Interchondrule Matrix Research Articles

Presolar Silicate and Oxide Grains Found in Lithic Clasts from Isheyevo and the Fine-grained Matrix of Northwest Africa 801

Abstract We report on the discovery of 33 oxygen-anomalous grains from the CH3/CBb3 chondrite Isheyevo and the CR2 chondrite Northwest Africa (NWA) 801. Oxygen isotopic compositions indicate the origin of the majority grains in stellar outflows of low-mass (∼1.2 to ∼2.2 M ⊙), solar-metallicity red giant or asymptotic giant branch stars, while highly 17O-enriched grains probably have nova origins. Isotopic compositions of the eight 18O-rich grains, including an extremely 18O-rich grain (∼16 times solar 18O/16O ratio), are reproduced by zone mixing of SNe II ejecta. Close-to-normal silicon, magnesium, and calcium isotopic compositions of grains are consistent with the isotope exchange in the interstellar medium or the meteorite parent body, while two grains with Si isotopic anomalies and one grain with Mg isotopic anomalies reflect the Galactic chemical evolution. An Isheyevo clast showed several hot spots with moderate to high 15N enrichments, including a hot spot with an extreme 15N excess of (7225 ± 316)‰. However, no correlation between 15N enrichment and presolar oxygen-rich grain abundance is found. Grains with elliptical shapes probably indicate primary condensation feature. Two complex grains possibly display decoupling of the isotopic and elemental compositions in the grain formation environments. The low silicate-to-oxide abundance ratio for the fine-grained chondrule rims in NWA 801 likely reflects the preferential destruction of silicates due to terrestrial weathering. In NWA 801, the presolar O-rich grain abundance in fine-grained chondrule rims is higher than in the interchondrule matrix, similar to the trend observed for some aqueously altered chondrites of petrologic type 2.

Noble gases in cluster chondrite clasts and their host breccias.

We measured noble gases in “cluster chondrite clasts” from nine unequilibrated ordinary chondrites (UOCs). For five meteorites, we also present data for so‐called “clastic matrix,” the impact‐brecciated material in which the angular to subrounded cluster chondrite clasts are often embedded. Cluster chondrite clasts are characterized by close‐fit texture of deformed and indented chondrules with lower amounts of fine‐grained interchondrule matrix than in other UOCs (Metzler 2012). They are ubiquitous in UOCs and may indicate accretion and compaction of hot and deformable chondrules within hours or days after formation. Clastic matrix of four of the five meteorites contains He and Ne implanted by the solar wind (SW), indicating that they are regolith breccias. In contrast, cluster chondrite clasts are essentially devoid of SW, confirming that they are fragments of “primary accretionary rocks” (Metzler 2012). Trapped Kr and Xe in all samples are essentially primordial (type “Q”). Trapped Xe concentrations in cluster chondrite clasts are similar to values in other UOCs of similar metamorphic grade despite their low fractions of primordial gas‐bearing fine‐grained materials. This possibly indicates that the interchondrule matrix in cluster chondrite clasts is more pristine than matrix of regular UOCs. Later loss of primordial gases during parent body metamorphism is mirrored in the decreasing concentrations of primordial noble gases with increasing petrologic type. Relative to cluster chondrite lithologies, clastic matrix often contains excesses of cosmogenic noble gases, most likely due to precompaction exposure in the parent body regolith.

Mineralogy, petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD = 12.8; Cr2O3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.

Mineralogy and petrology of Dominion Range 08006: A very primitive CO3 carbonaceous chondrite

Abstract Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach. The interchondrule matrix in DOM 08006 consists of unequilibrated mixtures of silicate, metal, and sulfide minerals and lacks Fe-rich rims on silicates indicating that DOM 08006 has only experienced minimal, if any, thermal metamorphism. This is also reflected by the Co/Ni ratios of Ni-rich and Ni-poor metal, a sensitive indicator of thermal metamorphism, and the presence of euhedral chrome-spinel grains, which typically become subhedral to anhedral during progressive metamorphism. DOM 08006 matrix shows minor evidence for aqueous alteration and while the presence of magnetite surrounding metal in chondrules indicates that there has been some interaction with fluid, much metal remains and none of the sulfides analyzed show evidence of being formed by aqueous alteration. Furthermore, the plagioclase of ∼50% of chondrules analyzed show resolvable excess silica indicating that these chondrules have experienced minimal, if any, reprocessing in the CO parent body. Noble gas data for DOM 08006 show that it contains the highest concentrations of trapped 36Ar and 132Xe of all CO chondrites analyzed to date, further indicating that DOM 08006 is the most primitive CO chondrite known. The cosmic ray exposure age of DOM 08006 is estimated to be ∼19 Ma. The minimally altered nature of DOM 08006 demonstrates that it is an extremely important sample for providing valuable insight into early Solar System conditions. At a total weight of 667 g, a significant amount of material is available for a wide array of future studies.

A study of presolar material in hydrated lithic clasts from metal‐rich carbonaceous chondrites

AbstractWe report on the investigation of presolar grain inventories of hydrated lithic clasts in three metal‐rich carbonaceous chondrites from the CR clan, Acfer 182 (CH3), Isheyevo (CH3/CBb3), and Lewis Cliff (LEW) 85332 (C3‐un), as well as the carbon‐ and nitrogen‐isotopic compositions of the fine‐grained clast material. Eleven presolar silicate grains as well as nine presolar silicon carbide (SiC) grains were identified in the clasts. Presolar silicate abundances range from 4 to 22 parts per million (ppm), significantly lower than in pristine meteorites and interplanetary dust particles (IDP), and comparable to recent findings for CM2s and CR2 interchondrule matrix. SiC concentrations lie between 9 and 23 ppm, and are comparable to the values for CI, CM, and CR chondrites. The results of our investigation suggest similar alteration pathways for the clast material, the interchondrule matrix of the CR2 chondrites, and the fine‐grained fraction of CM2 chondrites. Fine‐grained matter of all three meteorites contains moderate to high 15N‐enrichments (~50‰ ≤ δ15N ≤ ~1600‰) compared to the terrestrial value, indicating the presence of primitive organic material. We observed no correlation between 15N‐enrichments and presolar dust concentrations in the clasts. This is in contrast to the findings from a suite of primitive IDPs, which display in several cases enhanced bulk 15N/14N ratios and high presolar grain abundances of several hundred or even thousand ppm. The bulk 15N/14N ratios of the clasts are comparable to the range for primitive IDPs, suggesting a nitrogen carrier less susceptible to destruction by aqueous alteration than silicate stardust.

53Mn–53Cr radiometric dating of secondary carbonates in CR chondrites: Timescales for parent body aqueous alteration

We present 53Mn–53Cr ages of secondary carbonates in Renazzo-like (CR) chondrites, determined by secondary ion mass spectrometry. The timing of aqueous alteration in CR chondrites has been unconstrained in the literature. We measured 53Mn–53Cr isotope systematics in carbonates from three different CR-chondrite lithologies. Calcite in the interchondrule matrix of Renazzo, calcite in the matrix of GRO 95577, and dolomite in a dark inclusion of Renazzo all show excesses in 53Cr, interpreted as the daughter product from the decay of 53Mn. The Renazzo calcite yields an initial ratio of (53Mn/55Mn)0=(3.6±2.7)×10−6, and the Renazzo dark inclusion dolomite ranges from (53Mn/55Mn)0=(3.1±1.4)×10−6 (corrected to the RSF of a calcite standard) to (3.7±1.7)×10−6 (corrected to an inferred dolomite RSF). When anchored to the D’Orbigny angrite, the Renazzo carbonates yield ages between 4563.6 and 4562.6Ma, or ∼4.3–5.3Myr after the formation of CV CAIs. Calcite measured in the heavily altered specimen GRO 95577 yields a shallower slope of (53Mn/55Mn)0=(7.9±2.8)×10−7, corresponding to a much younger age of 4555.4Ma, or ∼12.6Myr after CAI formation. The two Renazzo ages are contemporaneous with Mn–Cr ages of carbonates in Tagish Lake, CI, and CM chondrites, but the GRO 95577 age is uniquely young. These findings suggest that early aqueous alteration on chondritic parent bodies was a common occurrence, likely driven by internal heating from 26Al decay after accretion. The young carbonate ages of GRO 95577 suggest that either the CR parent body was sufficiently large to sustain heating from 26Al for ∼8Myr, or that late-stage impact events supplied heat to the region where GRO 95577 originated.

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

AbstractCluster chondrites are characterized by close‐fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler ). 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.

Ultrarapid chondrite formation by hot chondrule accretion? Evidence from unequilibrated ordinary chondrites

Abstract– Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close‐fit texture of deformed and indented chondrules. These clasts, termed “cluster chondrites,” occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82–92 vol%) and only low amounts of fine‐grained interchondrule matrix and rims (3–9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3–5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co‐accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule‐forming conditions. Cluster chondrites would document local chondrule formation, where chondrule‐forming heating events and the accretion of chondritic bodies were closely linked in time and space.

Indicators of parent-body processes: Hydrated chondrules and fine-grained rims in the Mokoia CV3 carbonaceous chondrite

A petrographic and electron microscopic study of the Mokoia CV3 carbonaceous chondrite shows that all of the chondrules and inclusions (>400 μm in diameter) and most of their fine-grained rims studied (referred to as chondrules/rims) contain various amounts of hydrous phyllosilicates (mostly saponite) formed by aqueous alteration of anhydrous silicates. The rims mainly consist of fine-grained olivine and saponite in varying proportions and contain crosscutting veins of Fe-rich olivine. The boundaries between the chondrules and their rims are irregular and show abundant evidence of aqueous alteration interactions between them. In contrast, the host matrix contains very minor amounts of saponite and shows no evidence of such extensive aqueous alteration. The boundaries between the chondrules/rims and the matrix are sharp and show no traces of the matrix having been involved in the alteration of the chondrules/rims. These observations indicate that the aqueous alteration in the chondrules/rims did not occur in the present setting. We suggest that the chondrules/rims are actually clasts transported from a location on the meteorite parent body different from where the Mokoia meteorite was from. The aqueous alteration of the chondrules/rims probably occurred there. The veins in the rims were originally fractures produced in an interchondrule matrix by impacts; these were later filled by Fe-rich olivine during aqueous activity. This location was then involved in impact brecciation, and individual chondrules were ejected as clasts with remnants of the matrix surrounding them. During the continuing brecciation, those chondrule/rim clasts were transported, mixed with anhydrous matrix grains, and finally lithified to the present meteorite. Therefore, the rims are fragmented remnants of a former matrix. Textures characterized by fine-grained rims surrounding chondrules in chondrites have been widely thought to have formed in the solar nebula before they accreted into their parent bodies. However, our results suggest that some textures may not be explained by such an accretionary model; instead, the multi-stage parent-body process modeled for the Mokoia rim formation may be a more plausible explanation.

Chondritic lithic clasts in the CB/CH-like meteorite Isheyevo: Fragments of previously unsampled parent bodies

The CB/CH-like chondrite Isheyevo is characterized by the absence of fine-grained interchondrule matrix material; the only present fine-grained material is found as chondritic lithic clasts. In contrast to the pristine high-temperature components of Isheyevo, these clasts experienced extensive aqueous alteration in an asteroidal setting. Hence, the clasts are foreign objects that either accreted together with the high-temperature components or were added later to the final Isheyevo parent body during regolith gardening. In order to constrain the origin and secondary alteration of the clasts in Isheyevo, we studied their mineralogy, petrography, structural order of the polyaromatic carbonaceous matter, and oxygen isotopic compositions of carbonates. Three main groups of clasts were defined based on mineralogy and petrology. Group I clasts consist of phyllosilicates, carbonates, magnetite, and lath-shaped Fe,Ni-sulfides. Group II clasts contain different abundances of anhydrous silicates embedded in a hydrated matrix; sulfides, magnetite, and carbonates are rare. With only a few exceptions, groups I and II clasts did not experienced significant thermal metamorphism. Group III clasts are characterized by the absence of magnetite and the presence of Fe,Ni-metal. In addition to aqueous alteration, they experienced thermal metamorphism as reflected by the structure of their polyaromatic carbonaceous matter. While there are some similarities between the Isheyevo clasts, CI chondrites, and the matrices of CM and CR chondrites, on the whole, the characteristics of the clasts do not match those of any of these aqueously altered meteorite classes. Nor do they match those of similar material in various types of chondritic clasts present in other meteorite groups. We conclude that the Isheyevo clasts represent fragments of previously unsampled parent bodies.

Composition of matrix in the CR chondrite LAP 02342

We report evidence of interchondrule matrix heterogeneity on a scale of ∼50 μm in the well-preserved CR2 chondrite LAP 02342. Despite minor effects resulting from asteroidal aqueous alteration, the matrix in this CR chondrite seems to preserve much of the compositional record of nebular fines. We carried out electron-microprobe studies using a 3-μm-diameter beam; we analyzed 10 elements in 36- or 49-point grids on 11 ca. 50 × 50-μm rectangular areas of matrix. Each grid area has a distinct composition, inconsistent with a simple model of matrix material having a uniform composition throughout the nebular formation region of the CR chondrites. On S–Fe, Mg–Si, K–Na and K–Al scatter diagrams, the grid areas (i.e., different matrix patches) are largely separated from each other; plots of means with 95% confidence limits demonstrate that the compositions are resolvable. Five matrix areas were analyzed again in duplicate runs; excellent agreement was observed between duplicate studies. LAP 02342 experienced two forms of mild aqueous alteration – as patchy enrichments in Ca (inferred to reflect CaCO 3) and as regions in which sulfide laths are embedded within phyllosilicates. Despite this evidence of aqueous transport, the effect on the composition of matrix is not resolvable. For example, matrix points that were adjacent to points with high CaCO 3 contents show elemental concentrations similar to those in regions having only one or two points with a Ca enrichment. It appears that secondary minerals are found in areas where there are suitable precursor phases and voids into which new phases could grow unimpeded. Calcium appears to be unique in forming a phase that greatly lowers the Ca ++ content of the aqueous medium, thus enhancing the rate of diffusion. Because chondrules vary widely in bulk composition, the formation of chondrules in small sets (100 or less) could generate “smoke” and mesostasis spray with compositions unique to each set. However, if these nanoparticles were moving independently in the nebula, it would not have been possible to have preserved compositional variations. We therefore infer that the anomalous materials were preserved in small nebular structures, probably as porous chondrules formed by low degrees of melting.

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende meteorite

Elemental and isotopic abundances of lithium in chondrule constituents in the Allende CV3 meteorite were determined using secondary ion mass spectrometry. Olivines and mesostasis dominated by a feldspathic phase are depleted in Li ( < 1 μ g g - 1 and 0.1 – 0.6 μ g g - 1 , respectively). In contrast, low-Ca pyroxenes and mesostasis dominated by a Na-rich phase are enriched in Li ( ∼ 1 – 8 μ g g - 1 and 0.4 – 3.5 μ g g - 1 , respectively) and the interchondrule matrix is generally enriched in Li ( ∼ 2.0 μ g g - 1 on average). The Li isotopic abundance of olivine ranges from δ 7 Li ∼ - 32 to 21. The spatial distributions of elemental and isotopic abundances of Li in olivines within individual chondrules exhibit no systematic pattern. This suggests that the distribution of Li in olivine was not disturbed during aqueous alteration or thermal metamorphism on the Allende meteorite parent body. Although mesostasis is the last crystallizing phase from a chondrule melt and is expected to be enriched in Li, in the Allende meteorite it is generally depleted in Li. We suggest that during aqueous alteration on the CV asteroid, Li in mesostasis was leached out by aqueous fluids. The Li-enriched Na-rich mesostasis was probably produced later by infiltration of Na-rich fluids. It seems likely that aqueous fluids sequestered alkali elements from the Allende–chondrite region in the CV parent asteroid, although significant amounts of Li are preserved in ferrous olivine in the interchondrule matrix.

Iodine-Xenon dating of chondrules from the Qingzhen and Kota Kota enstatite chondrites

Initial 129I/ 127I values (I-Xe ages) have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites Qingzhen (EH3) and Kota Kota (EH3). In view of the absence of aqueous alteration and the low-peak metamorphic temperatures experienced by these meteorites, we suggest that the I-Xe ages for the chondrules record the event in which they were formed. These ages are within the range recorded for chondrules from ordinary chondrites, demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites. The timing of this chondrule-forming episode or episodes brackets the I-Xe closure age of planetesimal bodies such as the Shallowater aubrite parent body. Although chondrule formation need not have occurred close to planetesimals, the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process. Whichever mechanisms are proposed to form and transport chondrules, they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance.

Phyllosilicate-rich chondrule rims in the vigarano cv3 chondrite: evidence for parent-body processes

A petrographic and scanning electron microscopic study of the Vigarano CV3 carbonaceous chondrite reveals that a small proportion (∼5%) of the fine-grained rims surrounding chondrules consist mainly of phyllosilicates (saponite and serpentine) and minor but variable amounts of ferrihydrite. Most of the rims consist mainly of Fe-rich olivine. The phyllosilicate-rich rims and the chondrules that they enclose show abundant evidence of aqueous alteration. Low-Ca pyroxene and olivine at chondrule margins have been partially replaced by phyllosilicate-rich materials, producing embayments on chondrule surfaces. Fe-Ni metal, Fe sulfide and magnetite in the chondrules have also been partially replaced by Fe hydroxide. Due to preferential replacement of low-Ca pyroxene and olivine, sulfide, magnetite and Fe hydroxide remain exposed at chondrule margins. We discovered a clast (0.7 × 0.9 mm in size) containing several chondrules embedded in a phyllosilicate-rich matrix that is almost identical in mineralogy, texture and composition to the rims. We also found numerous small clasts (20–400 μm in size) consisting of a similar phyllosilicate-rich material in the host Vigarano meteorite; many of the clasts include fragments of chondrules, amoeboid olivine aggregates and Ca-Al-rich inclusions. These clasts also show abundant evidence of aqueous alteration, which is similar to that observed in the rims and their host chondrules. From these observations, we conclude that the chondrule/rim assemblages and the clasts are a part of the same precursor chondrite, which was fragmented during brecciation on the meteorite parent body. We interpret that a portion (or portions) of the Vigarano parent body was aqueously altered, and the interchondrule matrix was largely converted to phyllosilicates and ferrihydrite. This hydrated portion was later fragmented and brecciated by impacts. As fragmentation occurred preferentially along the phyllosilicate-rich porous matrix, most of the ejected clasts consisted of a chondrule rimmed by remnant matrix. The clasts were rounded by abrasion during brecciation and transportation and finally mixed into an anhydrous portion in the regolith, from which the Vigarano meteorite was derived. Some of the phyllosilicate-rich rims show evidence for partial dehydration during heating before final incorporation into the regolith. These results, taken together, suggest that the Vigarano parent body was spatially heterogeneous in degree of aqueous alteration and thermal metamorphism and the host Vigarano meteorite is a breccia consisting of numerous clasts from many different locations.

A transmission electron microscope study of iron‐nickel carbides in the matrix of the Semarkona unequilibrated ordinary chondrite

Abstract— The fine‐grained matrix of the Semarkona unequilibrated ordinary chondrite contains an assemblage of Fe carbides that are associated with oxides, sulfides, carbonates, and hydrated silicates. The carbides consist of fine‐scale epitactic intergrowths of cohenite (Fe3C) and Hägg carbide (Fe5C2) that formed as by‐products of gas‐solid reactions on the Semarkona parent body. The carbide intergrowths resulted from prograde reactions at moderate temperatures in the presence of a highly reducing carbon‐bearing gas. The carburization occurred prior to the aqueous alteration episode that produced the Fe‐rich phyllosilicates throughout the interchondrule matrix and the thin magnetite rims surrounding the carbide grains. The occurrence of the carbide intergrowths places an upper limit of &lt;500 °C on the maximum postaccretional thermal processing that occurred on the Semarkona parent body.

Trace element contents of chondrule rims and interchondrule matrix in ordinary chondrites

The abundances of forty elements have been measured in the rims and matrices of the unequilibrated ordinary chondrites Semarkona, Bishunpur, Chainpur, and Krymka. The rims appear to be mixtures of matrix-like material and material that was volatilized during chondrule formation. Iron, silicon, manganese, and copper all appear to have behaved as volatiles during chondrule formation and their enrichments are generally confined to the rims, suggesting recondensation of these elements was rapid and localized. This is supported by petrologic observations that show many chondrules were still partially molten or plastic when the rims accreted. Some rims also exhibit a decrease in both their FeO content and the degree of mineral re-equilibration away from the chondrule. The alkali metals are enriched in both the rims and matrices. The alkalis may have recondensed onto the matrix as well as the rims, but their mobility is such that they have probably undergone extensive redistribution after accretion. Some Na may even have been able to diffuse back into chondrules either during chondrule cooling or during metamorphism/alteration. Most refractory lithophiles are depleted with respect to A1 in the rims and matrices in all four UOCs. These depletions are probably due to postaccretional redistribution of these elements into phosphates and other secondary minerals that form in and around metal/sulfide grains. The matrix is probably composed of chondrule fragments and processed nebular dust, but the presence of carbonaceous presolar grains in the matrix indicates that there must be unprocessed presolar silicates present too. On average, the presolar silicates presumably have a CI-like composition but a CI-like signature is not seen in the matrix compositions. Simple estimates suggest that the abundances of carbonaceous presolar grains are consistent with there being at most 10 wt%o presolar silicates present in UOC matrix, which would be hard to detect in this study.

Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites

Optical microscopy, electron probe microanalysis, scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in Bishunpur, Krymka, Semarkona, Chainpur (all LL3), Tieschitz and Sharps (both H3). The size-distribution of grains in matrix in Bishunpur obeys a power law. Rims around chondrules tend to be finer grained than matrix and may be layered. All rims and matrices studied contain clasts of forsteritic olivine and low-Ca pyroxene. In Bishunpur the clasts may be cemented by amorphous “glue”, rich in normative albite, and partly altered to smectite. Rims in Krymka, Sharps, Chainpur and Tieschitz have a densely packed olivine groundmass with a grain-size of about 0.1 μm. In Krymka, groundmass olivine is Fo 20–30 and forms interlocking dendrites. Various Fe oxides are present in rims and matrices. Grain-size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules. From analyses, matrices/rims in the LL-group apparently are enriched in a feldspathic component. Tieschitz rims are slightly enriched, but here the feldspathic component is present as white matrix. Sharps rims are not enriched in a feldspathic component. Opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules, partly induced by contraction of protopyroxene on inversion to clinopyroxene. Fragmented silica-rich chondrule mesostases reacted with Fe oxides and Na to form the groundmass of fayalitic olivine and feldspathic “glue”. A low-temperature, nebular or pre-solar, component is limited to 3 vol.% of each meteorite, so equilibrium condensate was not the carrier of volatiles such as Tl and Bi.

The microstructure of Semarkona and Bishunpur

The ordinary chondrites, Semarkona and Bishunpur, appear to have experienced in situ hydrous alteration. Here we report the results of the first detailed TEM examination of them, which supports this conclusion. In Semarkona, all but the most magnesian mafic minerals have been destroyed, and primary sulphides, most likely troilite, have been partially altered to phyllosilicates and Ni-rich pyrrhotite. In Bishunpur, alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix. During hydrous alteration of both meteorites, the temperature probably did not exceed 260°C. The equilibrium composition of the gas present during the alteration of Semarkona was not solar but was dominated by H 2O. The formation of smectite does not require the presence of liquid water. However, evidence for the redistribution of elements, such as Ca, Zn, and Si, over tens of microns or more implies that there was at least some transient grain-boundary fluid present. The alteration in Semarkona and Bishunpur is far from complete. Thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the CM and CI carbonaceous chondrites. Superficially, the mineralogy, mineral chemistry, and bulk chemistry of Semarkona matrix-rims resemble those of some chondritic interplanetary dust particles (CIDPs). The ordinary chondrite parent bodies may, therefore, be a source of some CIDPs.

The Semarkona meteorite: First recorded occurrence of smectite in an ordinary chondrite, and its implications

Abstract Semarkona is an unshocked unequilibrated ordinary chondrite. Much of the interchondrule matrix and the mesostases of some chondrules and clasts have been hydrothermally altered. Analyses of altered materials and opaque matrix are presented. Ca and Fe have been redistributed from primary, anhydrous phases into calcite and Na, Fe smectite, respectively. In Semarkona the process possibly requires open system behavior inconsistent with the conventional view of “metamorphism” of the ordinary chondrites. The low petrologic type previously assigned to Semarkona is the result of retrograde metamorphism, under aqueous conditions, of a higher temperature mineral assemblage. Semarkona, and possibly Bishunpur, should be assigned to petrologic type 2.

Chromian-manganoan augite in the interchondrule matrix of the Tieschitz (H3) chondritic meteorite

AbstractThe Tieschitz chondrite is a disequilibrium assemblage of silicate, metal and sulphide occurring together or separately as chondrules and clasts that generally have opaque rims (matrix) of fine-grained material. Translucent, silicate-rich matrix fills channels that occur sporadically between chondrules and clasts. An angular fragment of twinned ‘clinoenstatite’ was found to be rimmed with chromium- and manganese-rich augite and surrounded by opaque matrix. The whole is set within the largest area of translucent (white) interchondrule matrix encountered. It is suggested that reaction between crystallizing undersaturated translucent matrix, opaque matrix, and clinoenstatite probably led to the partial replacement of clinoenstatite by chromian-manganoan augite at high temperature.