Chondrule cooling rates inferred from diffusive profiles in metal lumps from the Acfer 097 CR2 chondrite

Abstract— CR chondrites contain metal lumps (>300 μm) either attached to chondrule silicates or apparently isolated in the matrix. Here, laser ablation microanalysis of six metal lumps from a polished thin section of the Acfer 097 CR2 chondrite at 15 μm spatial resolution revealed zoning profiles for the volatile elements Cu and Ga. The mutual diffusivities of Cu and Ga were used to infer T ～ 1473 ± 100 K from the correlation of Cu versus Ga. The cooling rates of the metal lumps were calculated to be 0.5–50 K h^?1 for T_p ～ 1473 ± 100 K, with a maximum possible range of 0.1–400 K h^?1 for T_p ～ 1200–1800 K, overlapping the range of cooling rates inferred from petrological studies of type I chondrules (10–1000 K h^?1). Chondrule textures were established near the peak heating temperatures of chondrules (approximately 1900–2000 K), while the Cu and Ga diffusive profiles were established after solidification (T ～ 1500 K), consistent with nonlinear cooling. Furthermore, one chondrule (N2) has a more complex zoning profile that is modeled as a three‐stage cooling history involving initial cooling at approximately 1 K h^?1, followed by mild re‐heating (T ～ 1700 K) that re‐evaporated Cu and Ga from the outer approximately 100 μm of the metal lump and then cooled again at approximately 5 K h^?1. The thermal effects of parent body and other preaccretionary heating events on the Cu and Ga zoning profiles are examined. Although CR parent bodies have experienced aqueous alteration, the thermal effects of this process can neither produce nor erase the Cu and Ga diffusive profiles. Thus, metal lumps in CR chondrites record the solid‐state thermal history of chondrules as they travelled away from the chondrule‐forming region.     

Refractory inclusions in the unique carbonaceous chondrite Acfer 094

Abstract– Acfer 094 is an unshocked, nearly unaltered carbonaceous chondrite with an unusual suite of refractory inclusions. The refractory inclusions in a newly prepared thin section and a small aliquot of disaggregated material were studied to compare the population with previous work, and to report new or unusual inclusion types. A total of 289 Ca‐, Al‐rich inclusions in the thin section and 67 among the disaggregated material, having a total of 31 different mineral assemblages, were found. Inclusions are largely free of secondary alteration products, and are typically ≤200 μm across. The most common are gehlenitic melilite+spinel±perovskite, spinel+perovskite, and spinel with a thin, silicate rim, typically melilite±diopside. Such rims and (thicker) mantles are very common among Acfer 094 inclusions, and they exhibit a variety of zoning patterns with respect to åkermanite and FeO contents. In the thin section, about 13% of the inclusions contain hibonite and approximately 5% are grossite‐bearing; in the disaggregated material, the percentages are 14 and 9, respectively, comparable to previous work. Among the unusual inclusions are a fine‐grained, porous, Ti‐rich hibonite+spinel+perovskite+melilite inclusion with a compact, coarse, Ti‐poor hibonite+spinel+melilite clast; two inclusions in which hibonite has reacted to form grossite; two inclusions with FeO‐rich spinel; and a small object consisting of fassaite enclosing euhedral spinel, the first fragment of a Type B inclusion reported from Acfer 094. Inclusions similar to those found in CM or CV chondrites are rare; Acfer 094 contains a distinctive population of inclusions. The population, dominated by small, melilite‐bearing inclusions, is most similar to that of CO chondrites. A distinguishing feature is that in Acfer 094, almost every phase in almost every refractory inclusion contains 0.5–1.5 wt% FeO. A lack of diffusion gradients and the pristinity of the matrix imply that the inclusions experienced prolonged exposure to FeO‐bearing fluid prior to accretion into the Acfer 094 parent body. There are no known nebular conditions under which the refractory phases found in the present samples could acquire FeO enrichments to the observed levels. The most likely setting is therefore in an earlier, FeO‐rich parent body. The inclusions were ejected from this parent body, mixed with typical CAIs, chondrules, amoeboid olivine aggregates, and amorphous material, and incorporated into the Acfer 094 parent body.     

Unusual dark clasts in the Vigarano CV3 carbonaceous chondrite: Record of parent body process

Abstract— Two unusual dark clasts found in the Vigarano CV3 chondrite were examined using an optical microscope and a scanning electron microscope (SEM). Both clasts lack chondrules, Ca-Al-rich inclusions, and coarse-grained mineral fragments; they, instead, contain abundant inclusions that consist of fine grains (<1 μm) of homogeneous Fe-rich olivine, thus resembling the fine-grained variety of dark inclusions in CV3 chondrites. The external shapes of inclusions in the clasts bear a close resemblance to those of chondrules and chondrule fragments; some of the inclusions are surrounded by dark rims similar to chondrule rims. Our SEM observations reveal the following unusual characteristics: 1) the inclusions are not mere random aggregates of olivine grains but have peculiar internal textures, that is, assemblages of round or oval shaped outlines, which are suggestive of pseudomorphs after porphyritic olivine chondrules; 2) one of thick inclusion rims contains a network of vein-like strings of elongated olivine grains; 3) an Fe-Ni metal aggregate in one of the clasts has an Fe-, Ni-, S-rich halo suggesting a reaction between its precursor and the surrounding matrix; and 4) olivine in the clasts commonly shows a swirly, fibrous texture similar to that of phyllosilicate. These characteristics suggest that the dark clasts in Vigarano are not primary aggregates of dust in the solar nebula but were affected by aqueous alteration and subsequent dehydration by heating after accretion to the meteorite parent body. The fine olivine grains in these clasts were presumably produced by thermal transformation of phyllosilicate, as is the case with those in the two thermally metamorphosed Antarctic CM chondrites, Belgica-7904 and Yamato-86720. From textural and mineralogical similarities, some of the dark inclusions and clasts previously reported from CV3 chondrites and other types of meteorites may have origins common with these clasts in Vigarano.     

Carbonaceous chondrite clasts in the howardites Bholghati and EET87513

Abstract— Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.     

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine-pyroxene-phyllosilicate-sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel-phyllosilicate-diopside calcium-aluminum-rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine-grained rims of material resembling matrix. The opaque, fine-grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material. The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix-supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low-Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca-carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine-grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly-crystalline material with the same bulk composition as matrix phyllosilicate. There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C. We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM-like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type-2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.     

Acfer 217-A new member of the Rumuruti chondrite group (R)

Abstract— Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) ?75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes-type” chondrites (Weisberg et al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y-793575, Y-82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulze et al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R). Acfer 217 is a regolith breccia consisting of up to cm-sized clasts (～33 vol%) embedded in a fine-grained, well-lithified clastic matrix. The most abundant mineral is olivine (～72 vol%), which has a high Fa-content of 37–39 mol%. The major minerals (olivine, low-Ca pyroxene, Ca-pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa_{37.8 ± 5.7}, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2). The bulk composition of Acfer 217 and other R-meteorites show that the R-meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y-75302. In a δ¹⁷O vs. δ¹⁸O-diagram, the R-meteorites form a group well resolved from other chondrite groups. Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. Based on ²¹Ne, an exposure age of about 35 Ma was calculated.     

Silica‐rich objects in the Acfer 182 CH chondrite: A new view

Five silica‐rich objects (SRO) from Acfer 182 were studied. They have cryptocrystalline textures characterized by micro‐emulsion and amoeboid patterns that point toward the coexistence of pyroxene‐ and silica‐normative liquids that were quenched. Both objects have variable contents of refractory lithophile elements. Their positive Yb versus La correlation around primordial values suggests a cosmochemical process (e.g., a gas/liquid condensation) as responsible for SRO formation. The bulk trace element abundances of amoeboid‐ and emulsion‐type SRO as well as their fractionation do not support an origin through high temperature processes. Conversely, their formation might have taken place while cooling of the nebular gas in two different chondrule‐forming regions characterized by having different evolution paths. Cooling of these dust‐enriched regions might lead to the condensation of pyroxene‐rich liquids first, followed by formation of Mg‐rich and SiO₂‐rich liquids, provided irradiation and annealing were active in these regions. Irradiation could be the process involved both in the formation of cristobalite (with annealing ~1200 K) and in triggering a spinoidal decomposition causing unmixing of the enstatite liquid into two coexisting phases, such as Mg‐rich and SiO₂‐rich liquids, the precursors of the SRO in Acfer 182. Formation of emulsion‐ and amoeboid‐type objects may be the result of exposing those chondrule‐forming regions to different degrees of radiation.     

The GRO 95577 CR1 chondrite and hydration of the CR parent body

Abstract— We carried out a petrologic and instrumental neutron activation analysis (INAA) whole chondrite compositional study of Grosvenor Mountains (GRO) 95577. GRO 95577 has many petrological similarities to the CR chondrites. Although the INAA data show patterns indicative of terrestrial weathering, some of the elemental abundances are consistent with a relationship to CR chondrites. The oxygen isotopic composition of GRO 95577 plots close to the Renazzo CR chondrite on the three‐isotope diagram. However, GRO 95577 is remarkable in that the chondrules are completely hydrated, consisting almost entirely of phyllosilicates, magnetite, and sulfides. Although GRO 95577 is completely hydrated, the initial chondrule textures are perfectly preserved. The chondrules are in sharp contact with the matrix, their fine‐grained rims are clearly visible, and the boundaries of the dark inclusions can be easily discerned. Many chondrules in GRO 95577 have textures suggestive of type I chondrules, but the phenocrysts have undergone perfect pseudomorphic replacement by yellow to brownish serpentine‐rich phyllosilicate, with sharp original crystal outlines preserved. The chondrule mesostasis is a green aluminous chlorite‐rich material, most likely a hydration product of the feldspathic mesostasis commonly found in anhydrous type I chondrules. Some chondrules contain magnetite spheres, most likely formed by oxidation of metal. We propose that GRO 95577 be classified as a CR1 chondrite, making it the first known CR1 chondrite and expanding the range of alteration conditions on the CR parent body.     

Shapes of chondrules determined from the petrofabric of the CR2 chondrite NWA 801

The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X-ray computed tomography and best-fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro-deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3-D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation.     

Iron isotopic measurements in presolar silicate and oxide grains from the Acfer 094 ungrouped carbonaceous chondrite

We carried out Fe isotopic analyses on 21 O‐rich presolar grains from the Acfer 094 ungrouped carbonaceous chondrite. Presolar grains were identified on the basis of oxygen isotopic ratios, and elemental compositions were measured by Auger spectroscopy. The Fe isotopic measurements were carried out by analyzing the Fe isotopes as negative secondary oxides with the NanoSIMS to take advantage of the higher spatial resolution of the Cs⁺ primary ion beam. Our results demonstrate the effectiveness of this approach for measuring both ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe. The ion yield for FeO^– is significantly lower than for Fe⁺, but this is not a serious limitation for presolar silicate grains with Fe as a major element. Most of the grains analyzed are ferromagnesian silicates, but we also measured four oxide grains. Iron contents are high in all of the grains, ranging from 10 to 40 atom%. Three of the grains belong to oxygen isotope Group 4. All of them have ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe ratios that are solar within errors, consistent with an origin in the outer zones of a Type II supernova, as indicated by their oxygen isotopic compositions. The remaining grains belong to oxygen isotope Group 1, with origins in low‐mass AGB stars. The majority of these also have solar ⁵⁴Fe/⁵⁶Fe and ⁵⁷Fe/⁵⁶Fe ratios. However, four grains are depleted in ⁵⁷Fe; one is also slightly depleted in ⁵⁴Fe. Current AGB models predict excesses in ⁵⁷Fe with ⁵⁴Fe/⁵⁶Fe ratios that largely reflect the metallicity of the parent star. While the solar ⁵⁷Fe/⁵⁶Fe ratios are consistent with formation of the grains in early third dredge‐up episodes, these models cannot account for the grains with ⁵⁷Fe depletions. Comparison with galactic evolution models suggests formation of these grains from stars with significantly subsolar metallicity; however, these models also predict large depletions in ⁵⁴Fe, which are not observed in the grains. Thus, the isotopic compositions of these grains remain unexplained.     

Matrix and whole‐rock fractionations in the Acfer 094 type 3.0 ungrouped carbonaceous chondrite

Abstract– We used the electron microprobe to study matrix in the ungrouped type 3.0 carbonaceous chondrite Acfer 094 using 7 × 7‐point, focused‐beam arrays; data points attributable to mineral clasts were discarded. The grid areas show resolvable differences in composition, but differences are less pronounced than we observed in studies of CR2 LaPaz Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley [1993]). A key question is why Acfer shows an anomalously uniform composition of matrix compared with these other carbonaceous chondrites. Both whole‐rock and matrix samples of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination during hot desert weathering. By contrast, the whole‐rock abundance of Na is low. Although weathering effects are responsible for some fractionations, it appears that nebular effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we infer that the observed differences among different areas were inherited from the solar nebula and may have been carried by porous chondrules that experienced low (about 20%) degrees of melting. Acfer 094 has been comminuted by one or more impact events that may also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine in other respects. This evidence of comminution and an O‐isotopic composition similar to values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before being outgassed by the inferred impact event. Convection within the plume associated with the impact event probably also contributed to the homogenization of the Acfer 094 matrix.     

Chemical composition and iron oxidation state of amorphous matrix silicates in the carbonaceous chondrite Acfer 094

Nanoscale amorphous silicates are a major component in primitive carbonaceous chondrite matrices and anhydrous interplanetary dust particles. Owing to their metastability and sensitive response to reactions with water, this material is of particular interest in understanding nebular and parent body processes in the early solar system. Here we investigated the amorphous silicate matrix (ASM) in the ungrouped carbonaceous chondrite Acfer 094 regarding its texture, chemical composition, and Fe oxidation state. We applied transmission electron microscopy techniques on six, focused ion beam technique-prepared, electron-transparent lamellae of Acfer 094 to determine the textures of this material. Furthermore, we used energy-dispersive X-ray analysis and electron energy loss spectroscopy to quantify the Fe content and the Fe oxidation state of the ASM. Textural investigations reveal differences in sulfide content, porosity, and distribution of the ASM among the samples, as well as evidence for rare recrystallization of phyllosilicate fibers. The chemical composition reveals mobilization of Fe. Furthermore, the determined Fe³⁺/ΣFe ratios of the ASM in the six samples display a homogeneously high oxidation state (0.66–0.73). This high and homogeneous Fe oxidation state in the ASM of Acfer 094 disagrees with its formation as a primary phase in a reduced solar gas and must have been induced in a later stage process. Most likely, this process was aqueous alteration on the Acfer 094 parent body, which led to hydration and oxidation of the ASM, which is supported by textural and chemical evidence of aqueous alteration.     

Origin of fayalitic olivine rims and lath-shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa_{42 ± 2}) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa_{39 ± 2}) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa_{41 ± 1}) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.     

A detailed mineralogical,petrographic, and geochemical study of the highly reduced chondrite,Acfer 370

Among the many ungrouped meteorites, Acfer 370, NWA 7135, and El Médano 301—probably along with the chondritic inclusion in Cumberland Falls and ALHA 78113—represent a homogeneous grouplet of strongly reduced forsterite‐rich chondrites characterized by common textural, chemical, mineralogical, and isotopic features. All of these meteorites are much more reduced than OCs, with a low iron content in olivine and low‐Ca pyroxene. In particular, Acfer 370 is a type 4 chondrite that has olivine and low‐Ca pyroxene compositional ranges of Fa 5.2–5.8 and Fs 9.4–33.4, respectively. The dominant phase is low‐Ca pyroxene (36.3 vol%), followed by Fe‐Ni metal (16.3 vol%) and olivine (15.5 vol%); nevertheless, considering the Fe‐oxyhydroxide (due to terrestrial weathering), the original metal content was around 29.6 vol%. Finally, the mean oxygen isotopic composition Δ¹⁷O = +0.68‰ along with the occurrence of a silica phase, troilite, Ni‐rich phosphides, chromite, and oldhamite confirms that these ungrouped meteorites have been affected by strong reduction and are different from any other group recognized so far.     

On the origin of shocked and unshocked CM clasts in H‐chondrite regolith breccias

Abstract— CM chondrite clasts that have experienced different degrees of aqueous alteration occur in H‐chondrite and HED meteorite breccias. Many clasts are fragments of essentially unshocked CM projectiles that accreted at low relative velocities to the regoliths of these parent bodies. A few clasts were heated and dehydrated upon impact; these objects most likely accreted at higher relative velocities. We examined three clasts and explored alternative scenarios for their formation. In the first scenario, we assumed that the H and HED parent bodies had diameters of a few hundred kilometers, so that their high escape velocities would effectively prevent soft landings of small CM projectiles. This would imply that weakly shocked CM clasts formed on asteroidal fragments (family members) associated with the H and HED parent bodies. In the second scenario, we assumed that weakly shocked CM clasts were spall products ejected at low velocities from larger CM projectiles when they slammed into the H and HED parent bodies. In both cases, if most CM clasts turn out to have ancient ages (e.g., ?3.4‐4.1 Ga), a plausible source for their progenitors would be outer main belt objects, some which may have been dynamically implanted 3.9 Ga ago by the events described in the so‐called “Nice model.” On the other hand, if most CM clasts have recent ages (<200 Ma), a plausible source location for their parent body would be the inner main belt between 2.1–2.2 AU. In that case, the possible source of the CM‐clasts' progenitors' parent fragments would be the breakup ?160 Ma ago of the parent body 170 km in diameter of the Baptistina asteroid family (BAF).     

Origin of the metamorphosed clasts in the CV3 carbonaceous chondrite breccias of Graves Nunataks 06101, Vigarano,Roberts Massif 04143, and Yamato‐86009

We observed metamorphosed clasts in the CV3 chondrite breccias Graves Nunataks 06101, Vigarano, Roberts Massif 04143, and Yamato‐86009. These clasts are coarse‐grained polymineralic rocks composed of Ca‐bearing ferroan olivine (Fa_24–40, up to 0.6 wt% CaO), diopside (Fs_7–12Wo_44–50), plagioclase (An_52–75), Cr‐spinel (Cr/[Cr + Al] = 0.4, Fe/[Fe + Mg] = 0.7), sulfide and rare grains of Fe‐Ni metal, phosphate, and Ca‐poor pyroxene (Fs₂₄Wo₄). Most clasts have triple junctions between silicate grains. The rare earth element (REE) abundances are high in diopside (REE ~3.80–13.83 × CI) and plagioclase (Eu ~12.31–14.67 × CI) but are low in olivine (REE ~0.01–1.44 × CI) and spinel (REE ~0.25–0.49 × CI). These REE abundances are different from those of metamorphosed chondrites, primitive achondrites, and achondrites, suggesting that the clasts are not fragments of these meteorites. Similar mineralogical characteristics of the clasts with those in the Mokoia and Yamato‐86009 breccias (Jogo et al. 2012 ) suggest that the clasts observed in this study would also form inside the CV3 chondrite parent body. Thermal modeling suggests that in order to reach the metamorphosed temperatures of the clasts of >800 °C, the clast parent body should have accreted by ~2.5–2.6 Ma after CAIs formation. The consistency of the accretion age of the clast parent body and the CV3 chondrule formation age suggests that the clasts and CV3 chondrites could be originated from the same parent body with a peak temperature of 800–1100 °C. If the body has a peak temperature of >1100 °C, the accretion age of the body becomes older than the CV3 chondrule formation age and multiple CV3 parent bodies are likely.     

Origin of the differences in refractory-lithophile-element abundances among chondrite groups

Chondrite groups can be distinguished on the basis of their abundances of refractory lithophile elements (RLE). These abundances are, in part, functions of the mass fraction of Ca-Al-rich inclusions (CAIs) within the chondrites. Carbonaceous chondrites contain the most CAIs and the highest RLE abundances; they also contain modally abundant fine-grained matrix material that consists largely of modified nebular dust. The amount of dust varied throughout the solar nebula: enstatite and ordinary chondrites formed in low-dust regions in the inner part of the nebula, R chondrites formed in higher-dust zones at somewhat greater heliocentric distances, and carbonaceous chondrites formed in even dustier regions farther from the Sun. The amount of ambient dust peaked in the region where CV and CK chondrites accreted; these chondrites have abundant matrix, the highest modal abundances of CAIs, and the highest bulk RLE contents. Substantial amounts of nebular dust occurred in highly porous multi-millimeter-to-centimeter-size dustballs that were on the order of 100 times more massive than CAIs. Radial drift processes in the nebula affected these dustballs to approximately the same extent as the CAIs; both types of objects were aerodynamically concentrated in the same nebular regions. These regions maintained approximately the same relative amounts of dust through the periods of chondrule formation and chondrite accretion.     

Opaque assemblages in CR2 Graves Nunataks (GRA) 06100 as indicators of shock‐driven hydrothermal alteration in the CR chondrite parent body

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 Ni‐poor 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 impact‐driven 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 Ni‐rich, 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.     

Mineralogy and petrology of dark clasts in the Allan Hills 76005 polymict eucrite pairing group

The Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h^?1. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.     

Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites

Cluster chondrites are characterized by close‐fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012 ). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI‐normalized REE patterns of the clasts are flat, showing LL‐chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al₂O₃ and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3‐isotope diagram, interpreted as the result of O‐isotope exchange between chondrule melts and ¹⁸O‐rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ¹⁸O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ¹⁸O values of type I chondrules are correlated with concentrations of SiO₂ and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO₂‐ and MnO‐rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.