An amoeboid olivine inclusion (AOI) in CK3 NWA 1559, comparison to AOIs in CV3 Allende,and the origin of AOIs in CK and CV chondrites

An amoeboid olivine inclusion in CK3 NWA 1559 (0.54 × 1.3 mm) consists of a diopside‐rich interior (approximately 35 vol%) and an olivine‐rich rim (approximately 65 vol%). It is the first AOI to be described in CK chondrites; the apparent paucity of these inclusions is due to extensive parent‐body recrystallization. The AOI interior contains irregular 3–15 μm‐sized Al‐bearing diopside grains (approximately 70 vol%), 2–20 μm‐sized pores (approximately 30 vol%), and traces of approximately 2 μm plagioclase grains. The 75–160 μm‐thick rim contains 20–130 μm‐sized ferroan olivine grains, some with 120º triple junctions. A few coarse (25–50 μm‐sized) patches of plagioclase with 2–18 μm‐thick diopside rinds occur in several places just beneath the rim. The occurrence of olivine rims around AOI‐1 and around many AOIs in CV3 Allende suggests that CK and CV AOIs formed by the acquisition of porous forsteritic rims around fine‐grained, rimless CAIs that consisted of diopside, anorthite, melilite, and spinel. Individual AOIs in carbonaceous chondrites may have formed after transient heating events melted their olivine rims as well as portions of the underlying interiors. In AOI‐1, coarse plagioclase grains with diopside rinds crystallized immediately below the olivine rim. Secondary parent‐body alteration transformed forsterite in the rims of CV and CK AOIs into more‐ferroan olivine. Some of the abundant pores in the interior of AOI‐1 may have formed during aqueous alteration after fine‐grained melilite and anorthite were leached out. Chondrite groups with large chondrules tend to have large AOIs. AOIs that formed in dust‐rich nebular regions (where CV and CK chondrites later accreted) tend to be larger than AOIs from less‐dusty regions.     

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.     

Fractionated matrix composition in CV3 Vigarano and alteration processes on the CV parent asteroid

Abstract— Although CV3 Vigarano is one of the most primitive CV chondrites, it has lost most of the S from the matrix; matrix Na is also depleted relative to the concentration in bulk CV chondrites. We used a matrix‐grid technique to study thirteen 50 × 50 μm regions in Vigarano; in each area, we used an electron microprobe to gather data with an electron beam 3 μm in width. We found two end‐member types of matrix textures. One is coarse and porous, has lower Fe contents and lower analytical totals; it appears to be contaminated with comminuted chondrule debris. The other is finer grained and appears smooth; its mean composition has higher Fe, but lower S and Al contents, than the coarse matrix areas. Our tentative interpretation is that the larger grain size of the coarse areas resulted from the admixing of comminuted chondrule materials, and thus that the initial fraction of nebular fines was higher in the fine matrix regions. Aside from volatiles, the overall composition of Vigarano matrix is similar to that observed in matrix‐grid studies of other carbonaceous chondrites: Al, Si, Fe, and Mn have high whole‐chondrite‐normalized abundance ratios; Ca concentrations are low and highly variable. Because asteroidal alteration effects are present in our sample, it is difficult to resolve nebular signatures in the compositions of the grid areas.     

NWA 10214—An LL3 chondrite breccia with an assortment of metamorphosed,shocked, and unique chondrite clasts

NWA 10214 is an LL3‐6 breccia containing ~8 vol% clasts including LL5, LL6, and shocked‐darkened LL fragments as well as matrix‐rich Clast 6 (a new kind of chondrite). This clast is a dark‐colored, subrounded, 6.1 × 7.0 mm inclusion, consisting of 60 vol% fine‐grained matrix, 32 vol% coarse silicate grains, and 8 vol% coarse opaque grains. The large chondrules and chondrule fragments are mainly Type IB; one small chondrule is Type IIA. Also present are one 450 × 600 μm spinel‐pyroxene‐olivine CAI and one 85 × 110 μm AOI. Clast 6 possesses a unique set of properties. (1) It resembles carbonaceous chondrites in having relatively abundant matrix, CAIs, and AOIs; the clast's matrix composition is close to that in CV3 Vigarano. (2) It resembles type‐3 OC in its olivine and low‐Ca pyroxene compositional distributions, and in the Fe/Mn ratio of ferroan olivine grains. Its mean chondrule size is within 1σ of that of H chondrites. The O‐isotopic compositions of the chondrules are in the ordinary‐ and R‐chondrite ranges. (3) It resembles type‐3 enstatite chondrites in the minor element concentrations in low‐Ca pyroxene grains and in having a high low‐Ca pyroxene/olivine ratio in chondrules. Clast 6 is a new variety of type‐3 OC, somewhat more reduced than H chondrites or chondritic clasts in the Netschaevo IIE iron; the clast formed in a nebular region where aerodynamic radial drift processes deposited a high abundance of matrix material and CAIs. A chunk of this chondrite was ejected from its parent asteroid and later impacted the LL body at low relative velocity.     

Fayalitic olivine in CV3 chondrite matrix and dark inclusions: A nebular origin

Abstract— Fayalitic olivine (Fa₃₂) is the major component of the matrices and dark inclusions of CV3 and other unequilibrated chondrites. It occurs most commonly as rims, veins and halos in and around chondrule silicates in the Allende-type (CV3_OXA) chondrites and, to a much lesser extent, in the reduced (CV3R) and Bali-type (CV3OXB) chondrites. The olivines have distinctive platy, tabular and lath- or irregular-shaped crystals, with the ratio of the two types varying widely. In CV3_OXB chondrites, matrix fayalitic olivines range up to Fag_99.9; whereas, in the other CV3 chondrites, the range is much smaller. The platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor, and the nature of the occurrences suggests that CV3 matrices are unequilibrated mixtures of nebular materials. We argue that the parent body hydration/dehydration model has numerous inconsistencies that make this hypothesis highly unlikely. These include: (1) There is no direct evidence linking fayalitic olivine to precursor phyllosilicates. (2) Dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines. (3) Fayalitic olivine clearly predates the formation of the hydrous phases in CV3 chondrites and is one of the phases that breaks down to form phyllosilicates (Keller et al., 1994). (4) The unequilibrated nature of the matrix, including fine-scale zoning in 10 μm sized fayalitic olivine crystals, would not survive the parent body metamorphism required in the dehydration model. (5) A dark inclusion in the Ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material (Zolensky et al., 1997), which probably formed by radiation damage. This indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting. (6) Some Allende chondrules contain unaltered primary, anhydrous glassy mesostasis in contact with the host matrix (e.g., Ikeda and Kimura, 1995). Chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario. (7) Single platy and barrel-shaped crystals of fayalitic olivine are present in accretionary rims in calcium-aluminum-rich inclusions (CAIs) (MacPherson and Davis, 1997), which developed in the nebula. (8) Matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine (Scott et al., 1984), which indicates a nebular origin. (9) Oxygen isotopic compositions of Allende matrix and dark inclusions strongly indicate little or no hydration for Allende and its components (Clayton, 1997). We favor a nebular vaporization/recondensation model in which vaporization of chondritic dust produced a fayalite-rich vapor, followed by formation of the fayalitic olivine by direct recondensation from the vapor, epitactic growth on surfaces of existing forsterite and enstatite in chondrules, and replacement of existing forsterite and enstatite by gas-solid exchange.     

A clast of Bali‐like oxidized CV material in the reduced CV chondrite breccia Vigarano

Abstract— The CV (Vigarano‐type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali‐like and Allende‐like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997; Krot et al., 1998b). Some chondrules and calcium‐aluminum‐rich inclusions (CAIs) in the reduced CV chondrite Vigarano show alteration features similar to those in Allende: metal is oxidized to magnetite; low‐Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs_40–50); and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et al., 1993; Kimura and Ikeda, 1996, 1997, 1998). Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments, and lithic clasts of the Bali‐like oxidized CV materials. The largest lithic clast (about 1 times 2 cm in size) is composed of opaque matrix, type‐I chondrules (400–2000 μm in apparent diameter) surrounded by coarse‐grained and fine‐grained rims, and rare CAIs. The matrix‐chondrule ratio is about 1.1. Opaque nodules in chondrules in the clast consist of Cr‐poor and Cr‐rich magnetite, Ni‐ and Co‐rich metal, Ni‐poor and Ni‐rich sulfide; low‐Ni metal nodules occur only inside chondrule phenocrysts. Chromium‐poor magnetite is preferentially replaced by fayalite. Chondrule mesostases are replaced by phyllosilicates; low‐Ca pyroxene and olivine phenocrysts appear to be unaltered. Matrix in the clast consists of very fine‐grained (<1 μm) ferrous olivine, anhedral fayalite grains (Fa_80–100), rounded objects of porous Ca‐Fe‐rich pyroxenes (Fs_10–50Wo₅₀), Ni‐poor sulfide, Ni‐ and Co‐rich metal, and phyllosilicates; magnetite is rare. On the basis of the presence of the Bali‐like lithified chondritic clast—in addition to individual chondrules and CAIs of both Bali‐like and Allende‐like materials—in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously altered asteroid.     

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

Abstract— 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_<10) and a refractory, Ca, Al‐rich component. The AOAs are depleted in moderately volatile elements (Mn, Cr, Na, K), Fe, Ni‐metal and sulfides and contain no low‐Ca pyroxene. The refractory component consists of fine‐grained calcium‐aluminum‐rich inclusions (CAIs) composed of Al‐diopside, anorthite (An₁₀₀), and magnesium‐rich spinel (～1 wt% FeO) or fine‐grained intergrowths of these minerals; secondary nepheline and sodalite are very minor. This indicates that AOAs from the reduced CV chondrites are more pristine than those from the oxidized CV chondrites Allende and Mokoia. Although AOAs from the reduced CV chondrites show evidence for high‐temperature nebular annealing (e.g., forsterite grain boundaries form 120° triple junctions) and possibly a minor degree of melting of Al‐diopside‐anorthite materials, none of the AOAs studied appear to have experienced extensive (>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.     

Amorphous silicates as a record of solar nebular and parent body processes—A transmission electron microscope study of fine‐grained rims and matrix in three Antarctic CR chondrites

Renazzo‐type (CR) carbonaceous chondrites belong to one of the most pristine meteorite groups containing various early solar system components such as matrix and fine‐grained rims (FGRs), whose formation mechanisms are still debated. Here, we have investigated FGRs of three Antarctic CR chondrites (GRA 95229, MIL 07525, and EET 92161) by electron microscopy techniques. We specifically focused on the abundances and chemical compositions of the amorphous silicates within the rims and matrix by analytical transmission electron microscopy. Comparison of the amorphous silicate composition to a matrix area of GRA 95229 clearly shows a compositional relationship between the matrix and the fine‐grained rim, such as similar Mg/Si and Fe/Si ratios. This relationship and the abundance of the amorphous silicates in the rims strengthen a solar nebular origin and rule out a primary formation mechanism by parent body processes such as chondrule erosion. Moreover, our chemical analyses of the amorphous silicates and their abundance indicate that the CR rims experienced progressive alteration stages. According to our analyses, the GRA 95229 sample is the least altered one based on its high modal abundance of amorphous silicates (31%) and close‐to‐chondritic Fe/Si ratios, followed by MIL 07525 and finally EET 92161 with lesser amounts of amorphous silicates (12% and 5%, respectively) and higher Fe/Si ratios. Abundances and chemical compositions of amorphous silicates within matrix and rims are therefore suitable recorders to track different alteration stages on a submicron scale within variably altered CR chondrites.     

Microdistribution of primordial Ne and Ar in fine‐grained rims,matrices, and dark inclusions of unequilibrated chondrites—Clues on nebular processes

Abstract— The low temperature fine‐grained material in unequilibrated chondrites, which occurs as matrix, rims, and dark inclusions, carries information about the solar nebula and the earliest stages of planetesimal accretion. The microdistribution of primordial noble gases among these components helps to reveal their accretionary and alteration histories. We measured the Ne and Ar isotopic ratios and concentrations of small samples of matrix, rims, and dark inclusions from the unequilibrated carbonaceous chondrites Allende (CV3), Leoville (CV3), and Renazzo (CR2) and from the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1) to decipher their genetic relationships. The primordial noble gas concentrations of Semarkona, and—with certain restrictions—also of Leoville, Bishunpur, and Allende decrease from rims to matrices. This indicates a progressive accretion of nebular dust from regions with decreasing noble gas contents and cannot be explained by a formation of the rims on parent bodies. The decrease is probably due to dilution of the noble‐gas‐carrying phases with noble‐gas‐poor material in the nebula. Krymka and Renazzo both show an increase of primordial noble gas concentrations from rims to matrices. In the case of Krymka, this indicates the admixture of noble gas‐rich dust to the nebular region from which first rims and then matrix accreted. This also explains the increase of the primordial elemental ratio 36Ar/ 20Ne from rims to matrix. Larger clasts of the noble‐gas‐rich dust form macroscopic dark inclusions in this meteorite, which seem to represent unusually pristine material. The interpretation of the Renazzo data is ambiguous. Rims could have formed by aqueous alteration of matrix or—as in the case of Krymka—by progressive admixture of noble gas‐rich dust to the reservoir from which the Renazzo constituents accreted. The Leoville and Krymka dark inclusions, as well as one dark inclusion of Allende, show noble gas signatures different from those of the respective host meteorites. The Allende dark inclusion probably accreted from the same region as Allende rims and matrix but suffered a higher degree of alteration. The Leoville and Krymka dark inclusions must have accreted from regions different from those of their respective rims and matrices and were later incorporated into their host meteorites. The noble gas data imply a heterogeneous reservoir with respect to its primordial noble gas content in the accretion region of the studied meteorites. Further studies will have to decide whether these differences are primary or evolved from an originally uniform reservoir.     

LIME silicates in amoeboid olivine aggregates in carbonaceous chondrites: Indicator of nebular and asteroidal processes

MnO/FeO ratios in olivine from amoeboid olivine aggregates (AOAs) reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites. LIME (low‐iron, Mn‐enriched) olivine was identified in AOAs from Y‐81020 (CO3.05), Kaba (CV~3.1), and in Y‐86009 (CV3), Y‐86751 (CV3), NWA 1152 (CR/CV3), but was not identified in AOAs from Efremovka (CV3.1–3.4) or Allende (CV>3.6). According to thermodynamic models of nebular condensation, LIME olivine is stable at lower temperatures than Mn‐poor olivine and at low oxygen fugacities (dust enrichment <10× solar). Although this set of samples does not represent a single metamorphic sequence, the higher subtypes tend to have AOA olivine with lower Mn/Fe, suggesting that Mn/Fe decreases during parent body metamorphism. Y‐81020 has the lowest subtype and most forsteritic AOA olivine (Fo_>95) in our study, whereas Efremovka AOAs are slightly Fe‐rich (Fo_>92). AOA olivines from Kaba are mostly forsteritic, but rare Fe‐rich olivine precipitated from an aqueous fluid. A combination of precipitation of Fe‐rich olivine and diffusion of Fe into primary olivine grains resulted in iron‐rich compositions (Fo_97–59) in Allende AOAs. Variations from fine‐grained, nonporous matrix toward higher porosity and coarser lath‐like matrix olivine can be divided into six stages represented by (1) Y‐81020, Efremovka, NWA 1152; (2) Y‐86751 lithology B; (3) Y‐86009; (4) Kaba; (5) Y‐86751 lithology A; (6) Allende. These stages are inferred to represent general degree of metamorphism, although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain.     

Petrology and mineralogy of the Yamato-86751 CV3 chondrite

Abstract— The Y-86751 chondrite (CV3) consists of fine-grained Ca- and Al-rich inclusions (CAIs), amoeboid olivine inclusions (AOIs), spinel-rich inclusions, chondrules with and without dark rims, dark inclusions, isolated minerals, metal-sulfide aggregates, and matrix. Olivines in chondrules without dark rims and AOIs coexist with magnetite and show strong zoning from a magnesian core to a ferroan rim. Spinels in spinel-rich inclusions show similar zoning. This zoning seems to be caused by exchange reaction of olivine and spinel with an oxidized nebular gas prior to the accretion onto the parent body, and the Mg/Fe diffusion in olivines and spinels took place at a temperature of about 830–860 K. At the same time, enstatite in chondrules without dark rims was replaced by ferroan olivine at the grain boundaries. This feature suggests that chondrules without dark rims, fine-grained CAIs, spinel-rich inclusions, and AOIs have experienced oxidation in an oxidizing nebular gas. The oxygen fugacity of the oxidized nebular gas was >10^?27.3 bars at about 830 K, being more than 10⁴x larger than that of the canonical nebular gas. Magnetite occurs in the Y-86751 matrix in close association with Ni-rich taenite and Co-rich metal, and it was produced under a condition with the oxygen fugacity of ～10^?38 bars at a temperature of about 620–650 K. On the other hand, olivines in chondrules with dark rims and dark inclusions are magnesian and rich in MnO. They do not show such strong zoning. Probably they were in equilibrium with a nebular gas under a redox condition different from the oxidized nebular gas that produced the zoned olivines in chondrules without dark rims.     

Chondrules: Precursors and interactions with the nebular gas

Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.     

Aqueous alteration in the matrix of the Vigarano (CV3) carbonaceous chondrite

Abstract The matrix of Vigarano, a meteorite which belongs to the reduced subgroup of the CV3 chondrites, contains small amounts (<10%) of ferrihydrite and smectite. These hydrous minerals occur together as fine fibrous intergrowths between anhydrous silicate and oxide grains. Coarser crystals of ferrihydrite fill fractures that cut matrix minerals, and smectite also lines narrow channels within olivine grains. These channels may have formed by preferential alteration of olivines along (100)-parallel defects. Formation of ferrihydrite and smectite in the matrix of Vigarano was the result of mild aqueous alteration in a low-temperature (<150 °C), oxidising parent body environment. Partial equilibration of matrix olivines indicates that alteration was followed by thermal metamorphism with a peak temperature of 400–500 °C. Mineralogically similar alteration products, which also were formed by parent body processes, have previously been described from the matrices of four CV meteorites: Bali, Grosnaja, Kaba and Mokoia, all of which belong to the oxidised subgroup. This discovery of the products of oxidative aqueous alteration in Vigarano has important consequences for understanding the chemical and thermal history of the CV class of meteorites.     

Mineralogical characterization of primitive,type‐3 lithologies in Rumuruti chondrites

Abstract— Rumuruti (R) chondrites constitute a new, well‐established chondrite group different from the carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas‐rich regolith breccias showing the typical light‐dark structure and consist of abundant fragments of various parent‐body lithologies embedded in a fine‐grained olivine‐rich matrix. Unequilibrated type‐3 lithologies among these fragments have frequently been mentioned in various publications. In this study, detailed mineralogical data on seven primitive fragments from the R‐chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from ～300 μ in size up to several millimeters. Generally, the main characteristics can be summarized as follows: (1) Unequilibrated type‐3 fragments have a well‐preserved chondritic texture with a chondrule‐to‐matrix ratio of ～1:1. Chondrules and chondrule fragments are embedded in a fine‐grained olivine‐rich matrix. Thus, the texture is quite similar to that of type‐3 carbonaceous chondrites. (2) In all cases, matrix olivines in type‐3 fragments have a significantly higher Fa content (44–57 mol%) than olivines in other (equilibrated) lithologies (38–40 mol% Fa). (3) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa_0–65 and Fs_0–33, respectively) and, generally, more magnesian than those found in equilibrated R chondrites. Agglomerated material of the R‐chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg‐rich silicate constituents, and Fe‐rich highly oxidized fine‐grained materials. The dominating phase of this fine‐grained material may have been Fa‐rich olivine from the beginning. The brecciated whole rocks, the R‐chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg‐rich olivines and pyroxenes in primitive type‐3 fragments.     

Petrology and origin of amoeboid olivine aggregates in CR chondrites

Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.     

Northwest Africa 11024—A heated and dehydrated unique carbonaceous (CM) chondrite

Based on the high abundance of fine‐grained material and its dark appearance, NWA 11024 was recognized as a CM chondrite, which is also confirmed by oxygen isotope measurements. But contrary to known CM chondrites, the typical phases indicating aqueous alteration (e.g., phyllosilicates, carbonates) are missing. Using multiple analytical techniques, this study reveals the differences and similarities to known CM chondrites and will discuss the possibility that NWA 11024 is the first type 3 CM chondrite. During the investigation, two texturally apparent tochilinite–cronstedtite intergrowths were identified within two thin sections. However, the former phyllosilicates were recrystallized to Fe‐rich olivine during a heating event without changing the textural appearance. A peak temperature of 400–600 °C is estimated, which is not high enough to destroy or recrystallize calcite grains. Thus, calcites were never constituents of the mineral paragenesis. Another remarkable feature of NWA 11024 is the occurrence of unknown clot‐like inclusions (UCLIs) within fine‐grained rims, which are unique in this clarity. Their density and S concentration are significantly higher than of the surrounding fine‐grained rim and UCLIs can be seen as primary objects that were not formed by secondary alteration processes inside the rims. Similarities to chondritic and cometary interplanetary dust particles suggest an ice‐rich first‐generation planetesimal for their origin. In the earliest evolution, NWA 11024 experienced the lowest degree of aqueous alteration of all known CM chondrites and subsequently, a heating event dehydrated the sample. We suggest to classify the meteorite NWA 11024 as the first type 3 CM chondrite similar to the classification of CV3 chondrites (like Allende) that could also have lost their matrix phyllosilicates by thermal dehydration.     

Chondritic ingredients: I. Usual suspects and some oddballs in the Leoville CV3 meteorite

Abstract– Reduced CV3 chondrites are relatively pristine rocks and prime candidates for studies exploring processes that predated planet formation. We closely examined the petrographic features and trace elemental composition of different CV3 constituents in the accretionary breccia Leoville. The petrographic results are presented here. Our sample (2.2 cm²) is not brecciated. The main ingredient—about 65 area%—is fine‐ to coarse‐grained ferromagnesian type I chondrules. Minor constituents (in order of 2‐D abundance) include refractory inclusions, Al‐rich chondrules, and very fine‐crystalline clasts of moderately volatile composition. Type II chondrules and metal nuggets occur sporadically. The chondrule–matrix ratio is approximately 3:1. Medium‐ and coarse‐grained chondrules exhibit porphyritic textures, probably caused by incomplete melting, and frequent, partial or continuous, recrystallized dust rims. The fine‐grained population most likely represents randomly sectioned dust rims. The rim material and some of the medium‐grained objects are relatively troilite‐rich. Iron‐nickel metal is rare. In addition, almost all constituents show strikingly ragged or convoluted outlines. Only a few, rim‐less components exhibit smooth contours. Evidence for incomplete melting and the formation of recrystallized or igneous rims in carbonaceous chondrites is well established, suggesting that both processes were widespread events. The observed features in Leoville support this conclusion. In addition, our findings indicate that surface abrasion in a turbulent dust‐filled regime may have taken place after the consolidation of dust rims. Alternatively, the irregular, convoluted nature of at least the rimmed chondrules may have been inherent to the dust accretion event and was not erased by subsequent heating.     

Khatyrka,a new CV3 find from the Koryak Mountains,Eastern Russia

A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay‐rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel‐rich iron‐nickel metal, nepheline, and calcic pyroxene [diopside‐hedenbergite solid solution]) of oxidized‐subgroup CV3 chondrites. A few grains are fine‐grained spinel‐rich calcium‐aluminum‐rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper‐aluminum‐iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu‐Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3‐like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu‐Al‐Fe metal alloys and the alloy‐bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper‐aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.     

Arcuate band texture in a dark inclusion from the Vigarano CV3 chondrite: Possible evidence for early sedimentary processes

Abstract— A dark inclusion in the Vigarano CV3 carbonaceous chondrite consists almost exclusively of small (<5 μm in diameter) grains of Fe-rich olivine and is devoid of chondrules, Ca-Al-rich inclusions (CAIs) and their pseudomorphs. In backscattered electron images, this dark inclusion shows an unusual texture comprising a network of arcuate bands. Two or more bands occur roughly parallel, forming a set of succesive parallel bands, some crosscutting one another. The bands contain slightly higher amounts of relatively small (<1 μm) olivine grains and so are more densely packed than other areas. The olivine grains in the bands are slightly more Fe-rich than those in other areas. The bands commonly show gradation on the concave side due to a decrease in the abundance of the small Fe-rich olivine grains. Texturally, the arcuate bands closely resemble “dish structures” that are commonly observed in siltstones and sandstones on Earth. Dish structures are characterized by thin, dark-colored, subhorizontal to concave-upward laminations that are rich in relatively fine-grained material. On Earth, dish structures form during compaction and dewatering of unconsolidated fine-grained sediments; they are one of the characteristic sedimentary structures formed through fluidization of fine grains. The dark inclusion in Vigarano, therefore, provides the first evidence that sedimentary processes due to water migration may have taken place within planetesimals and further suggests that fluidization may have played a significant role in the formation of the carbonaceous chondrites.     

Fine‐grained,spinel‐rich inclusions from the reduced CV chondrites Efremovka and Leoville: I. Mineralogy,petrology, and bulk chemistry

Abstract— Fine‐grained, spinel‐rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al‐diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite‐rich accretionary rims. In contrast to heavily altered fine‐grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine‐grained CAIs are ¹⁶O‐enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine‐grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al‐diopside. Other fine‐grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite‐free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al‐diopside. The mantles are calcium‐enriched, magnesium‐depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine‐grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel‐perovskite‐melilite ± hibonite gas‐solid condensates from a reservoir that was ¹⁶O‐enriched but depleted in the most refractory REEs. These aggregates later experienced low‐temperature gas‐solid nebular reactions with gaseous SiO and Mg to form Al‐diopside and ±anorthite. The zoned structures of many of the fine‐grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi‐stage formation history of fine‐grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse‐grained CAIs in CV chondrites.