Primary feldspar in the Semarkona LL3.00 chondrite: Constraints on chondrule formation and secondary alteration

Feldspar in ordinary chondrites (OCs) is often associated with thermal metamorphism, as a secondary mineral that forms from the crystallization of matrix and chondrule mesostasis. However, studies of feldspar in equilibrated OCs show that there is a range of plagioclase compositions within chondrules, some of which may be primary products of chondrule crystallization. It is important to recognize primary feldspar within chondrules because it can be used to help understand the secondary effects of thermal metamorphism and aqueous alteration. The presence of primary feldspar also provides important petrologic constraints on chondrule formation time scales. We undertook a careful study of Semarkona (LL3.00) and observed feldspar in 18% of chondrules. The feldspar is plagioclase covering a wide range of compositions (An₂–An₉₉) with little K‐feldspar component (<Or₃). We show that plagioclase is a primary igneous phase, based on grain morphology and compositions consistent with growth from a melt having the bulk compositions of the host chondrules. Based on experimental studies, the presence of plagioclase suggests chondrules cooled slowly at temperatures close to the solidus. We also observed several secondary features consistent with the aqueous alteration. These features include zoning of Na and Ca in plagioclase, heterogeneity in plagioclase compositions in altered chondrules, development of porosity from the dissolution of chondrule glass, and alteration of glass to phyllosilicates. Alteration of major Al‐bearing phases, like plagioclase and glass, has important implications for interpretations of ages derived from Al‐Mg dating of chondrules, if they have been affected by secondary processes.     

A neutron activation analysis of iridium concentration in Yamato carbonaceous chondrite

Iridium concentration in extra-terrestrial bodies is an important quantity in relation to Ir-rich geological layers. Ir concentration of a Yamato carbonaceous chondrite (Y-793321) has been measured by a neutron activation method. The measurement yields a value (0.57 ± 0.06) g per gramme for the chondrite.     

Silica-rich orthopyroxenite in the Bovedy chondrite

Abstract— A large (≥4.5 × 7 × 4 mm), igneous-textured clast in the Bovedy (L3) chondrite is notable for its high bulk SiO₂ content (57.5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol%), tridymite (6.2 %), an intergrowth of feldspar (5.8 %) and sodic glass (3.1 %), pigeonite (1.0 %), and small amounts of chromite (0.2 %), augite, and Fe, Ni-metal; it is best described as a silica-rich orthopyroxenite. The oxygen-isotopic composition of the clast is similar, but not identical, to Bovedy and other ordinary chondrites. The clast has a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, pyroxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a siliceous melt that cooled fast enough to prevent complete attainment of equilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggest that it is an igneous differentiate from an asteroid or planetesimal that formed in the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the source region of the clast, metallic and silicate partial melt were largely-to-completely lost during a relatively low degree of melting, and that during a higher degree of melting, olivine and low-Ca pyroxene separated from the remaining liquid, which ultimately solidified to form the clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.     

Mechanisms of ringwoodite formation in shocked meteorites: Evidence from L5 chondrite Dhofar 1970

The formation of the high‐pressure compositional equivalents of olivine and pyroxene has been well‐documented within and surrounding shock‐induced veins in chondritic meteorites, formed by crystallization from a liquid‐ or solid‐state phase transformation. Typically polycrystalline ringwoodite grains have a narrow range of compositions that overlap with those of their olivine precursors, whereas the formation of iron‐enriched ringwoodite has been documented from only a handful of meteorites. Here, we report backscattered electron images, quantitative wavelength‐dispersive spectrometry (WDS) analyses, qualitative WDS elemental X‐ray maps, and micro‐Raman spectra that reveal the presence of Fe‐rich ringwoodite (Fa_44‐63) as fine‐grained (500 nm), polycrystalline rims on olivine (Fa_24‐25) wall rock and as clasts engulfed by shock melt in a previously unstudied L5 chondrite, Dhofar 1970. Crystallization of majorite + magnesiowüstite in the vein interior and metastable mineral assemblages within 35 μm of the vein margin attest to rapid crystallization of a superheated shock melt (>2300 K) from 20─25 GPa to ambient pressure and temperature. The texture and composition of bright polycrystalline ringwoodite rims (Fa_44‐63; MnO 0.01─0.08 wt%) surrounding dark polycrystalline olivine (Fa_8‐14; MnO 0.56─0.65 wt%) implies a solid‐state transformation mechanism in which Fe was preferentially partitioned to ringwoodite. The spatial association between ringwoodite and shock melt suggests that the rapidly fluctuating thermal regimes experienced by chondritic minerals in contact with shock melt are necessary to both drive phase transformation but also to prevent back‐transformation.     

The Bencubbin chondrite breccia and its relationship to CR chondrites and the ALH85085 chondrite

Abstract— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (～40 vol.%) and host metal (～60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO₂, Al₂O₃ and Cr₂O₃ contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in ¹⁵N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.     

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

Platinum group element (PGE) concentrations have been determined in situ in ordinary chondrite kamacite and taenite grains via laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Results demonstrate that PGE concentrations in ordinary chondrite metal (kamacite and taenite) are similar among the three ordinary chondrite groups, in contrast to previous bulk metal studies in which PGE concentrations vary in the order H < L < LL. PGE concentrations are higher in taenite than kamacite, consistent with preferential PGE partitioning into taenite. PGE concentrations vary between and within metal grains, although average concentrations in kamacite broadly agree with results from bulk studies. The variability of PGE concentrations in metal decreases with increasing petrologic type; however, variability is still evident in most type six ordinary chondrites, suggesting that equilibration of PGEs does not occur between metal grains, but rather within individual metal grains via self‐diffusion during metamorphism. The constant average PGE concentrations within metal grains across different ordinary chondrite groups are consistent with the formation of metal via nebular condensation prior to the accretion of ordinary chondrite parent bodies. Post‐condensation effects, including heating during chondrule‐formation events, may have affected some element ratios, but have not significantly affected average metal PGE concentrations.     

Structure,composition, and location of organic matter in the enstatite chondrite Sahara 97096 (EH3)

Abstract– The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine‐grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar δ¹⁵N and δ¹³C; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium‐heterogeneity and having mean H‐ and N‐isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer‐sized areas highly enriched in ¹⁵N (δ¹⁵N up to 1600‰). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.     

Alteration processes in the CV chondrite parent body based on analysis of NWA 2086 meteorite

Analysis of the NWA 2086 CV3 chondrite showed a matrix/chondrule ratio of 52%, similar to Bali, Mokoia, and Grosanaja. Nearly twice as many chondrule fragments as intact ones demonstrate that an early fragmentation phase occurred prior to final accretion. After this event, no substantial mechanical change or redeposition is evident. Rims with double‐layered structures were identified around some chondrules, which, in at least one case, is attributed to an accretionary origin. The rim's outer parts with a diffuse appearance were formed by in situ chemical alteration. During this later process, Mg content decreased, Fe content increased, and olivine composition was homogenized, producing a rim composition close to that of the matrix. This alteration occasionally happened along fractures and at confined locations, and was probably produced by fluid interactions. Iron oxides are the best candidate for a small grain‐sized alteration product; however, technical limitations in the available equipment did not allow exact phase identification. These results suggest that NWA 2086 came from a location (possible more deeply buried) in the CV parent body than Mokoia or Bali, and suffered less impact effects—although there is no evidence of sustained thermal alteration. This meteorite may represent a sample of the CV parent asteroid interior and provide a useful basis for comparison with other CV meteorites in the future.     

Microstructural evidence for complex formation histories of amoeboid olivine aggregates from the ALHA77307 CO3.0 chondrite

The microstructures and compositions of olivine and refractory components in six amoeboid olivine aggregates (AOAs) in the Allan Hills A77307 CO3.0 chondrite have been characterized in detail using the focused ion beam sample preparation technique with transmission electron microscopy. In the AOAs, refractory components (perovskite, melilite, spinel, anorthite, and Al‐Ti‐bearing diopside) provide evidence of a high degree of textural and compositional heterogeneity, suggesting that these phases have formed by disequilibrium gas–solid condensation at high temperatures under highly dynamic conditions. We infer different possible reactions of early‐condensed solid minerals (perovskite and spinel) with a nebular gas, forming diopside with wide ranges of Al and Ti contents and/or anorthite. The progressive, incomplete consumption of spinel in these reactions may have resulted in the Cr enrichment in the remaining, unreacted spinel in the AOAs. In contrast to the refractory components, olivines in the AOAs have equilibrated textures with 120° triple junctions, indicating that the AOAs were subjected to high‐temperature annealing after agglomeration of olivine and refractory components. Because the AOAs consist of fine‐grained olivine grains with numerous pores, the annealing is constrained by experimental data to have occurred for a short duration of the order of a few hours to tens of hours depending on the annealing temperature. In comparison, the effects of annealing on the refractory components are minimal, probably due to pinning of grain boundaries in the multiphase assemblages that inhibited grain growth.     

Parent body histories recorded in Rumuruti chondrite sulfides: Implications for the onset of oxidized,sulfur-rich core formation

Models of planetary core formation beginning with melting of Fe,Ni metal and troilite are not readily applicable to oxidized and sulfur-rich chondrites containing only trace quantities of metal. Cores formed in these bodies must be dominated by sulfides. Siderophile trace elements used to model metallic core formation could be used to model oxidized, sulfide-dominated core formation and identify related meteorites if their trace element systematics can be quantified. Insufficient information exists regarding the behavior of these core-forming elements among sulfides during metamorphism prior to anatexis. Major, minor, and trace element concentrations of sulfides are reported in this study for petrologic type 3–6 R chondrite materials. Sulfide-dominated core-forming components in such oxidized chondrites (ƒO₂ ≥ iron-wüstite) follow metamorphic evolutionary pathways that are distinct from reduced, metal-bearing counterparts. Most siderophile trace elements partition into pentlandite at approximately 10× chondritic abundances, but Pt, W, Mo, Ga, and Ge are depleted by 1–2 orders of magnitude relative to siderophile elements with similar volatilities. The distribution of siderophile elements is further altered during hydrothermal alteration as pyrrhotite oxidizes to form magnetite. Oxidized, sulfide-dominated core formation differs from metallic core formation models both physically and geochemically. Incongruent melting of pentlandite at 865°C generates melts capable of migrating along solid silicate grains, which can segregate to form a Ni,S-rich core at lower temperatures compared to reduced differentiated parent bodies and with distinct siderophile interelement proportions.     

Petrology and mineralogy of the Ningqiang carbonaceous chondrite

Abstract— We report detailed chemical, petrological, and mineralogical studies on the Ningqiang carbonaceous chondrite. Ningqiang is a unique ungrouped type 3 carbonaceous chondrite. Its bulk composition is similar to that of CV and CK chondrites, but refractory lithophile elements (1.01 × CI) are distinctly depleted relative to CV (1.29 × CI) and CK (1.20 × CI) chondrites. Ningqiang consists of 47.5 vol% chondrules, 2.0 vol% Ca,Al‐rich inclusions (CAIs), 4.5 vol% amoeboid olivine aggregates (AOAs), and 46.0 vol% matrix. Most chondrules (95%) in Ningqiang are Mg‐rich. The abundances of Fe‐rich and Al‐rich chondrules are very low. Al‐rich chondrules (ARCs) in Ningqiang are composed mainly of olivine, plagioclase, spinel, and pyroxenes. In ARCs, spinel and plagioclase are enriched in moderately volatile elements (Cr, Mn, and Na), and low‐Ca pyroxenes are enriched in refractory elements (Al and Ti). The petrology and mineralogy of ARCs in Ningqiang indicate that they were formed from hybrid precursors of ferromagnesian chondrules mixed with refractory materials during chondrule formation processes. We found 294 CAIs (55.0% type A, 39.5% spinel‐pyroxene‐rich, 4.4% hibonite‐rich, and several type C and anorthite‐spinel‐rich inclusions) and 73 AOAs in 15 Ningqiang sections (equivalent to 20 cm²surface area). This is the first report of hibonite‐rich inclusions in Ningqiang. They are texturally similar to those in CM, CH, and CB chondrites, and exhibit three textural forms: aggregates of euhedral hibonite single crystals, fine‐grained aggregates of subhedral hibonite with minor spinel, and hibonite ± Al,Ti‐diopside ± spinel spherules. Evidence of secondary alteration is ubiquitous in Ningqiang. Opaque assemblages, formed by secondary alteration of pre‐existing alloys on the parent body, are widespread in chondrules and matrix. On the other hand, nepheline and sodalite, existing in all chondritic components, formed by alkali‐halogen metasomatism in the solar nebula.     

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.     

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.     

Unusual forms of magnetite in the Orgueil carbonaceous chondrite

Abstract— The Orgueil CI carbonaceous chondrite contains magnetite (Fe₃O₄) that displays a rich and seemingly unique variety of morphologies. While many have already been described, recent images provide far greater definition and, thus, provide more unambiguous identification of trapezohedral crystals and possibly also trisoctahedral forms. Although it is an extremely abundant terrestrial mineral and commonly occurs in well-developed crystals, neither of these forms have been recognized previously from magnetite. The barrel-shaped arrays of magnetite discs (“plaquettes”) are also unique. It appears as if the circumstances for the formation of these unusual crystals were somehow exceptional. Their morphologies and association with sulfates and carbonates suggest an aqueous origin.     

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

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

High‐pressure polymorphs in Yamato‐790729 L6 chondrite and their significance for collisional conditions

Shock pressure recorded in Yamato (Y)‐790729, classified as L6 type ordinary chondrite, was evaluated based on high‐pressure polymorph assemblages and cathodoluminescence (CL) spectra of maskelynite. The host‐rock of Y‐790729 consists mainly of olivine, low‐Ca pyroxene, plagioclase, metallic Fe‐Ni, and iron‐sulfide with minor amounts of phosphate and chromite. A shock‐melt vein was observed in the hostrock. Ringwoodite, majorite, akimotoite, lingunite, tuite, and xieite occurred in and around the shock‐melt vein. The shock pressure in the shock‐melt vein is about 14–23 GPa based on the phase equilibrium diagrams of high‐pressure polymorphs. Some plagioclase portions in the host‐rock occurred as maskelynite. Sixteen different CL spectra of maskelynite portions were deconvolved using three assigned emission components (centered at 2.95, 3.26, and 3.88 eV). The intensity of emission component at 2.95 eV was selected as a calibrated barometer to estimate shock pressure, and the results indicate pressures of about 11–19 GPa. The difference in pressure between the shock‐melt vein and host‐rock might suggest heterogeneous shock conditions. Assuming an average shock pressure of 18 GPa, the impact velocity of the parent‐body of Y‐790729 is calculated to be ~1.90 km s^?1. The parent‐body would be at least ~10 km in size based on the incoherent formation mechanism of ringwoodite in Y‐790729.     

The nature of the L chondrite parent body's disruption as deduced from high-pressure phases in the Sixiangkou L6 chondrite

The disruption of the L chondrite parent body (LCPB) at ~470 Ma is currently the best-documented catastrophic celestial impact event, based on the large number of L chondritic materials associated with this event. Uranium-lead (U-Pb) dating of apatite and its high-pressure decomposition product, tuite, in the Sixiangkou L6 chondrite provides a temporal link to this event. The U-Pb system of phosphates adjacent to shock melt veins was altered to varying degrees and the discordance of the U-Pb system correlates closely with the extent of apatite decomposition. This suggests that the U-Pb system of apatite could be substantially disturbed by high-temperature pulse during shock compression from natural impacts, at least on the scale of mineral grains. Although many L chondrites can be temporally related to the catastrophic LCPB impact event, the shock conditions experienced by each individual meteorite vary. This could be due to the different geologic settings of these meteorites on their parent body. The shock pressure and duration derived from most meteorites may only reflect local shock features rather than the impact conditions, although they could provide lower limits to the impact conditions. The Sixiangkou shock duration (~4 s), estimated from high-pressure transformation kinetics, provides a lower limit to the high-pressure pulse of the LCPB disruption impact. Combined with available literature data of L chondrites associated with this impact event, our results suggest that the LCPB suffered a catastrophic collision with a large projectile (with a diameter of at least 18–22 km) at a low impact velocity (5–6 km s⁻¹). This is consistent with astronomical estimates based on the dynamical evolution of L chondritic asteroids.     

Redistribution of elements in the heavily shocked Yanzhuang chondrite

Abstract— Compositions of metal, sulfide, olivine, pyroxene, and plagioclase/plagioclase glass were studied for the melted and unmelted parts of the heavily shocked H6(S6) chondrite‐Yanzhuang. We found that the partitioning of some trace elements significantly changed between the 2 parts; compared with the corresponding minerals in the unmelted part, Ga is enriched in the metal, Co, Cr, and Zn are enriched in the sulfide, Cr is enriched in olivine and pyroxene, and Ti is enriched in the plagioclase glass of the melt pocket. These detailed studies of the mineral phases put constraints on 3 important parameters (temperature, pressure, and duration) associated with the post‐shock melting process. The coexistence of melted and unmelted olivine in the melt pocket of Yanzhuang implies a peak temperature after shock that approaches the melting point of olivine. The lack of Ni in the olivine crystallized from a melt suggests crystallization of olivine at pressures below 10 kbar. The resetting of Ga partitioning between metal and silicate in the melt pocket indicates that the interval from the peak temperature after shock to the crystallization of metal‐sulfide and plagioclase glass in the melted part of Yanzhuang is longer than 500 sec.     

Fall,recovery and description of the Coleman chondrite

Abstract— A 0.5 kg stony meteorite associated with a bright bolide seen over southeastern Michigan on 1994 October 20 has been recovered. The circumstances of the fall and recovery of this chondrite, named Coleman, are presented. The most likely trajectory from the observations of the event implies preatmospheric orbital parameters typical of meteorites. Gamma-ray spectrometry of the cosmogenic radionuclides showed that the recovered mass was an interior fragment of a larger body and revealed abnormally high ²²Na and ²⁶Al activity. Electron microprobe analysis yielded compositions of Fa_24.1 and Fs_20.3, which are consistent with an L-chondrite classification. Petrographically, the presence of chondrules, the observed mineralogy and the degree of chondrule-matrix integration suggests assignment to petrologic type 6.