of an active sun. The alternative and possibly more plausible explanation is the heterogeneous addition of presolar material. The presolar component, enriched in 54Cr, is mostly contained in the matrix of carbonaceous chondrites. The relative amount of matrix decreases in the sequence CI > CM > CO,CV. A large proportion of Mn is associated with the matrix while Cr preferentially resides in the chondrules. Thus, the Mn/Cr ratio also follows the sequence CI > CM > CO > CV and is correlated with 54Cr. The acid-resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54Cr and moderate deficits of 53Cr. The magnitude of these excesses and deficits decreases in the sequence CI, CM, CV and may imply that Cr in the CM, and CV residues is increasingly more equilibrated with Cr from the rest of the meteorites. The 53Cr/ 52Cr and 54Cr/ 52Cr ratios in the residues are anti-correlated indicating that there are at least two Cr components of possibly presolar origin. All residues have large excesses of the most neutron-rich Ti isotope, 50Ti ⁎. The pattern of 50Ti ⁎ does not exactly follow that observed for 54Cr ⁎ but it is generally similar. The Cr isotope systematic of the pallasite Eagle Station indicates that the precursor of this meteorite was a CV-type material. The 53Mn– 53Cr system indicates that the Cr isotopes equilibrated in this meteorite 4557.5 ± 0.6 Ma ago.

Occurrence and possible significance of rare Ti oxides (Magnéli phases) in carbonaceous chondrite matrices

Abstract— Rare, ultrafine‐grained Ti oxides (Ti3O5 and the Magnéli phases, Ti5O9 and Ti8O15) have been identified by transmission electron microscopy in the CM2 carbonaceous chondrite, Bells and a carbonaceous chondrite matrix clast from the Nilpena polymict ureilite. In both meteorites the Ti oxides occur in the matrix as isolated grains and clusters of two or more grains. They are euhedral in shape and have grain sizes of 0.05–0.3 μm. Magnéli phases have been recently shown to be a common component in some interplanetary dust particles, but this is the first reported occurrence in a meteorite. The morphological properties and grain size of the Ti oxides are consistent with formation by vapor phase condensation either within the solar nebula or possibly in a presolar environment.

CI chondrite-like clasts in the Nilpena polymict ureilite: Implications for aqueous alteration processes in CI chondrites

We have carried out a detailed petrographic and mineralogical study of carbonaceous chondrite matrix clasts in the Nilpena polymict ureilite. The bulk compositions of a number of clasts, determined by electron microprobe, show that they have close affinities to CI chondrite matrix and differ substantially from CM matrix material. Transmission electron microscopy studies show that the mineralogy of the clasts is also consistent with CI chondrite matrix, indicated by the presence of saponitic smectite clays, serpentine, magnetite, pentlandite, pyrrhotite, and ferrihydrite. Phases such as tochilinite and cronstedtite, diagnostic of CM matrix, are absent. Based on the oxygen isotopic composition of the matrix, the phase chemistry, and the bulk clast chemistry, we suggest that the matrix clasts in Nilpena represent material which is less altered than Orgueil. The difference in oxygen isotopic composition of the Nilpena matrix clasts from CI chondrites may be because the clasts underwent alteration at a lower water/rock ratio. Increased alteration of Nilpena-type material should result in the oxidation of Fe-bearing saponite and sulfides to produce ferrihydrite and more Mg-rich saponite, such as that in Orgueil matrix. Although the presence of CI-like matrix clasts in the polymict ureilites suggests that CI matrix may have been a precursor to the ureilites, in detail this seems highly unlikely. The trace element and carbon isotopic compositions of ureilites are incompatible with a significant CI chondrite precursor. We suggest that the CI-like matrix clasts in Nilpena represent a component of a late-accreting veneer on the ureilite parent body.

Iron, magnesium, and silicon in dust from Comet Halley

We have used mass spectra from the Vega-1 and Giotto spacecraft's PUMA-1 and PIA impact mass spectrometers to study the Mg, Si, and Fe composition of Comet Halley's dust grains. Automated and manual methods were developed to select only the highest quality mass spectra, rejecting the many spectra with excessive noise, double peaks, data format errors, or molecular contamination. The rigorous selection algorithms accepted a total of 433 compressed (low resolution) PUMA-1 spectra, 29 of the PUMA-1 uncompressed (high resolution) spectra, and 32 of the PIA uncompressed spectra. This selected data set gives results agreeing in general with results of other authors, but differing in some details such as dispersions of ion ratios. Relative to Si, a fairly constant Mg abundance is found, while Fe varies over a very large range. The majority of Mg appears to be contained in silicates, but the wider range for Fe indicates that it is contained in a range of materials such as metal, magnetite, sulfides, and silicates of varying Fe composition. The lack of sharp clustering in the data reveals that (with the exception of some FeS particles) none of the detected particles are single mineral grains. Halley solids are either composed of mineral grains smaller than the typical particle size, or they contain abundant glass. The distribution of Fe/(Fe + Mg) approximately matches the broad, flat distribution of submicron grains in some types of interplanetary dust particles collected in the stratosphere and is quite different from the narrow distribution found in carbonaceous chondrite matrix. The best match is with interplanetary particles that contain high-temperature, Mg-rich silicates and that are dominated by anhydrous minerals. We tentatively suggest that by analogy Halley is a discordant mixture of ice and high-temperature anhydrous minerals. This contrasts with the carbon-rich carbonaceous chondrites that are dominated by hydrous minerals.

The composition of asteroid 2 Pallas and its relation to primitive meteorites

High resolution spectroscopic observations of asteroid 2 Pallas from 1.7-3.5 μm are reported. These data are combined with previous measurements from 0.4-1.7 μm to interpret Pallas' surface mineralogy. Evidence is found for low-Fe 2+ hydrated silicates, opaque components, and low-Fe 2+ anhydrous silicates. This assemblage is very similar to carbonaceous chondrite matrix material such as is found in type CI and CM meteorites, but it has been subjected to substantial aqueous alteration and there is a major extraneous anhydrous silicate component. This composition is compared to that of asteroid 1 Ceres. Although there are substantial differences in their broad band spectral reflectances, it appears that both asteroids are genetically related to known carbonaceous chondrites.

Ordered mixed-layer structures in the Mighei carbonaceous chondrite matrix

High resolution transmission electron microscopy of the Mighei carbonaceous chondrite matrix has revealed the presence of a new mixed layer structure material. This mixed-layer material consists of an ordered arrangement of serpentine-type (S) and brucite-type (B) layers in the sequence … SBBSBB. … Electron diffraction and imaging techniques show that the basal periodicity is ~ 17 Å. Discrete crystals of SBB-type material are typically curved, of small size (<1 μm) and show structural variations similar to the serpentine group minerals. Mixed-layer material also occurs in association with planar serpentine. Characteristics of SBB-type material are not consistent with known terrestrial mixed-layer clay minerals. Evidence for formation by a condensation event or by subsequent alteration of preexisting material is not yet apparent.

Infrared spectroscopy of interplanetary dust in the laboratory

Diagnostic infrared spectra of individual nanogram-sized interplanetary dust particles (IDPs) collected in the Earth's stratosphere have been obtained. A mount containing three crushed “chondritic” IDPs shows features near 1000 and 500 cm −1, suggestive of crystalline pyroxene, and different from those of crystalline olivine, amorphous olivine, or meteoritic clay minerals. The structural diversity of chondritic IDPs and possible effects of atmospheric heating must be considered when comparing this spectrum with astrophysical spectra of interplanetary and cometary dust. Transmission electron microscope (TEM) and infrared observations are also reported on one member of the rare subset of IDPs which resemble hydrated carbonaceous chondrite matrix material. The infrared spectrum of this particle between 4000 and 400 cm −1 closely matches that of the C2 meteorite Murchison. TEM observations suggest that this class of particles might serve as a thermometer for the process of heating on atmospheric entry.

The composition of the Trojan asteroids

Physical studies have shown that most asteroids are covered with material of low visual albedo (pv∼0.04) having flat, nearly featureless reflection spectra between 0.4 and 1.1 µm (ref. 1). This large class of objects, the C asteroids, accounts for a progressively larger fraction of minor planets as one goes from the inner to the outer regions of the asteroid belt, a trend which has been interpreted as evidence of a decrease in effective condensation temperature in the original solar nebula with increasing distance from the proto-Sun2. We suggest here that the very low albedos and red spectra of some Trojan asteroids can be explained by the presence of kerogen-like organic compounds. Materials containing these types of carbonaceous substances, rather than those found in the more familiar carbonaceous chondrite matrix, may have been the primary rocky condensate in the outer Solar System and may therefore be typical of the rocky component of comet nuclei.

The composition of carbonaceous chondrite matrix

Matrix compositions of 32 carbonaceous chondrites have been analyzed by an electron microprobe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. Weight per cent Mg/Si in matrices is constant (0.82 ± 0.05) but less than ratios derived from bulk analyses. Matrices in metamorphosed meteorites are Mg-depleted relative to those of other chondrites. Al Rais and Renazzo (anomalous by any classification scheme) have Mg-enriched matrices. Average matrix compositions cluster into chemical subgroups similar to those based on bulk chemical and petrographie criteria [C1, C2, C3(0), C3(V)]. C1 matrices are particularly variable in composition from point to point within the same meteorite, but points within individual breccia clasts appear to be more compositionally uniform. Cl matrices are depleted in Na, S, and Ca relative to solar and C2 matrix values, probably as a result of leaching. Matrix Ca/A1 ratios are highly variable and generally fall below the accepted meteoritic value. The only strong interelement correlation is for Fe, Ni, and S in C2 matrices, suggesting mixing of variable proportions of two components: Mg-rich phyllosilicate and a Ni-bearing chalcophile phase. The amount of magnetite associated with C2 matrix appears to vary systematically with matrix composition. Isotopic, chemical, and mineralogical constraints suggest that matrix, although appreciably altered in some meteorites, is chiefly a solar system condensation product which contains an admixture of unprocessed interstellar dust.