Petrologic Type Research Articles

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

AbstractMagnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.

39Ar‐40Ar chronology of the enstatite chondrite parent bodies

AbstractAr‐Ar isochron ages of EL chondrites suggest closure of the K‐Ar system at 4.49 ± 0.01 Ga for EL5 and 6 chondrites, and 4.45 ± 0.01 Ga for EL3 MAC 88136. The high‐temperature release regimes contain a mixture of radiogenic 40Ar* and trapped primordial argon (solar or Q‐type) with 40Ar/36ArTR ~ 0, which does not affect the 40Ar budget. The low‐temperature extractions show evidence of an excess 40Ar component. The 40Ar/36Ar is 180–270; it is defined by intercept values of isochron regression. Excess 40Ar is only detectable in petrologic types >4/5. These lost most of their primordial 36Ar from low‐temperature phases during metamorphism and retrapped excess 40Ar. The origin of this excess 40Ar component is probably related to metamorphic Ar mobilization, homogenization of primordial and in situ radiogenic Ar, and trapping of Ar by distinct low‐temperature phases. Ar‐Ar ages of EH chondrites are more variable and show clear evidence of a major impact‐induced partial resetting at about 2.2 Ga ago or alternatively, prolonged metamorphic decomposition of major K carrier phases. EH impact melt LAP 02225 displayed the highest Ar‐Ar isochron age of 4.53 ± 0.01 Ga. This age sets a limit of about 25–45 Ma for the age bias between the K‐Ar and U‐Pb decay systems.

Petrology and bulk chemistry of Yamato‐82094, a new type of carbonaceous chondrite

AbstractCarbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y‐82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y‐82094 contains 3 vol% Ca‐Al‐rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y‐82094 is within the range of CO and CV chondrites. Therefore, Y‐82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO‐rich rims on AOA olivine and the mode of occurrence of Ni‐rich metal in the chondrules indicate that Y‐82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe‐Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high‐temperature conditions, and accreted to the parent body of Y‐82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y‐82094 may represent a new, previously unsampled, asteroidal body.

He and Ne in individual chromite grains from the regolith breccia Ghubara (L5): Exploring the history of the L chondrite parent body regolith

AbstractWe analyzed He and Ne in chromite grains from the regolith breccia Ghubara (L5), to compare it with He and Ne in sediment‐dispersed extraterrestrial chromite (SEC) grains from mid‐Ordovician sediments. These SEC grains arrived on Earth as micrometeorites in the aftermath of the L chondrite parent body (LCPB) breakup event, 470 Ma ago. A significant fraction of them show prolonged exposure to galactic cosmic rays for up to several 10 Ma. The majority of the cosmogenic noble gases in these grains were probably acquired in the regolith of the LCPB (Meier et al. ). Ghubara, an L chondritic regolith breccia with an Ar‐Ar shock age of 470 Ma, is a sample of that regolith. We find cosmic‐ray exposure ages of up to several 10 Ma in some Ghubara chromite grains, confirming for the first time that individual chromite grains with such high exposure ages indeed existed in the LCPB regolith, and that the >10 Ma cosmic‐ray exposure ages found in recent micrometeorites are thus not necessarily indicative of an origin in the Kuiper Belt. Some Ghubara chromite grains show much lower concentrations of cosmogenic He and Ne, indicating that the 4π (last‐stage) exposure age of the Ghubara meteoroid lasted only 4–6 Ma. This exposure age is considerably shorter than the 15–20 Ma suggested before from bulk analyses, indicating that bulk samples have seen regolith pre‐exposure as well. The shorter last‐stage exposure age probably links Ghubara to a small peak of 40Ar‐poor L5 chondrites of the same exposure age. Furthermore, and quite unexpectedly, we find a Ne component similar to presolar Ne‐HL in the chromite grains, perhaps indicating that some presolar Ne can be preserved even in meteorites of petrologic type 5.

NATURE'S STARSHIPS. I. OBSERVED ABUNDANCES AND RELATIVE FREQUENCIES OF AMINO ACIDS IN METEORITES

The class of meteorites called carbonaceous chondrites are examples of material from the solar system which have been relatively unchanged from the time of their initial formation. These meteorites have been classified according to the temperatures and physical conditions of their parent planetesimals. We collate available data on amino acid abundance in these meteorites and plot the concentrations of different amino acids for each meteorite within various meteorite subclasses. We plot average concentrations for various amino acids across meteorites separated by subclass and petrologic type. We see a predominance in the abundance and variety of amino acids in CM2 and CR2 meteorites. The range in temperature corresponding to these subclasses indicates high degrees of aqueous alteration, suggesting aqueous synthesis of amino acids. Within the CM2 and CR2 subclasses, we identify trends in relative frequencies of amino acids to investigate how common amino acids are as a function of their chemical complexity. These two trends (total abundance and relative frequencies) can be used to constrain formation parameters of amino acids within planetesimals. Our organization of the data supports an onion shell model for the temperature structure of planetesimals. The least altered meteorites (type 3) and their amino acids originated near cooler surface regions. The most active amino acid synthesis likely took place at intermediate depths (type 2). The most altered materials (type 1) originated furthest toward parent body cores. This region is likely too hot to either favor amino acid synthesis or for amino acids to be retained after synthesis.

Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies

Ordinary chondrites contain two phosphate minerals, merrillite and chlorapatite, both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies. We have studied the phosphate mineralogy of four LL chondrites, of petrologic types 3.9–6, in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed. Characterization of merrillite and apatite includes textural observations, mineral compositions determined by electron probe microanalysis, and ion microprobe analyses of trace element and volatile anion elemental abundances. Initial formation of phosphate minerals during mild metamorphism, to petrologic type 4 conditions, resulted in oxidation of P that was originally incorporated in metal, and growth of merrillite as inclusions within metal grains. Subsequent development of both phosphate minerals occurred in response to diffusional equilibration, possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids. Porosity and vein-filling textures in both merrillite and chlorapatite, as well as textures indicating replacement of merrillite by chlorapatite, support a model in which fluid played a significant role and suggest an interface-coupled dissolution–reprecipitation mechanism during metasomatism. Some associations of phosphate minerals with chromite-plagioclase assemblages suggest that phosphate minerals could also be related to impact processes, either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt. Fluid compositions may have been water-bearing initially, at low temperatures of metamorphism, but later evolved to become halogen-rich and very dry. Late-stage halogen-rich fluids that dominated during cooling of LL chondrite material may have been derived from vaporization of partial melts in the interior of the parent body. Overall, the LL chondrite parent body underwent a complex chemical evolution, in which metasomatism played a significant role.

Metamorphosed calcium‐aluminum‐rich inclusions in CK carbonaceous chondrites

AbstractCK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium‐aluminum‐rich inclusion (CAI)‐poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine‐grained CAIs recrystallized as irregular assemblages of plagioclase + Ca‐rich pyroxene ± olivine ± Ca‐poor pyroxene ± magnetite. Coarse‐grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca‐rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al‐Mg isotopic systematics measured in fine‐ and coarse‐grained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al‐Mg isotopic signatures obtained in the core of a coarse‐grained CAI (CG1‐CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.

Fe–Ni and Al–Mg isotope records in UOC chondrules: Plausible stellar source of 60Fe and other short-lived nuclides in the early Solar System

The short-lived now-extinct nuclide 60Fe, present in the early Solar System, is a unique product of stellar nucleosynthesis. Even though the first hint for its presence in the early Solar System was obtained more than two decades back, a robust value for Solar System initial (SSI) 60Fe/56Fe is yet to be established. A combined study of 26Al–26Mg and 60Fe–60Ni isotope systematics in chondrules from unequilibrated ordinary chondrites of low petrologic type, Semarkona (LL3.0), LEW 86134 (L3.0), and Y 791324 (L3.1), has been conducted to infer the value of SSI 60Fe/56Fe. Seven of the analysed chondrules host resolved radiogenic excess in both 60Ni and 26Mg resulting from in situ decay of the short-lived nuclides 60Fe and 26Al, respectively. The initial 26Al/27Al values for these chondrules range from (6.9±5.8)×10−6 to (3.01±1.78)×10−5 that suggest their formation between 2.1 and 0.6Ma after CAIs. The initial 60Fe/56Fe at the time of formation of these chondrules ranges from (3.2±1.3)×10−7 to (1.12±0.39)×10−6 and show a good correlation with their initial 26Al/27Al values suggesting co-injection of the two short-lived nuclides, 60Fe and 26Al, into the protosolar cloud from the same stellar source. Considering 26Al as a reliable early Solar System chronometer, this data set yield a SSI 60Fe/56Fe value of (7.0±1.2)×10−7, if we adopt a half-life value of 2.6Ma for 60Fe reported in a recent study. Model stellar nucleosynthesis yields suggest that both a high mass (5–6.5M⊙) Asymptotic Giant Branch (AGB) star or a supernova (SN) could be the source of 60Fe and 26Al present in the early Solar System. A high mass (∼25M⊙) SN appears more plausible because of the much higher probability of its close association with the protosolar molecular cloud than a high mass AGB star. Such a SN can also account for SSI abundance of 26Al and its correlated presence with 60Fe in chondrules.

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

AbstractWe 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.

The first data on the oxygen, carbon, and sulfur isotope composition of the Chelyabinsk meteorite

Meteorites are the main source of extraterrestrial material for investigation of regularities of the forma� tion and evolution of solar system matter. The genetic connection of a concrete meteorite to an assumed parental body is based on meteorite classification, which is mostly supported by study of their mineralog� ical and petrographic characteristics, as well as inves� tigation of the bulk chemical composition [1]. In 1973 R. Clayton et al. [2] discovered an anomalous concen� tration of the 17 O isotope in meteorites in comparison with terrestrial rocks. The main differences between terrestrial and extraterrestrial material are controlled by primary isotope heterogeneities in the early solar system. Further investigations of the relative concen� tration of stable isotopes 16 О, 17 О, and 18 O provided the oxygen isotope classification of meteorites and planets and allowed characterization of two important processes of the evolution of the solar system: chemi� cal fractionation and mixing of the components of protoplanetary materials with various isotope compo� sitions [3]. In recent years special attention has been paid to investigation of variations in the stable isotopes in meteorites during genetic identification of meteor� ites and study of the evolution of the matter of the solar system. For this purpose we investigated the oxygen, sulfur, and carbon isotope compositions of a fragment of Chelyabinsk meteorite. Chelyabinsk meteorite rain was registered on Feb� ruary 15, 2013, in Chelyabinsk oblast (Russia). The meteorite was related to the LL group of ordinary chondrites, petrological type 5, shock fraction S4, and weathering degree W0 [4]. An individual fragment of the Chelyabinsk meteorite with a weight of 7 g was used for preparation of bulk samples for the oxygen and carbon isotope analysis. The bulk sample with a total weight of 800 mg was collected from the light part of the meteorite and ground in an agate mortar. None of the chemical methods of treating was applied, since the weathering degree of the Chelyabinsk meteorite was zero (W0). A section with a thickness of 3 mm was cut from the meteorite fragment and polished for local analysis of the sulfur isotope composition.

Chromium valences in ureilite olivine and implications for ureilite petrogenesis

Ureilites are a group of ultramafic achondrites commonly thought to be residues of partial melting on a carbon-rich asteroid. They show a large variation in FeO content (olivine Fo values ranging from ∼74 to 95) that cannot be due to igneous fractionation and suggests instead variation in oxidation state. The presence of chromite in only a few of the most ferroan (Fo 75–76) samples appears to support such a model. MicroXANES analyses were used in this study to determine the valence states of Cr (previously unknown) in olivine cores of 11 main group ureilites. The goal of this work was to use a method that is independent of Fo to determine the oxidation conditions under which ureilites formed, in order to evaluate whether the ureilite FeO-variation is correlated with oxidation state, and whether it is nebular or planetary in origin. Two of the analyzed samples, LEW 88774 (Fo 74.2) and NWA 766 (Fo 76.7) contain primary chromite; two others, LAP 03587 (Fo 74.4) and CMS 04048 (Fo 76.2) contain sub-micrometer-sized exsolutions of chromite+Ca-rich pyroxene in olivine; and one, EET 96328 (Fo 85.2) contains an unusual chromite grain of uncertain origin. No chromite has been observed in the remaining six samples (Fo 77.4–92.3).Chromium in olivine in all eleven samples was found to be dominated by the divalent species, with valences ranging from 2.10±0.02 (1σ) to 2.46±0.04. The non-chromite-bearing ureilites have the most reduced Cr, with a weighted mean valence of 2.12±0.01, i.e., Cr2+/Cr3+=7.33. All low-Fo chromite-bearing ureilites have more oxidized Cr, with valences ranging from 2.22±0.03 to 2.46±0.04. EET 96328, whose chromite grain we interpret as a late-crystallizing phase, yielded a reduced Cr valence of 2.15±0.07, similar to the non-chromite-bearing samples. Based on the measured Cr valences, magmatic (1200–1300°C) oxygen fugacities (fO2) of the non-chromite-bearing samples were estimated to be in the range IW−1.9 to IW−2.8 (assuming basaltic melt composition), consistent with fO2 values obtained by assuming olivine-silica-iron metal (OSI) equilibrium. For the primary chromite-bearing-ureilites, the corresponding fO2 were estimated (again, assuming basaltic melt composition) to be ∼IW to IW+1.0, i.e., several orders of magnitude more oxidizing than the conditions estimated for the chromite-free ureilites. In terms of Fo and Cr valence properties, ureilites appear to form two groups rather than a single “Cr-valence (or fO2) vs. Fo” trend. The chromite-bearing ureilites show little variation in Fo (∼74–76) but significant variation in Cr valence, while the non-chromite-bearing ureilites show significant variation in Fo (∼77–95) and little variation in Cr valence. These groups are unrelated to petrologic type (i.e., olivine–pigeonite, olivine–orthopyroxene, or augite-bearing). The chromite-bearing ureilites also have lower contents of Cr in olivine than most non-chromite-bearing ureilites, consistent with predictions based on Cr olivine/melt partitioning in spinel saturated vs. non-spinel-saturated systems.Under the assumption that at magmatic temperatures graphite–gas equilibria controlled fO2 at all depths on the ureilite parent body, we conclude: (1) that ureilite precursor materials having the Fo and Cr valence properties now observed in ureilites are unlikely to have been preserved during planetary processing; and (2) that the Fo and Cr valence properties now observed in ureilites are consistent with having been established by high-temperature carbon redox control over a range of depths on a plausible-sized ureilite parent body. The apparent limit on ureilite Fo values around 74–76 suggests that the precursor material(s) had bulk mg# ⩾ that of LL chondrites.

Petrography and mineralogy of the ungrouped type 3 carbonaceous chondrite Dar al Gani 978

AbstractDar al Gani (DaG) 978 is an ungrouped type 3 carbonaceous chondrite. In this study, we report the petrography and mineralogy of Ca,Al‐rich inclusions (CAI), amoeboid olivine aggregates (AOAs), chondrules, mineral fragments, and the matrix in DaG 978. Twenty‐seven CAIs were found: 13 spinel‐diopside‐rich inclusions, 2 anorthite‐rich inclusions, 11 spinel‐troilite‐rich inclusions, and 1 spinel‐melilite‐rich inclusion. Most CAIs have a layered texture that indicates a condensation origin and are most similar to those in R chondrites. Compound chondrules represent a high proportion (approximately 8%) of chondrules in DaG 978, which indicates a local dusty chondrule‐forming region and multiple heating events. Most spinel and olivine in DaG 978 are highly Fe‐rich, which corresponds to a petrologic type of >3.5 and a maximum metamorphic temperature of approximately 850–950 K. This conclusion is also supported by other observations in DaG 978: the presence of coarse inclusions of silicate and phosphate in Fe‐Ni metal, restricted Ni‐Co distributions in kamacite and taenite, and low S concentrations in the matrix. Mineralogic records of iron‐alkali‐halogen metasomatism, such as platy and porous olivine, magnetite, hedenbergite, nepheline, Na‐rich in CAIs, and chlorapatite, are present, but relatively limited, in DaG 978. The fine‐grained, intergrowth texture of spinel‐troilite‐rich inclusions was probably formed by reaction between pre‐existing Al‐rich silicates and shock‐induced, high‐temperature S‐rich gas on the surface of the parent body of DaG 978. A shock‐induced vein is present in the matrix of DaG 978, which indicates that the parent body of DaG 978 at least experienced a shock event with a shock stage up to S3.

Thermal history of the H-chondrite parent body: Implications for metamorphic grade and accretionary time-scales

Multiple temperature-age constraints for eight H-chondrite samples have been used to provide insight into the thermal history of their parent-body through combination with numerical models of thermal evolution assuming internal heating by 26Al and conductive cooling. The effect of spreading accretion out over time is the principal focus of this work. A wide range of body size and date of accretion is systematically tested for different values of accretion rate in order to quantify and illustrate the parameter space that is consistent with the available thermo-chronological data. We conclude that the H-chondrite samples considered have a thermal history consistent with a parent body that at some stage had a concentric ‘onion-shell’ internal structure. That body had a radius no larger than 130km, and accretion most probably took place over a time interval on the order of 0.0–0.2Myr, approximately 2Myr after CAI condensation. In any case, the time interval of accretion is unlikely to have been more than 0.5Myr supporting evidence in favour of rapid accretion, possibly through reassembly of the fragments of an earlier generation of bodies. Furthermore, the H-chondrites studied here are inferred to have come from a wide depth range within the body where they experienced metamorphism, indicating that preservation of the onion-shell structure is unlikely. The presence of an insulating regolith does not modify this conclusion, as appropriate thermal histories for the three H6 samples considered cannot be reproduced at depths near the surface. Asteroid 6-Hebe may be the parent body of the H-chondrites, but the high bulk density of the latter is difficult to reconcile with a ‘rubble-pile’ structure of pure H-chondrite material. Finally, optimized thermal histories are used to constrain the temperatures characterizing boundaries between petrological types (800, 1000, and 1140K for the H3/4, H4/5, and H5/6 boundaries respectively). In detail, the type 6 samples studied here are inferred to have experienced a similar peak temperature (on the order of 1180K), but spent different times at that temperature, while samples of lower metamorphic grade experienced different peak temperature, but spent more or less the same time within 10K of peak temperature (on the order of 1Ma). These results constitute a quantitative framework within which variations in petrographic and textural properties may be interpreted.

Compositional and petrographic similarities of CV and CK chondrites: A single group with variations in textures and volatile concentrations attributable to impact heating, crushing and oxidation

Greenwood et al. (2010) gathered data on O-isotopic and elemental compositions and reevaluated literature data for CV and CK chondrites. They concluded that these two chondrite groups originated on the same parent asteroid, with CK chondrites being metamorphosed CV chondrites (which are otherwise missing types 4 through 6). To test this interpretation we have gathered new instrumental neutron-activation-analysis (INAA) data for CV and CK chondrites and reexamined their petrographic features. The new INAA data like the older data show scatter attributable to weathering effects, but we conclude that the refractory lithophile abundances are the same in CV and CK, in agreement with the Greenwood et al. interpretation. Several volatile elements are significantly lower in CK than in CV chondrites. Among the elements we determine, the greatest difference between CV and CK is found for Br, for which the CV/CK ratio is ∼4; As and Sb are about 20% lower in CK than CV and smaller differences are observed for Zn, Ga and Se. It seems likely that volatiles were lost during impact-heating events that also provided the heat responsible for metamorphic recrystallization. Within statistical uncertainty, chondrules in CV and CK chondrites are the same size and have similar textural distributions. A significant petrographic difference between CK and CV chondrites cited by Kallemeyn et al. (1991) was the much higher percentage of igneous rims around CV chondrules. However, we now recognize that many chondrules in CK3.8 NWA 1559 have igneous rims and in CK4 chondrites, igneous rims are recognizable by their associated sulfide-rich rings; there are no quantifiable CV–CK differences in igneous-rim abundances. We used Ca and Al maps to show that CK chondrites have CAI abundances similar to those of CV chondrites. It thus appears that there are no resolvable pre-metamorphic petrographic differences between CV and CK chondrites. We recommend that the “CK” designation be abandoned and that the CV group be acknowledged as spanning the range of petrologic types 3–6. We suggest that CK3 chondrites be designated CV3OxK. Most CK4 chondrites are highly fragmental; collisional crushing appears to be much more common than in CV chondrites. It seems likely that CK chondrites formed from reduced CV3 materials after the latter was impacted, buried, aqueously altered and annealed.

The chlorine isotope composition of chondrites and Earth

The chlorine isotope composition of chondrites provides information about isotopic reservoirs that were present in the solar nebula and the effects of secondary processes on their isotopic composition. We analyzed chlorine isotope ratios of 19 chondrites that included both unaltered (type 3) and altered (types 1,2 and 4–6) chondrites from the carbonaceous, ordinary, and enstatite classes as well as an enstatite chondrite impact melt breccia and a partial melt residue. Chlorine isotope compositions (δ37Cl, ‰ vs. SMOC) and 1σ uncertainty () are as follows (excluding melt residue and anomalous Parnallee):MaterialCarbonaceous chondritesOrdinary chondritesEnstatite chondritesStructurally-bound Cl−0.3(0.6)−0.4(0.6)0.4(0.4)Water-soluble Cl−0.9(0.9)0.3(2.1)0.4(0.6)Bulk (total) Cl−0.2(0.6)−0.4(0.7)0.4(0.3)Petrologic type 3 chondrites are the least equilibrated and are those most likely to retain primary nebular signatures. Type 3 carbonaceous chondrites have bulk δ37Cl values that average −0.3±0.5‰ and structurally-bound Cl values of −0.3±0.3‰. These values are indistinguishable from the δ37Cl values of the bulk Earth (−0.2‰) and close to the lowest measured value of the Moon (−0.7‰). From this similarity, we conclude that the inner regions of the early solar nebula had a homogeneous chlorine isotope reservoir. For samples that have undergone secondary processing, δ37Clbulk values range from −1.2‰ to 0.8‰ for metamorphosed samples (average 0.0‰) and −0.3‰ to 0.3‰ (average 0.1‰) for aqueously-altered samples (Orgueil, Ivuna, Murray, and Murchison). Most of the data can be explained in terms of fractionation processes similar to those that occur on the Earth, such as interaction with pore waters. No apparent correlations exist with Cl concentration data. Ordinary chondrites have the lowest and most restricted range of Cl concentration, but the largest range in δ37Cl values. Carbonaceous and enstatite chondrites have wide ranges of Cl concentration and more restricted ranges of δ37Cl values.We have analyzed additional terrestrial mantle-derived materials of the Earth in order to better constrain the δ37Cl value of this reservoir and compare with the chondrite data. The mantle data cluster at −0.3‰ to −0.1‰, and support the idea that the most pristine type C chondrites and the bulk Earth have the same δ37Cl value, indicating that no fractionation by evaporative processes occurred during the formation of the Earth. The discrepancy between our data and previously published results is discussed.

H-chondrite parent asteroid: A multistage cooling, fragmentation and re-accretion history constrained by thermometric studies, diffusion kinetic modeling and geochronological data

We present a detailed thermometric study and cooling history analysis of selected H-chondrites from the petrologic types 4–6 on the basis of their mineralogical properties, and integrate these data with other available constraints on the cooling rates to develop a comprehensive model for the cooling, fragmentation and re-accretion history of the parent asteroid. Temperatures have been determined on the basis of two-pyroxene (2-Px) and spinel (Spnl)–orthopyroxene (Opx)/olivine (Ol) thermometers using the average of line scans and distributed spot analysis of coexisting pairs in each set. All of these minerals have been found to be compositionally homogeneous from ∼1 to 2μm from the interface within the resolution of microprobe analysis. The thermometric results for the H5 (Allegan and Richardton) and H6 (Guarena and Kernouvé) samples are very similar. Also, while the 2-Px temperature increases by ∼90°C from H4 to H5/6, a reverse trend is observed for the Spnl–Opx/Ol temperatures implying compositional resetting of these pairs during cooling. For the H4 sample (Forest Vale) all thermometric results are similar. The cooling rates calculated from numerical modeling of the compositional profiles in Opx–Cpx pairs in H5 and H6, corrected for the spatial averaging or convolution effect in microprobe analysis, are ∼25–100°C/ky, which are 3–4 orders of magnitude higher than the cooling rates implied by in situ cooling in an onion-shell parent body model. Similar numerical simulation of the compositional profile in Opx–Spnl pair in H4 yields a cooling rate ∼50°C/ky, which is in very good agreement with recent metallographic cooling rate of this sample and geochronological constraints on the cooling T–t path. Numerical simulation suggests that the slow cooling of the H5/6 samples at a rate of ∼15°C/My, as deduced by recent metallographic study, could not have commenced at a temperature above ∼700°C since, otherwise, the simulated compositional profile fails to match the observed profile. For the H5 samples, the T–t path constructed on the basis of Hf–W age of peak metamorphism and two stage cooling model satisfies the constraints imposed by the Pb–Pb ages of the phosphates and Ar–Ar ages of the feldspars vs. their respective closure temperatures, whereas that for H6 samples constructed using the same approach fails to satisfy these geochronological data. A second stage of even slower cooling at ∼3°C/My needs to be invoked to satisfy the geochronological age vs. closure temperature relations. We present a model of fragmentation and re-accretion history of the parent body that could lead to the reconstructed T–t paths of the H-chondrite samples studied in this work, and discuss some of their broader implications. The cooling rates retrieved from the available data on the Fe–Mg ordering states of orthopyroxenes of some other H5 and H6 samples are orders of magnitude faster than the metallographic cooling rates that are applicable to similar low temperature domain. It is, thus, likely that all samples of the same petrologic type did not share a common cooling history.

Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk

Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62±0.04Myr) in chondrites (60Fe/56Fe∼6×10−7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5±2.6)×10−9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consistent with derivation of 60Fe from galactic background (60Fe/56Fe≈2.8×10−7 in the interstellar medium from γ-ray observations) and can be reconciled with high 26Al/27Al∼5×10−5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars (e.g., a Wolf–Rayet star) rich in 26Al and poor in 60Fe. We present the first chronological application of the 60Fe–60Ni decay system to establish the time of core formation on Vesta at 3.7−1.7+2.5Myr after condensation of calcium–aluminum-rich inclusions (CAIs).

Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

Varre-Sai: The Recent Brazilian Fall

Varre-Sai, the most recent Brazilian meteorite fall, on June 19th, 2010 at Varre-Sai, in Rio de Janeiro State, Brazil (20°51′41″S; 41°44′.80″W). At least eight masses (total ~3.5 kg) were recovered. Most are totally covered by fusion crust. The exposed interior is of light-grey colour with a few dark shock veins. Five thin polished and etched sections were prepared from a slice weighing 35 g on deposit at the National Museum/UFRJ. It consists mostly of chondrules ranging in size from 0.35 to ~2.2 mm, and chondrule fragments enclosed in a crystalline matrix. The matrix consists of tiny isolated subhedral and anhedral crystals and opaque minerals that are intergrown with broken chondrules. The chondritic texture is poorly defined with chondrule textures that vary from non-porphyritic to porphyritic ones. The essential minerals are olivine (Fa25±0.2) and low-Ca pyroxene (Fa21.66±0.2Wo1.4). Accessory minerals are plagioclase, apatite, Fe–Ni metal phases, troilite, chromite and magnetite. Mossbauer spectroscopy analysis confirms that the mineral phases are olivine, pyroxene, troilite and kamacite/taenite. Chemical data indicate that Varre-Sai is a member of the low iron L chondrite group. The observed texture and mineral phases led us to classify Varre-Sai as an equilibrated petrologic type 5. The shock features of the minerals (undulatory extinction, planar structure and numerous cracks), as well as plagioclase partial or totally transformed to maskelynite, suggest a shock stage S4. Also, some post-impact metamorphic processes could be inferred from the meta-sulfide conjoint grains that show complex mixtures of kamacite–taenite–tetrataenite and troilite. The occurrence of veins crosscutting the studied sections indicates that Varre-Sai was affected by a late fracturing event. Sealing of these fractures must have been a fast process, as shown by troilite globule textures pointing towards rapid solidification. The meteorite name was approved by the Nomenclature Committee of the Meteoritical Society (Meteoritic Bulletin, no 99).

Post-impact thermal evolution of porous planetesimals

Impacts between planetesimals have largely been ruled out as a heat source in the early Solar System, by calculations that show them to be an inefficient heat source and unlikely to cause global heating. However, the long-term, localized thermal effects of impacts on planetesimals have never been fully quantified. Here, we simulate a range of impact scenarios between planetesimals to determine the post-impact thermal histories of the parent bodies, and hence the importance of impact heating in the thermal evolution of planetesimals. We find on a local scale that heating material to petrologic type 6 is achievable for a range of impact velocities and initial porosities, and impact melting is possible in porous material at a velocity of >4km/s. Burial of heated impactor material beneath the impact crater is common, insulating that material and allowing the parent body to retain the heat for extended periods (∼millions of years). Cooling rates at 773K are typically 1–1000K/Ma, matching a wide range of measurements of metallographic cooling rates from chondritic materials. While the heating presented here is localized to the impact site, multiple impacts over the lifetime of a parent body are likely to have occurred. Moreover, as most meteorite samples are on the centimeter to meter scale, the localized effects of impact heating cannot be ignored.