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Thorsten Kleine - One of the best experts on this subject based on the ideXlab platform.
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hf w chronology of cr Chondrites implications for the timescales of chondrule formation and the distribution of 26 al in the solar nebula
Geochimica et Cosmochimica Acta, 2018Co-Authors: Gerrit Budde, Thomas S Kruijer, Thorsten KleineAbstract:Abstract Renazzo-type carbonaceous (CR) Chondrites are distinct from most other Chondrites in having younger chondrule 26Al-26Mg ages, but the significance of these ages and whether they reflect true formation times or spatial variations of the 26Al/27Al ratio within the solar protoplanetary disk are a matter of debate. To address these issues and to determine the timescales of metal-silicate fractionation and chondrule formation in CR Chondrites, we applied the short-lived 182Hf-182W chronometer to metal, silicate, and chondrule separates from four CR Chondrites. We also obtained Mo isotope data for the same samples to assess potential genetic links among the components of CR Chondrites, and between these components and bulk Chondrites. All investigated samples plot on a single Hf-W isochron and constrain the time of metal-silicate fractionation in CR Chondrites to 3.6 ± 0.6 million years (Ma) after the formation of Ca-Al-rich inclusions (CAIs). This age is indistinguishable from a ∼3.7 Ma Al-Mg age for CR chondrules, suggesting not only that metal-silicate fractionation and chondrule formation were coeval, but also that these two processes were linked to each other. The good agreement of the Hf-W and Al-Mg ages, combined with concordant Hf-W and Al-Mg ages for angrites and CV chondrules, provides strong evidence for a disk-wide, homogeneous distribution of 26Al in the early solar system. As such, the young Al-Mg ages for CR chondrules do not reflect spatial 26Al/27Al heterogeneities but indicate that CR chondrules formed ∼1–2 Ma later than chondrules from most other chondrite groups. Metal and silicate in CR Chondrites exhibit distinct nucleosynthetic Mo and W isotope anomalies, which are caused by the heterogeneous distribution of the same presolar s-process carrier. These data suggest that the major components of CR Chondrites are genetically linked and therefore formed from a single reservoir of nebular dust, most likely by localized melting events within the solar protoplanetary disk. Taken together, the chemical, isotopic, and chronological data for components of CR Chondrites imply a close temporal link between chondrule formation and chondrite accretion, indicating that the CR chondrite parent body is one of the youngest meteorite parent bodies. The relatively late accretion of the CR parent body is consistent with its isotopic composition (for instance the elevated 15N/14N) that suggests a formation at a larger heliocentric distance, probably beyond the orbit of Jupiter. As such, the accretion age of the CR chondrite parent body of ∼3.6 Ma after CAI formation provides the earliest possible time at which Jupiter's growth could have led to scattering of carbonaceous meteorite parent bodies from beyond its orbit into the inner solar system.
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thermal history modeling of the h chondrite parent body
Astronomy and Astrophysics, 2012Co-Authors: Stephan Henke, Hanspeter Gail, M Trieloff, Winfried H Schwarz, Thorsten KleineAbstract:Context. The cooling histories of individual meteorites can be empirically reconstructed by using ages obtained from different radioisotopic chronometers having distinct closure temperatures. For a given group of meteorites derived from a single parent body such data permit the detailed reconstruction of the cooling history of that body. Particularly suited for this purpose are H Chondrites because (i) all of them are thought to derive from a single parent body (possibly asteroid (6) Hebe) and (ii) for several specimens precise radiometric ages over a wide range of closure temperatures are available. Aims. A thermal evolution model for the H chondrite parent body is constructed by using the cooling histories of all H Chondrites for which at least three different precise radiometric ages are available. The thermal model thus obtained is then used to constrain some important basic properties of the H chondrite parent body. Methods. Thermal evolution models are calculated using our previously developed code, which incorporates the effects of sintering and uses new thermal conductivity data for porous materials. Several key parameters determining the thermal evolution of the H chondrite parent body are varied together with the unknown original location of the H Chondrites within their parent body until an optimal fit between the radiometric age data and the properties of the model is obtained. The fit is performed in an automated way based on an “evolution algorithm” to allow for a simultaneous fit of a large number of data, which depend in a complex way on several parameters. Empirical data for the cooling history of H Chondrites are taken from the literature and the thermal model is optimised for eight samples for which radiometric ages are available for at least three different closure temperatures. Results. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data for the cooling histories of six of the examined eight H Chondrites. For two of the samples significant discrepancies exist between model and empirical data, most likely reflecting inconsistencies in the empirical cooling data. The new thermal model constrains the radius and formation time of the H chondrite parent body, and the initial burial depths of the individual H Chondrites. In addition, the model provides an estimate for the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial 60 Fe content of the H chondrite parent body.
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si isotope systematics of meteorites and terrestrial peridotites implications for mg si fractionation in the solar nebula and for si in the earth s core
Earth and Planetary Science Letters, 2009Co-Authors: Caroline Fitoussi, Bernard Bourdon, Thorsten Kleine, Felix Oberli, B C ReynoldsAbstract:Abstract High precision Si isotope ratios have been measured for a series of meteorites and terrestrial samples using high-resolution multi-collector ICP-MS. Our results differ from those reported in an earlier study [Georg et al., 2007. Si in the Earth's core. Nature 447, 1102-1106] in two important aspects. First, our data set reveals systematic differences in δ 30 Si between different chondrite groups that are correlated with their Mg/Si elemental ratio. Second, in agreement with the previous study, δ 30 Si for the terrestrial samples are higher than values for Chondrites, but the difference between the Bulk Silicate Earth (BSE) and the carbonaceous Chondrites (Δ 30 Si BSE − c arbonaceous Chondrites = 0.08 ± 0.04‰ (1 standard deviation)) is about a factor of 2 smaller than previously reported. The δ 30 Si versus Mg/Si trend defined by the chondrite groups can be explained by reaction of olivine with a SiO-rich vapor to form enstatite, starting from a carbonaceous chondrite composition. In contrast, the difference between the BSE and carbonaceous Chondrites must reflect a different process, and can be explained by incorporation of Si into the Earth's core during metal–silicate equilibration in a deep magma ocean. The observed Si isotope fractionation is consistent with the temperatures and pressures of metal–silicate equilibration derived from siderophile element abundances in the Earth's mantle.
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hf w thermochronometry closure temperature and constraints on the accretion and cooling history of the h chondrite parent body
Earth and Planetary Science Letters, 2008Co-Authors: Thorsten Kleine, Bernard Bourdon, Mathieu Touboul, James A Van Orman, Colin Maden, Klaus Mezger, Alex N HallidayAbstract:Abstract We obtained Hf–W metal-silicate isochrons for several H Chondrites of petrologic types 4, 5, and 6 to constrain the accretion and high-temperature thermal history of the H chondrite parent body. The silicate fractions have 180 Hf/ 184 W ratios up to ∼ 51 and 182 W/ 184 W ratios up to ∼ 33 e units higher than the whole-rock. These high 180 Hf/ 184 W and radiogenic W isotope ratios result in highly precise Hf–W ages. The Hf–W ages of the H Chondrites become younger with increasing metamorphic grade and range from Δt CAI = 1.7 ± 0.7 Ma for the H4 chondrite Ste. Marguerite to Δt CAI = 9.6 ± 1.0 Ma for the H6 Chondrites Kernouve and Estacado. Closure temperatures for the Hf–W system in H Chondrites were estimated from numerical simulations of W diffusion in high-Ca pyroxene, the major host of radiogenic 182 W in H Chondrites, and range from 800 ± 50 °C for H4 Chondrites to 875 ± 75 °C for H6 Chondrites. Owing to these high closure temperatures, the Hf–W system closed early and dates processes associated with the earliest evolution of the H chondrite parent body. Consequently, the high-temperature interval of ∼ 8 Ma as defined by the Hf–W ages is much shorter than intervals obtained from Rb–Sr and Pb–Pb dating. For H4 Chondrites, heating on the parent body probably was insufficient to cause W diffusion in high-Ca pyroxene, such that the Hf–W age of Δt CAI = 1.7 ± 0.7 Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al–Mg ages of ∼ 2 Ma for L and LL chondrules and indicates that chondrules from all ordinary Chondrites formed contemporaneously. The Hf–W ages for H5 and H6 Chondrites of Δt CAI = 5.9 ± 0.9 Ma and Δt CAI = 9.6 ± 1.0 Ma correspond closely to the time of the thermal peak within the H chondrite parent body. Combined with previously published chronological data the Hf–W ages reveal an inverse correlation of cooling rate and metamorphic grade: shortly after their thermal peak H6 Chondrites cooled at ∼ 10 °C/Ma, H5 Chondrites at ∼ 30 °C/Ma and H4 Chondrites at ∼ 55 °C/Ma. These Hf–W age constraints are most consistent with an onion-shell structure of the H chondrite parent body that was heated internally by energy released from 26 Al decay. Parent body accretion started after chondrule formation at 1.7 ± 0.7 Ma and probably ended before 5.9 ± 0.9 Ma, when parts of the H chondrite parent body already had cooled from their thermal peak. The well-preserved cooling curves for the H Chondrites studied here indicate that these samples derive from a part of the H chondrite parent body that remained largely unaffected by impact disruption and reassembly but such processes might have been important in other areas. The H chondrite parent body has a 180 Hf/ 184 W ratio of 0.63 ± 0.20, distinctly lower than the 180 Hf/ 184 W = 1.21 ± 0.06 of carbonaceous chondrite parent bodies. This difference reflects Hf–W fractionation within the first ∼ 2 Ma of the solar system, presumably related to processes in the solar nebula.
D T Britt - One of the best experts on this subject based on the ideXlab platform.
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stony meteorite porosities and densities a review of the data through 2001
Meteoritics & Planetary Science, 2003Co-Authors: D T Britt, G J S J ConsolmagnoAbstract:In this review, we summarize the data published up to December 2001 on the porosity and density of stony meteorites. These data were taken from 925 samples of 454 different meteorites by a variety of techniques. Most meteorites have densities on the order of 3 to 4 g/cm^3, with lower densities only for some volatile-rich carbonaceous meteorites and higher densities for stony irons. For the vast majority of stones, porosity data alone cannot distinguish between different meteorite compositions. Average porosities for most meteorite classes are around 10%, though individual samples can range as high as 30% porosity. Unbrecciated basaltic aChondrites appear to be systematically less porous unless vesicles are present. The measured density of ordinary Chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment. A theoretical grain density based on composition can model "pre-weathered" porosities. The average model porosity for H and LL Chondrites is 10%, while L chondrite model porosities average only 6%, a statistically significant difference.
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Reflection spectra of shocked ordinary Chondrites and their relationship to asteroids
Icarus, 1992Co-Authors: Klaus Keil, Jeffrey F. Bell, D T BrittAbstract:Abstract Although ordinary Chondrites are the most common meteorites falling on Earth, reflectance spectra of only a few rare asteroids resemble those of powdered Chondrites measured in the laboratory. Therefore, “space weathering” processess which may have altered the surfaces of ordinary chondrite asteroids so that their spectra resemble those of the abundant S asteroids have been suggested. Recently, Britt et al. (1989, Lunar Planet. Sci. Conf. 19th , 537–545; and 1989, Lunar and Planet. Sci. XX , 111–112) and Britt and Pieters (1989, Lunar Planet. Sci. XX , 109–110) measured spectra of “shock-blackened” ordinary Chondrites which possess much lower reflectance and shallower absorption bands than those of “normal” ordinary Chondrites and, in some cases, resemble those of carbonaceous Chondrites and C asteroids. They therefore propose that surfaces of ordinary chondrite asteroids may have been shock-blackened by impact, and that these asteroids may be hidden among the C asteroids. We measured the spectral reflectance of a number of mineralogically well-characterized, shock-blackened ordinary Chondrites exhibiting four major types of black, shock-produced features: opaque melt veins (shock veins), melt pockets and irregular interconnected melt veins, melt dikes, and black Chondrites, Stoffler, Keil, and Scott (1991, Geochim. Cosmochim. Acta 55, 3845–3867.) We confirm that their spectra resemble those of C asteroids. However, the occurence of these materials in impact crater basements and floors rather than on the surface, their low abundance in craters relative to brecciated and ejected material, and their low abundance among ordinary chondrite falls suggest that the surfaces of ordinary chondrite parent bodies are not likely to be covered by vast amountt of such shock-blackened materials. Thus, these materials cannot be responsible for significant large-scale spectral alterations of the parent asteroids of ordinary Chondrites, and they cannot be called upon in support of the hypothesis that ordinary chondrite asteroids are hidden among C asteroids. If this hypothesis is to be upheld, then recourse may have to be taken to the suggestion of Britt and Pieters (1991, Lunar Planet. Sci. XXII , 139–142) that the surfaces of ordinary chondrite parent asteroids appear spectrally similar to those of C asteroids because they are covered by a hypothetical, thin layer of fine-grained material similar to that present in the dark portions of solar wind-bearing regolith breccias.
Frédéric Moynier - One of the best experts on this subject based on the ideXlab platform.
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Tin and zinc stable isotope characterisation of Chondrites and implications for early Solar System evolution
Chemical Geology, 2019Co-Authors: John B. Creech, Frédéric MoynierAbstract:Abstract Moderately volatile elements show variable depletion in terrestrial planets compared to the Sun. Isotopic ratios can be used as a signature of the processes at the origin of this depletion. Using a new method, the Sn stable isotope composition and elemental abundance in 36 primitive meteorites (Chondrites) have been characterised to high precision. Significant mass-dependent Sn isotope variations are found within Chondrites. The widest isotopic range is observed for the ordinary Chondrites (−1.1 ‰ to +0.5 ‰ in δ122/118Sn, representing the difference in the 122Sn/118Sn ratio of the sample relative to our in-house standard, Sn_IPGP), with the ordinary chondrite groups extending to lighter isotopic compositions in the order H > L > LL, while carbonaceous and enstatite Chondrites are heavier and occupy narrower compositional ranges. Tin and Zn isotope and concentration data are strongly correlated, particularly in ordinary Chondrites, from which both sets of data were obtained on the same rock powders. Given the difference in geochemical behaviour (Zn lithophile/chalcophile and Sn chalcophile/siderophile) of these elements, this suggests that the primary control on the isotope and abundance variations is volatility. Chondrite groups show variability increasing with petrographic types, suggesting a secondary control from parent-body metamorphism. The isotopic composition of the bulk silicate Earth (BSE; δ 122/118Sn = 0.49 ± 0.11 ‰) overlaps with the carbonaceous Chondrites (δ 122/118Sn = 0.43 ± 0.12 ‰; excl. CR and CK). Despite isotopic similarities for almost all isotopic systems, EH Chondrites have Sn isotope compositions that are distinct from the bulk silicate Earth (δ 122/118Sn = 0.18 ± 0.21 ‰). Therefore, an enstatite chondrite-like bulk Earth requires that isotopically light Sn was lost from the silicate Earth, possibly into the metallic core or a sulphide matte, or by evaporative loss from Earth or its precursors.
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nature of volatile depletion and genetic relationships in enstatite Chondrites and aubrites inferred from zn isotopes
Geochimica et Cosmochimica Acta, 2011Co-Authors: Frédéric Moynier, Randal C Paniello, Matthieu Gounelle, Francis Albarede, P Beck, Frank A Podosek, B. ZandaAbstract:Enstatite meteorites include the undifferentiated enstatite Chondrites and the differentiated enstatite aChondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite Chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well. For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL Chondrites, EH Chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 Chondrites. The different groups of meteorites are isotopically distinct and give the following values (parts per thousand): aubrite main group (-7.08 < delta Zn-66 <-0.37); EH3 Chondrites (0.15 < delta Zn-66 <0.31); EH4 Chondrites (0.15 < delta Zn-66 <0.27); EH5 Chondrites (delta Zn-66 = 0.27 +/- 0.09; n = 1); EL3 Chondrites (0.01 < delta Zn-66 <0.63); the Shallowater aubrite (1.48< delta Zn-66 <2.36); EL6 Chondrites (2.26 < delta Zn-66 < 7.35); and the impact-melt enstatite chondrite Happy Canyon (delta Zn-66 = 0.37). The aubrite Pena Blanca Spring (delta Zn-66 = 7.0 parts per thousand) and the EL6 North West Forrest (delta Zn-66 = 7.35%) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of Chondrites and terrestrial samples (-1.5 < delta Zn-66 < 1 parts per thousand) is much smaller (Luck et al., 2005; Herzog et al., 2009). EH and EL3 Chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite Chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 Chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL Chondrites and they are enriched in light isotopes (delta Zn-66 down to -7.04 parts per thousand). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubritcs evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.
Michael K. Weisberg - One of the best experts on this subject based on the ideXlab platform.
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The unequilibrated enstatite Chondrites
Geochemistry, 2012Co-Authors: Michael K. Weisberg, Makoto KimuraAbstract:Abstract The enstatite Chondrites formed under highly reducing (and/or sulfidizing) conditions as indicated by their mineral assemblages and compositions, which are sharply different from those of other chondrite groups. Enstatite is the major silicate mineral. Kamacite is Si-bearing and the enstatite Chondrites contain a wide variety of monosulfide minerals that are not present in other chondrite groups. The unequilibrated enstatite Chondrites are comprised of two groups (EH3 and EL3) and one anomalous member (LEW 87223), which can be distinguished by differences in their mineral assemblages and compositions. EH3 Chondrites have >1.8 wt.% Si in their kamacite and contain the monosulfide niningerite (MgS), whereas EL3 Chondrites have less than 1.4 wt.% Si in their kamacite and contain the monosulfide alabandite (MnS). The distinct mineralogies, compositions and textures of E3 Chondrites make comparisons with ordinary Chondrites (OCs) and carbonaceous Chondrites (CCs) difficult, however, a range of recrystallization features in the E3s are observed, and some may be as primitive as type 3.1 OCs and CCs. Others, especially the EL3 Chondrites, may have been considerably modified by impact processes and their primary textures disturbed. The chondrules in E3 Chondrites, although texturally similar to type I pyroxene-rich chondrules, are sharply different from chondrules in other chondrite groups in containing Si-bearing metal, Ca- and Mg–Mn-rich sulfides and silica. This indicates formation in a reduced nebular environment separate from chondrules in other Chondrites and possibly different precursor materials. Additionally the oxygen isotope compositions of E3 chondrules indicate formation from a unique oxygen reservoir. Although the abundance, size distribution, and secondary alteration minerals are not always identical, CAIs in E3 Chondrites generally have textures, mineral assemblages and compositions similar to those in other groups. These observations indicates that CAIs in O, C and E Chondrites all formed in the reservoir under similar conditions, and were redistributed to the different chondrite accretion zones, where the secondary alteration took place. Thus, chondrule formation was a local process for each particular chondrite group, but all CAIs may have formed in the similar nebular environment. Lack of evidence of water (hydrous minerals), and oxygen isotope compositions similar to Earth and Moon suggest formation of the E Chondrites in the inner solar system and make them prime candidates as building blocks for the inner planets.
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thermal history of the enstatite Chondrites from silica polymorphs
Meteoritics & Planetary Science, 2005Co-Authors: Makotoki Kimura, Michael K. Weisberg, Yangting Lin, Akio Suzuki, Eiji Ohtani, Ryuji OkazakiAbstract:Here we report the results of our petrologic and mineralogical study of enstatite (E) Chondrites in order to explore their thermal histories. We studied silica phases in 20 E Chondrites by laser micro Raman spectroscopy to determine the silica polymorphs they contain. Silica phases are commonly present in E Chondrites and their polymorphs reflect the physical conditions of formation. The samples studied here include EH3-5, EL3-6, E chondrite melt rocks, and an anomalous E chondrite. We identified quartz, tridymite, cristobalite, and silica glass in the samples studied. EH4 5 and EH melt rocks are divided into high and low temperature classes based on niningeritealabandite solid solutions. EH3, EL3, and some EH melt rocks of the high temperature class contain tridymite and cristobalite. We suggest that tridymite and cristobalite crystallized in chondrules and E chondrite melts, followed by rapid cooling, leading to the survival of these silica polymorphs. EH4 and EL4 Chondrites also contain tridymite and cristobalite in their chondrules, indicating that these silica polymorphs survived low temperature metamorphism (as estimated from opaque mineral geothermometers) because of the sluggishness of the transition to a more stable polymorph. Tridymite and cristobalite in EL6 Chondrites reflect the high temperature processes experienced by these meteorites. On the other hand, some EH5 Chondrites and EH melt rocks of the low temperature class contain quartz, which may be a product of the transition from tridymite or cristobalite during a long period of low temperature metamorphism. Although the thermal history of E Chondrites have been previously estimated from opaque minerals, such compositions mainly reflect low temperature processes. However, we can reconstruct the primordial thermal processes and subsequent cooling histories of E Chondrites from their silica polymorphs. The E Chondrites have complicated thermal histories, which produced the observed variations among them.
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A new metal-rich chondrite grouplet
Meteoritics & Planetary Science, 2001Co-Authors: Michael K. Weisberg, Robert N. Clayton, Martin Prinz, Toshiko K. Mayeda, Naoji Sugiura, Shigeo Zashu, Mitsuru EbiharaAbstract:Abstract-A new grouplet of primitive, metal-rich Chondrites, here called the CB (C, carbonaceous; B, bencubbinite) Chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah a1 Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH Chondrites, all of which are members of the more inclusive CR clan. CB Chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB Chondrites, as well as other CR clan Chondrites, have much heavier nitrogen (higher 15N/14N) than that in other chondrite groups. The primitive characteristics of the CB Chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 9555 1, is petrographically similar to the CB Chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.
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The K (Kakangari) chondrite grouplet
Geochimica et Cosmochimica Acta, 1996Co-Authors: Michael K. Weisberg, Robert N. Clayton, Martin Prinz, Toshiko K. Mayeda, Monica M. Grady, Ian A. Franchi, Colin Pillinger, Gregory W. KallemeynAbstract:Abstract The Kakangari, LEW 87232, and Lea Co. 002 Chondrites have a similar set of petrologic and oxygen isotopic characteristics that distinguishes them from other chondrite groups. They are here established to constitute a single chondrite grouplet—the K (after Kakangari) Chondrites. The K Chondrites have (1) high matrix abundances (33–77 vol%) as do carbonaceous Chondrites, (2) metal abundances (6–10 vol%) that are similar to the H group ordinary Chondrites, (3) average mafic silicate compositions (average Kakangari olivine = Fa2.2; enstatite Fs4.4) that indicate an oxidation state intermediate between H and E Chondrites, (4) matrix that differs from that in other chondrite groups in being enstatite-rich with compositions more Mg-rich (average = Fs3) than those in the chondrules, (5) refractory lithophile element abundances (
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Carbonates in the Kaidun Chondrite
Meteoritics, 1994Co-Authors: Martin Prinz, Michael K. Weisberg, Michael E. Zolensky, A. V. IvanovAbstract:Kaidun is a remarkable chondrite breccia fall containing lithic clasts that span a wide range of chondrite groups including C and E Chondrites, as well as having clasts with characteristics not yet found in existing chondrite samples. The dominant lithology in Kaidun appears to be CR chondritic, consonant with recent O isotope data. The carbonates in Kaidun are presented as one mineralogical basis for comparing it to the other hydrated Chondrites and to better understand its relative alteration history. The four polished thin sections of Kaidun studied contained a variety of lithologies that we classified into four groups -- CR, E, CM-like, and dark inclusions (DIs). DIs contain sulfide and magnetite morphologies that superficially resemble CI Chondrites, and some of the previously reported CI lithologies in Kaidun may be what we term DIs. Carbonates were found in all lithologies studied. Carbonates in Kaidun are similar in composition to those in CR Chondrites. Some of the DIs in Kaidun, previously characterized as CI, have carbonates similar to those in CR Chondrites and are unlike those in CI or CM Chondrites. Most carbonates in Kaidun and CR Chondrites are calcites, some of which formed at temperatures above 250 C. Dolomite is less common and some may be metastable. Alteration temperatures in the Renazzo CR chondrite were estimated to be approximately 300 C, based on O isotope fractionation between phyllosilicates and magnetite. Temperatures of up to 450 C were proposed for the alteration of a CR-like dark inclusion in Kaidun, based on the presence of hydrothermal pentlandite veins. The alteration temperatures for Kaidun and the other CR Chondrites are considerably higher than those suggested for CI or CM parent bodies.
Muhammad Humayun - One of the best experts on this subject based on the ideXlab platform.
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high precision osmium isotopes in enstatite and rumuruti Chondrites
Geochimica et Cosmochimica Acta, 2011Co-Authors: David Van Acken, A D Brandon, Muhammad HumayunAbstract:Abstract Isotopic heterogeneity within the solar nebula has been a long-standing issue. Studies on primitive Chondrites and chondrite components for Ba, Sm, Nd, Mo, Ru, Hf, Ti, and Os yielded conflicting results, with some studies suggesting large-scale heterogeneity. Low-grade enstatite and Rumuruti Chondrites represent the most extreme ends of the chondrite meteorites in terms of oxidation state, and might thus also present extremes if there is significant isotopic heterogeneity across the region of chondrite formation. Osmium is an ideal tracer because of its multiple isotopes generated by a combination of p-, r-, and s-process and, as a refractory element; it records the earliest stages of condensation. Some grade 3–4 enstatite and Rumuruti Chondrites show similar deficits of s-process components as revealed by high-precision Os isotope studies in some low-grade carbonaceous and ordinary Chondrites. Enstatite Chondrites of grades 5–6 have Os isotopic composition identical within error to terrestrial and solar composition. This supports the view of digestion-resistant presolar grains, most likely SiC, as the major carrier of these anomalies. Destruction of presolar grains during parent body processing, which all high-grade enstatite Chondrites, but also some low-grade Chondrites seemingly underwent, makes the isotopically anomalous Os accessible for analysis. The magnitude of the anomalies is consistent with the presence of a few ppm of presolar SiC with a highly unusual isotopic composition, produced in a different stellar environment like asymptotic giant branch stars (AGB) and injected into the solar nebula. The presence of similar Os isotopic anomalies throughout all major chondrite groups implies that carriers of Os isotopic anomalies were homogeneously distributed in the solar nebula, at least across the formation region of Chondrites.
