Indigenous amino acids in primitive CR meteorites

Abstract— –CR chondrites are among the most primitive meteorites. In this paper, we report the first measurements of amino acids in Antarctic CR meteorites. Three CRs, Elephant Moraine (EET) 92042, Graves Nunataks (GRA) 95229, and Grosvenor Mountains (GRO) 95577, were analyzed for their amino acid content using high‐performance liquid chromatography with UV fluorescence detection (HPLC‐FD) and gas chromatography–mass spectrometry (GC‐MS). Our data show that EET 92042 and GRA 95229 are the most amino acid–rich chondrites ever analyzed, with total amino acid concentrations ranging from 180 ppm to 249 ppm. The most abundant amino acids present in the EET 92042 and GRA 95229 meteorites are the α‐amino acids glycine, isovaline, α‐aminoisobutyric acid (α‐AIB), and alanine, with δ¹³C values ranging from +31.6‰ to +50.5‰. The carbon isotope results together with racemic enantiomeric ratios determined for most amino acids strongly indicate an extraterrestrial origin for these compounds. Compared to Elephant Moraine (EET) 92042 and GRA 95229, the more aqueously altered GRO 95577 is depleted in amino acids. In both CRs and CMs, the absolute amino acid abundances appear to be related to the degree of aqueous alteration in their parent bodies. In addition, the relative abundances of α‐AIB and β‐alanine in the Antarctic CRs also appear to depend on the degree of aqueous alteration.     

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.     

Presolar silicate and oxide abundances and compositions in the ungrouped carbonaceous chondrite Adelaide and the K chondrite Kakangari: The effects of secondary processing

Abstract– Although it has been suggested that the ungrouped carbonaceous chondrite Adelaide and the K chondrite Kakangari could be considered highly primitive, our study of their presolar grain abundances shows that both have experienced more secondary processing than other primitive chondrites with high presolar grain abundances. Presolar grains are rare in Kakangari and are present in reduced abundances in Adelaide (approximately 70 ppm for O‐anomalous grains). Thermal annealing has led to widespread crystallization of their fine‐grained matrices, and accounts for the partial to complete destruction of presolar grains. In addition, presolar silicates in Adelaide show elevated Fe abundances and Fe‐rich rims indicative of infiltration of Fe into the grains from the surrounding matrix. This process probably also took place during annealing, most likely in the solar nebula, in a region with an enhanced dust‐to‐gas ratio. The most primitive meteorites, with the highest presolar grain abundances, appear to be those whose matrices contain abundant amorphous material that has escaped any significant thermal or aqueous alteration.     

Fine‐grained rims in the Allan Hills 81002 and Lewis Cliff 90500 CM2 meteorites: Their origin and modification

Abstract— Antarctic CM meteorites Allan Hills (ALH) 81002 and Lewis Cliff (LEW) 90500 contain abundant fine‐grained rims (FGRs) that surround a variety of coarse‐grained objects. FGRs from both meteorites have similar compositions and petrographic features, independent of their enclosed objects. The FGRs are chemically homogeneous at the 10 μm scale for major and minor elements and at the 25 μm scale for trace elements. They display accretionary features and contain large amounts of volatiles, presumably water. They are depleted in Ca, Mn, and S but enriched in P. All FGRs show a slightly fractionated rare earth element (REE) pattern, with enrichments of Gd and Yb and depletion of Er. Gd is twice as abundant as Er. Our results indicate that those FGRs are not genetically related to their enclosed cores. They were sampled from a reservoir of homogeneously mixed dust, prior to accretion to their parent body. The rim materials subsequently experienced aqueous alteration under identical conditions. Based on their mineral, textural, and especially chemical similarities, we conclude that ALH 81002 and LEW 90500 likely have a similar or identical source.     

Multiple precursors of secondary mineralogical assemblages in CM chondrites

We report a petrographic and mineralogical survey of tochilinite/cronstedtite intergrowths (TCIs) in Paris, a new CM chondrite considered to be the least altered CM identified to date. Our results indicate that type‐I TCIs consist of compact tochilinite/cronstedtite rims surrounding Fe‐Ni metal beads, thus confirming kamacite as the precursor of type‐I TCIs. In contrast, type‐II TCIs are characterized by complex compositional zoning composed of three different Fe‐bearing secondary minerals: from the outside inwards, tochilinite, cronstedtite, and amakinite. Type‐II TCIs present well‐developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors. The results demonstrate that type‐II TCIs formed by pseudomorphism of the anhydrous silicates, olivine, and pyroxene. Hence, there is no apparent genetic relationship between type‐I and type‐II TCIs. In addition, the complex chemical zoning observed within type‐II TCIs suggests that the alteration conditions evolved dramatically over time. At least three stages of alteration can be proposed, characterized by alteration fluids with varying compositions (1) Fe‐ and S‐rich fluids; (2) S‐poor and Fe‐ and Si‐rich fluids; and (3) S‐ and Si‐poor, Fe‐rich fluids. The presence of unaltered silicates in close association with euhedral type‐II TCIs suggests the existence of microenvironments during the first alteration stages of CM chondrites. In addition, the absence of Mg‐bearing secondary minerals in Paris TCIs suggests that the Mg content increases during the course of alteration.     

Chemical compositions and classifica tion of five thermally altered carbonaceous chondrites

To establish the chemical group provenance of the five thermally altered carbonaceous chondrites Asuka (A‐) 881551, Asuka‐882113, Elephant Moraine (EET) 96026, Mulga (west), and Northwest Africa (NWA) 3133, we quantified 44 trace elements in each of them. We also analyzed Larkman Nunatak (LAR) 04318 (CK4), Miller Range (MIL) 090001 (CR2), Roberts Massif (RBT) 03522 (CK5) as reference samples as their chemical group affinity is already recognized. We conclude that Asuka‐881551, Asuka‐882113, and Mulga (west) are thermally metamorphosed CK chondrites. Compositionally, Elephant Moraine 96026 most resembles the CV chondrites. NWA 3133 is the most significantly thermally altered carbonaceous chondrite in our suite of samples. It is completely recrystallized (no chondrules or matrix remain), but its bulk composition is consistent with a CV–CK clan provenance. The thermally labile element (e.g., Se, Te, Zn, and Bi) depletion in NWA 3133 indicates a chemically open system during the heating episode. It remains unclear if the heat necessary for its thermal alteration of NWA 3133 was due to the decay of ²⁶Al or was impact related. Finally, we infer that MIL 090001, Mulga (west), and NWA 3133 show occasional compositional signatures indicative of terrestrial alteration. The alteration is especially evident within the elements Sr, Ba, La, Ce, Th, U, and possibly Sb. Despite the alteration, we can still confidently place each of the altered chondrites within an established chemical group or clan.     

Compound‐specific carbon,nitrogen, and hydrogen isotopic ratios for amino acids in CM and CR chondrites and their use in evaluating potential formation pathways

Abstract– Stable hydrogen, carbon, and nitrogen isotopic ratios (δD, δ¹³C, and δ¹⁵N) of organic compounds can reveal information about their origin and formation pathways. Several formation mechanisms and environments have been postulated for the amino acids detected in carbonaceous chondrites. As each proposed mechanism utilizes different precursor molecules, the isotopic signatures of the resulting amino acids may indicate the most likely of these pathways. We have applied gas chromatography with mass spectrometry and combustion isotope ratio mass spectrometry to measure the compound‐specific C, N, and H stable isotopic ratios of amino acids from seven CM and CR carbonaceous chondrites: CM1/2 Allan Hills (ALH) 83100, CM2 Murchison, CM2 Lewis Cliff (LEW) 90500, CM2 Lonewolf Nunataks (LON) 94101, CR2 Graves Nunataks (GRA) 95229, CR2 Elephant Moraine (EET) 92042, and CR3 Queen Alexandra Range (QUE) 99177. We compare the isotopic compositions of amino acids in these meteorites with predictions of expected isotopic enrichments from potential formation pathways. We observe trends of decreasing δ¹³C and increasing δD with increasing carbon number in the α‐H, α‐NH₂ amino acids that correspond to predictions made for formation via Strecker‐cyanohydrin synthesis. We also observe light δ¹³C signatures for β‐alanine, which may indicate either formation via Michael addition or via a pathway that forms primarily small, straight‐chain, amine‐terminal amino acids (n‐ω‐amino acids). Higher deuterium enrichments are observed in α‐methyl amino acids, indicating formation of these amino acids or their precursors in cold interstellar or nebular environments. Finally, individual amino acids are more enriched in deuterium in CR chondrites than in CM chondrites, reflecting different parent‐body chemistry.     

Forsterite‐rich accretionary rims around calcium‐aluminum‐rich inclusions from the reduced CV3 chondrite Efremovka

Abstract— It was suggested that multilayered accretionary rims composed of ferrous olivine, andradite, wollastonite, salite‐hedenbergitic pyroxenes, nepheline, and Ni‐rich sulfides around Allende calcium‐aluminum‐rich inclusions (CAIs) are aggregates of gas‐solid condensates which reflect significant fluctuations in physico‐chemical conditions in the slowly cooling solar nebula and grain/gas separation processes. In order to test this model, we studied the mineralogy of accretionary rims around one type A CAI (E104) and one type B CAI (E48) from the reduced CV3 chondrite Efremovka, which is less altered than Allende. In contrast to the Allende accretionary rims, those in Efremovka consist of coarse‐grained (20–40 μm), anhedral forsterite (Fa_1–8), Fe, Ni‐metal nodules, amoeboid olivine aggregates (AOAs) and fine‐grained CAIs composed of Al‐diopside, anorthite, and spinel, ± forsterite. Although the fine‐grained CAIs, AOAs and host CAIs are virtually unaltered, a hibonite‐spinel‐perovskite CAI in the E48 accretionary rim experienced extensive alteration, which resulted in the formation of Fe‐rich, Zn‐bearing spinel, and a Ca, Al, Si‐hydrous mineral. Forsterites in the accretionary rims typically show an aggregational nature and consist of small olivine grains with numerous pores and tiny inclusions of Al‐rich minerals. No evidence for the replacement of forsterite by enstatite was found; no chondrule fragments were identified in the accretionary rims. We infer that accretionary rims in Efremovka are more primitive than those in Allende and formed by aggregation of high‐temperature condensates around host CAIs in the CAI‐forming regions. The rimmed CAIs were removed from these regions prior to condensation of enstatite and alkalies. The absence of andradite, wollastonite, and hedenbergite from the Efremovka rims may indicate that these rims sampled different nebular regions than the Allende rims. Alternatively, the Ca, Fe‐rich silicates rimming Allende CAIs may have resulted from late‐stage metasomatic alteration, under oxidizing conditions, of original Efremovka‐like accretionary rims. The observed differences in O‐isotope composition between forsterite and Ca, Fe‐rich minerals in the Allende accretionary rims (Hiyagon, 1998) suggest that the oxidizing fluid had an ¹⁶O‐poor oxygen isotopic composition.     

Deciphering the nebular and asteroidal record of silicates and organic material in the matrix of the reduced CV3 chondrite Vigarano

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.     

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.     

Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments

Tochilinite/cronstedtite intergrowths are commonly observed as alteration products in CM chondrite matrices, but the conditions under which they formed are still largely underconstrained due to their scarcity in terrestrial environments. Here, we report low temperature (80 °C) anoxic hydrothermal experiments using starting assemblages similar to the constituents of the matrices of the most pristine CM chondrite and S‐rich and S‐free fluids. Cronstedtite crystals formed only in S‐free experiments under circumneutral conditions with the highest Fe/Si ratios. Fe‐rich tochilinite with chemical and structural characteristics similar to chondritic tochilinite was observed in S‐bearing experiments. We observed a positive correlation between the Mg content in the hydroxide layer of synthetic tochilinite and temperature, suggesting that the composition of tochilinite is a proxy for the alteration temperature in CM chondrites. Using this relation, we estimate the mean precipitation temperatures of tochilinite to be 120–160 °C for CM chondrites. Given the different temperature ranges of tochilinite and cronstedtite in our experiments, we propose that Fe‐rich tochilinite crystals resulted from the alteration of metal beads under S‐bearing alkaline conditions at T = 120–160 °C followed by cronstedtite crystals formed by the reaction of matrix amorphous silicates, metal beads, and water at a low temperature (50–120 °C).     

Amorphous silicates in the matrix of Semarkona: The first evidence for the localized preservation of pristine matrix materials in the most unequilibrated ordinary chondrites

We have investigated the fine‐grained matrix of the least‐altered unequilibrated ordinary chondrite (UOC) Semarkona (LL3.00) using different electron microscope techniques. Unlike previous studies, which found that the matrix of Semarkona was extensively altered to phyllosilicates, we have discovered the widespread occurrence of much more pristine amorphous silicates in the sample that we have studied. Detailed TEM study shows that these materials occur pervasively in the matrix as (1) continuous groundmass; (2) distinct, circular to subrounded features, which contain nanometric‐size sulfides and carbides; or (3) distinct objects containing parallel, linear features composed of sulfides and voids. These amorphous silicates have many textural and compositional similarities to the occurrences of amorphous silicates found in pristine carbonaceous chondrites (CCs); however, minor differences were also identified. Most of the textural and chemical differences suggest that these materials formed at different times and locations in the solar nebula, compared to matrix materials in CCs. Nevertheless, their occurrence suggests that the amorphous silicates in Semarkona formed by similar processes to those proposed for amorphous silicates in CCs, that is, rapid cooling that favored disequilibrium condensation of material evaporated during chondrule‐forming events. In addition, the occurrence of minimally altered amorphous silicates in Semarkona demonstrates that the effects of aqueous alteration, which have been widely described in this meteorite, are not pervasive. Instead, our new observations demonstrate that aqueous alteration has affected Semarkona heterogeneously and that locally, regions of much more pristine matrix that have escaped extensive alteration are still preserved within this meteorite. Such materials provide significant new insights into the pristine characteristics of ordinary chondrite matrix material that has not been previously available.     

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.     

Fine‐grained dust rims in the Tagish Lake carbonaceous chondrite: Evidence for parent body alteration

Abstract— The Tagish Lake carbonaceous chondrite consists of heavily aqueously altered chondrules, CAIs, and larger mineral fragments in a fine‐grained, phyllosilicate‐dominated matrix. The vast majority of the coarse‐grained components in this meteorite are surrounded by continuous, 1.5 to >200 μm wide, fine‐grained, accretionary rims, which are well known from meteorites belonging to petrological types 2 and 3 and whose origin and modification is still a matter of debate. Texturally, the fine‐grained rims in Tagish Lake are very similar throughout the entire meteorite and independent of the nature of the enclosed object. They typically display sharp boundaries to the core object and more gradational contacts to the meteorite matrix. Compared to the matrix, the rims are much more finegrained and characterized by a significantly lower porosity. The rims consist of an unequilibrated assemblage of phyllosilicates, Fe,Ni sulfides, magnetites, low‐Ca pyroxenes, and forsteritic olivines, and are, except for a much lower abundance of carbonates, very similar to the Tagish Lake matrix. Electron microprobe and synchrotron X‐ray microprobe analyses show that matrix and rims are also very similar in composition and that the rims differ significantly from matrix and bulk meteorite only by being depleted in Ca. X‐ray elemental mapping and mineralogical observations indicate that Ca was lost during aqueous alteration from the enclosed objects and preferentially crystallized as carbonates in the porous matrix. The analyses also show that Ca is strongly fractionated from Al in the rims, whereas there is no fractionation of the Ti/Al‐ratios. Our data suggest that the fine‐grained rims in Tagish Lake initially formed by accretion in the solar nebula and were subsequently modified by in situ alteration on the parent body. This pervasive alteration removed any potential evidence for pre‐accretionary alteration but did not change the overall texture of the Tagish Lake meteorite.     

The presolar grain inventory of fine‐grained chondrule rims in the Mighei‐type (CM) chondrites

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in ¹⁷O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in ¹⁷O and significantly depleted in ¹⁸O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced ¹⁸O/¹⁶O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca₂Mg[Si₂O₇]) or a an åkermanite‐diopside (MgCaSi₂O₆) intergrowth.     

Si‐bearing metal and niningerite in Almahata Sitta fine‐grained ureilites and insights into the diversity of metal on the ureilite parent body

A detailed mineralogical and chemical study of Almahata Sitta fine‐grained ureilites (MS‐20, MS‐165, MS‐168) was performed to shed light on the origin of these lithologies and their sulfide and metal. The Almahata Sitta fine‐grained ureilites (silicates <30 μm grain size) show textural and chemical evidence for severe impact smelting as described for other fine‐grained ureilites. Highly reduced areas in Almahata Sitta fine‐grained ureilites show large (up to ～1 mm) Si‐bearing metal grains (up to ～4.5 wt% Si) and niningerite [Mg_>0.5,(Mn,Fe)_<0.5S] with some similarities to the mineralogy of enstatite (E) chondrites. Overall, metal grains show a large compositional variability in Ni and Si concentrations. Niningerite grains probably formed as a by‐product of smelting via sulfidation. The large Si‐Ni variation in fine‐grained ureilite metal could be the result of variable degrees of reduction during impact smelting, inherited from coarse‐grained ureilite precursors, or a combination of both. Large Si‐bearing metal grains probably formed via coalescence of existing and newly formed metal during impact smelting. Bulk and in situ siderophile trace element abundances indicate three distinct populations of (1) metal crystallized from partial melts in MS‐20, (2) metal resembling bulk chondritic compositions in MS‐165, and (3) residual metal in MS‐168. Almahata Sitta fine‐grained ureilites developed their distinctive mineralogy due to severe reduction during smelting. Despite the presence of E chondrite and ureilite stones in the Almahata Sitta fall, a mixing relation of E chondrites or their constituents and ureilite material in Almahata Sitta can be ruled out based on isotopic, textural, and mineral‐chemical reasons.     

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.     

X‐ray fluorescence measurements of the surface elemental composition of asteroid 433 Eros

Abstract— We report major element ratios determined for the S‐class asteroid 433 Eros using remote‐sensing x‐ray fluorescence spectroscopy with the near‐Earth asteroid rendezvous Shoemaker x‐ray spectrometer (XRS). Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended “quiet Sun” periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact‐induced volatilization and/or photo‐ or ion‐induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree of melting with loss of an FeS‐rich partial melt. Size‐sorting processes could lead to segregation of Fe‐Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the x‐ray spectrometer are not representative of the bulk Eros composition.     

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.     

Absence of matrix‐like chondrule rims in CR2 LAP 02342

In numerous past papers, it was concluded that the fine (<1 μm) matrix immediately adjacent to, and radially symmetric around, chondrules in primitive chondrites consists of compact (low‐porosity) rims that were attached in the solar nebula. We present here textural and compositional evidence that no matrix‐like (or accretionary) rims around chondrules are present in the well‐preserved CR2 chondrite LAP 02342. Fine‐grained matrix‐rich regions (i.e., candidate “rims”) at the edges of chondrules were studied with an electron‐microprobe‐based matrix‐grid technique; comparison of the “rims” data for matrix regions near these chondrules showed the candidate “rims” to be compositionally heterogeneous, inconsistent with origins as radially symmetric, matrix‐like rims formed by gradual accretion. This evidence (together with simulations and laboratory studies indicating that accretionary processes produced highly porous aggregates) strongly suggests that nebular processes did not produce compact matrix‐like rims around chondrules in any chondrite group.