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Prior to becoming chondritic meteorites, primordial solids were a poorly consolidated mix of mm-scale igneous inclusions (chondrules) and high-porosity sub-μm dust (matrix). We used high-resolution numerical simulations to track the effect of impact-induced compaction on these materials. Here we show that impact velocities as low as 1.5 km s−1 were capable of heating the matrix to >1,000 K, with pressure–temperature varying by >10 GPa and >1,000 K over ~100 μm. Chondrules were unaffected, acting as heat-sinks: matrix temperature excursions were brief. As impact-induced compaction was a primary and ubiquitous process, our new understanding of its effects requires that key aspects of the chondrite record be re-evaluated: palaeomagnetism, petrography and variability in shock level across meteorite groups. Our data suggest a lithification mechanism for meteorites, and provide a ‘speed limit’ constraint on major compressive impacts that is inconsistent with recent models of solar system orbital architecture that require an early, rapid phase of main-belt collisional evolution.

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Abstract-We have studied the petrologic characteristics of sulfide-metal lodes, polymineralic Fe-Ni nodules, and opaque assemblages in the CR2 chondrite Graves Nunataks (GRA) 06100, one of the most altered CR chondrites. Unlike low petrologic type CR chondrites, alteration of metal appears to have played a central role in the formation of secondary minerals in GRA 06100. Differences in the mineralogy and chemical compositions of materials in GRA 06100 suggest that it experienced higher temperatures than other CR2 chondrites. Mineralogic features indicative of high temperature include: (1) exsolution of Nipoor and Ni-rich metal from nebular kamacite; (2) formation of sulfides, oxides, and phosphates; (3) changes in the Co/Ni ratios; and (4) carbidization of Fe-Ni metal. The conspicuous absence of pentlandite may indicate that peak temperatures exceeded 600°C. Opaques appear to have been affected by the action of aqueous fluids that resulted in the formation of abundant oxides, Fe-rich carbonates, including endmember ankerite, and the sulfide-silicate-phosphate scorzalite. We suggest that these materials formed via impactdriven metamorphism. Mineralogic features indicative of impact metamorphism include (1) the presence of sulfide-metal lodes; (2) the abundance of polymineralic opaque assemblages with mosaic-like textures; and (3) the presence of suessite. Initial shock metamorphism probably resulted in replacement of nebular Fe-Ni metal in chondrules and in matrix by Nirich, Co-rich Fe metal, Al-Ti-Cr-rich alloys, and Fe sulfides, while subsequent hydrothermal alteration produced accessory oxides, phosphates, and Fe carbonates. An extensive network of sulfide-metal veins permitted effective exchange of siderophile elements from pre-existing metal nodules with adjacent chondrules and matrix, resulting in unusually high Fe contents in these objects.

Abstract-We have conducted scanning electron microscope (SEM) and transmission electron microscope (TEM) studies of a variety of occurrences of matrix in the reduced CV3 chondrite breccia Vigarano. Matrix, which occurs as clastic interchondrule material and finer-grained rims, is dominated by morphologically variable olivines that host submicron, hercynitic spinel, and carbonaceous inclusions. Clastic matrix and fine-grained rims show significant differences in their olivine morphologies, abundance, and composition of olivine inclusions, and characteristics of the carbonaceous matter. We suggest that these differences are the result of different degrees of alteration of clastic matrix and rims and are not due to variability in their precursor materials. Textural and compositional characteristics of olivine in the matrix are consistent with formation by growth, possibly from an amorphous precursor material during asteroidal metamorphism, in the presence of limited quantities of aqueous fluids. Spinel inclusions in olivine may be nebular condensates that acted as seeds for nucleation of olivine or may have formed during metamorphism and were subsequently overgrown by olivine. Carbonaceous material occurs as nanometer-sized inclusions within olivine in both fine-grained rims and clastic matrix, but is most abundant as 100-200 nm grains, interstitial to matrix olivines. Most carbonaceous material is amorphous, but poorly graphitized carbon (PGC) also occurs as a minor component in both olivine inclusions and interstitial C. The widespread occurrence of fine-grained amorphous carbon grains in the interstitial regions between olivine grains may preserve the distribution and grain size of nebular organic material. No clear textural relationships exist between carbonaceous grains and the other mineralogical components of Vigarano matrix that could help constrain the origin of the organic grains (i.e., evidence for Fischer-Tropsch-type reactions). Finally, there are considerable differences between matrix olivines in Vigarano in comparison with those in oxidized CV3 chondrites. In particular, the mineralogy and morphology of the matrix olivines and the nature, composition, and distribution of inclusions in the olivine grains are distinct. Based on these differences, we conclude that matrix in the oxidized CV3 chondrites could not have formed by thermal processing of Vigarano-like material.

The water‐soluble organic compounds in carbonaceous chondrite meteorites constitute a record of the synthetic reactions occurring at the birth of the solar system and those taking place during parent body alteration and may have been important for the later origins and development of life on Earth. In this present work, we have developed a novel methodology for the simultaneous analysis of the molecular distribution, compound‐specific δ13C, and enantiomeric compositions of aliphatic monocarboxylic acids (MCA) extracted from the hot‐water extracts of 16 carbonaceous chondrites from CM, CR, CO, CV, and CK groups. We observed high concentrations of meteoritic MCAs, with total carbon weight percentages which in some cases approached those of carbonates and insoluble organic matter. Moreover, we found that the concentration of MCAs in CR chondrites is higher than in the other meteorite groups, with acetic acid exhibiting the highest concentration in all samples. The abundance of MCAs decreased with increasing molecular weight and with increasing aqueous and/or thermal alteration experienced by the meteorite sample. The δ13C isotopic values of MCAs ranged from −52 to +27‰, and aside from an inverse relationship between δ13C value and carbon straight‐chain length for C3–C6 MCAs in Murchison, the 13C‐isotopic values did not correlate with the number of carbon atoms per molecule. We also observed racemic compositions of 2‐methylbutanoic acid in CM and CR chondrites. We used this novel analytical protocol and collective data to shed new light on the prebiotic origins of chondritic MCAs.

Abstract-Studies of two separate stones of the CV3 chondrite Vigarano have revealed the presence of previously unreported occurrences of calcite. In the first stone, calcite occurs as thin veins in a type B CAI. In contrast, observations of the second stone, which was recovered one month after its fall, show three calcite occurrences: networks of veins, vesicle fillings in the fusion crust, and pseudomorphic replacement of augite associated with a porphyritic olivine chondrule. The most common occurrence is as veins ranging in thickness from <1 μm to 25 μm and extending for more than several hundred μm. Some veins crosscut the fusion crust and are connected to a carbonate coating on the exterior of the meteorite. Extensive minor element zoning occurs in carbonate masses, indicating variations in the fluid composition and/or redox potential during carbonate growth. Based on the textural evidence and a comparative study with carbonate veins in the CV3 chondrite Leoville, we conclude that the veins are terrestrial in origin. We propose a model for rapid carbonate formation in which calcite precipitation is driven by hydrolysis and oxidation in the meteorite interior that move the fluid composition to alkaline values. In addition, both stones also contain minor occurrences of carbonate that are not readily explained by terrestrial alteration. Minor carbonate in a type B CAI occurs in the first stone and calcite occurs as pseudomorphic replacement of augite in the second stone. Both of these occurrences appear to be preterrestrial, probably asteroidal in origin.