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1.
The Agoudal IIAB iron meteorite exhibits only kamacite grains (~6 mm across) without any taenite. The kamacite is homogeneously enriched with numerous rhabdite inclusions of different size, shape, and composition. In some kamacite domains, this appears frosty due to micron‐scale rhabdite inclusions (~5 to 100 μm) of moderate to high Ni content (~26 to 40 wt%). In addition, all the kamacite grains in matrix are marked with a prominent linear crack formed during an atmospheric break‐up event and subsequently oxidized. This feature, also defined by trails of lowest Ni‐bearing (mean Ni: 23 wt%) mm‐scale rhabdite plates (fractured and oxidized) could be a trace of a pre‐existing γ–α interface. Agoudal experienced a very slow rate of primary cooling ~4 °C Ma?1 estimated from the binary plots of true rhabdite width against corresponding Ni wt% and the computed cooling rate curves after Randich and Goldstein (1978). Chemically, Agoudal iron (Ga: 54 ppm; Ge: 140 ppm; Ir: 0.03 ppm) resembles the Ainsworth iron, the coarsest octahedrite of the IIAB group. Agoudal contains multiple sets of Neumann bands that are formed in space and time at different scales and densities due to multiple impacts with shock magnitude up to 130 kb. Signatures of recrystallization due to postshock low temperature mild reheating at about 400 °C are also locally present.  相似文献   

2.
Fossil iron meteorites are extremely rare in the geological sedimentary record. The paleometeorite described here is the first such finding at the Cretaceous‐Paleogene (K‐Pg) boundary. In the boundary clay from the outcrop at the Lechówka quarry (Poland), fragments of the paleometeorite were found in the bottom part of the host layer. The fragments of meteorite (2–6 mm in size) and meteoritic dust are metallic‐gray in color and have a total weight of 1.8181 g. Geochemical and petrographic analyses of the meteorite from Lechówka reveal the presence of Ni‐rich minerals with a total Ni amount of 2–3 wt%. The identified minerals are taenite, kamacite, schreibersite, Ni‐rich magnetite, and Ni‐rich goethite. No relicts of silicates or chromites were found. The investigated paleometeorite apparently represents an independent fall and does not seem to be derived from the K‐Pg impactor. The high degree of weathering did not permit the chemical classification of the meteorite fragments. However, the recognized mineral inventory, lack of silicates, and their pseudomorphs and texture may indicate that the meteorite remains were an iron meteorite.  相似文献   

3.
Abstract— We have measured the size of the high‐Ni particles in the cloudy zone and the width of the outer taenite rim in eight low shocked and eight moderately to heavily shocked IVA irons using a transmission electron microscope (TEM). Thin sections for TEM analysis were produced by a focused ion beam instrument. Use of the TEM allowed us to avoid potential artifacts which may be introduced during specimen preparation for SEM analysis of high Ni particles <30 nm in size and to identify microchemical and microstructural changes due to the effects of shock induced reheating. No cloudy zone was observed in five of the eight moderately to highly shocked (>13 GPa) IVA irons that were examined in the TEM. Shock induced reheating has allowed for diffusion from 20 nm to 400 nm across kamacite/taenite boundaries, recrystallization of kamacite, and the formation, in Jamestown, of taenite grain boundaries. In the eleven IVA irons with cloudy zone microstructures, the size of the high‐Ni particles in the cloudy zone increases directly with increasing bulk Ni content. Our data and the inverse correlation between cooling rate and high‐Ni particle size for irons and stony‐irons show that IVA cooling rates at 350‐200 °C are inversely correlated with bulk Ni concentration and vary by a factor of about 15. This cooling rate variation is incompatible with cooling in a metallic core that was insulated with a silicate mantle, but is compatible with cooling in a metallic body of radius 150 ± 50 km. The widths of the tetrataenite regions next to the cloudy zone correlate directly with high‐Ni particle size providing another method to measure low temperature cooling rates.  相似文献   

4.
Abstract– The single‐piece iron meteorite Javorje, with a mass of 4920 g, is the heaviest and largest meteorite found in the territory of Slovenia. The meteorite Javorje is a medium octahedrite with kamacite bandwidth of 0.85 ± 0.26 mm. The bulk composition of Ni (7.83 wt%), Co (0.48 wt%) and trace elements Ga (25 μg/g), Ge (47 μg/g), Ir (7.6 μg/g), As (5.8 μg/g), Au (0.47 μg/g), and Pt (13.4 μg/g) indicates that the meteorite Javorje belongs to the chemical group IIIAB. Mineral and bulk chemical compositions are consistent with other reported group IIIAB meteorites. The presence of numerous rhabdites, carlsbergite, sparse troilite, and chromite and abundance of daubréelites are in accordance with low‐Ni and low‐P IIIAB iron meteorites. The severely weathered surface and secondary weathering products in the interior of the meteorite suggest its high terrestrial age.  相似文献   

5.
Abstract— The microstructure of an iron meteorite which fell near Akyumak, East Anatolia, Turkey on 2 August 1981 has been examined and its composition determined. The Ni content is 7.7% and kamacite bandwidth is 0.32 ± 0.06 mm. The kamacite contains Neumann bands and some daubreelite inclusions. Taenite and plessite account for about 45 to 50% of the metal; finger, cellular and net plessite are observed. Gallium (1.9 ppm), Ge (< 40 ppm) and Ir (1.81 ppm) were determined by neutron activation. The microstructural observations and chemical data show Akyumak to be a fine octahedrite and a member of group IVA.  相似文献   

6.
Abstract– The microstructures of six reheated iron meteorites—two IVA irons, Maria Elena (1935), Fuzzy Creek; one IVB iron, Ternera; and three ungrouped irons, Hammond, Babb’s Mill (Blake’s Iron), and Babb’s Mill (Troost’s Iron)—were characterized using scanning and transmission electron microscopy, electron‐probe microanalysis, and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids. Maria Elena and Hammond were heated below approximately 700–750 °C, so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite. Both meteorites retained a record of the original Widmanstätten pattern. The other four, which show no trace of their original microstructure, were heated above 600–700 °C and recrystallized to form 10–20 μm wide homogeneous taenite grains. On cooling, kamacite formed on taenite grain boundaries with their close‐packed planes aligned. Formation of homogeneous 20 μm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 °C or approximately 1 h at 1300 °C. All six irons contain approximately 5–10 μm wide taenite grains with internal microprecipitates of kamacite and nanometer‐scale M‐shaped Ni profiles that reach approximately 40% Ni indicating cooling over 100–10,000 yr. Un‐decomposed high‐Ni martensite (α2) in taenite—the first occurrence in irons—appears to be a characteristic of strongly reheated irons. From our studies and published work, we identified four progressive stages of shock and reheating in IVA irons using these criteria: cloudy taenite, M‐shaped Ni profiles in taenite, Neumann twin lamellae, martensite, shock‐hatched kamacite, recrystallization, microprecipitates of taenite, and shock‐melted troilite. Maria Elena and Fuzzy Creek represent stages 3 and 4, respectively. Although not all reheated irons contain evidence for shock, it was probably the main cause of reheating. Cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays. If the reheated irons that we studied are representative, the IVA irons may have been shocked soon after they cooled below 200 °C at 4.5 Gyr in an impact that created a rubblepile asteroid with fragments from diverse depths. The primary cooling rates of the IVA irons and the proposed early history are remarkably consistent with the Pb‐Pb ages of troilite inclusions in two IVA irons including the oldest known differentiated meteorite ( Blichert‐Toft et al. 2010 ).  相似文献   

7.
We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main‐group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe‐Ni matrix with 9–10 wt% Ni and low‐Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe‐Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low‐Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main‐group pallasites, including clinopyroxene (augite), tridymite, K‐rich felsic glass, and an unknown Ca‐Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two‐pyroxene barometry (0.39 ± 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main‐group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius.  相似文献   

8.
Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ～25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.  相似文献   

9.
Abstract— Instrumental neutron activation analysis (INAA) was used to determine Ni, Co, Cu, Ga, As, Au, W, Re and Ir in taenite lamellae isolated by acid dissolution from eight iron meteorites from groups IA, IIIAB and IVA. Taenite is enriched in Ni, Cu, Ga, As, Au, W, Re and Ir relative to kamacite, whereas taenite is depleted in Co. Taenite/kamacite partition ratios in slowly cooled IAB meteorites are farther from unity than those in rapidly cooled IVA meteorites. Taenite/kamacite partition ratios for Cu, Ir, Au and Co may be sensitive cooling rate indicators.  相似文献   

10.
Abstract— –Meridiani Planum is the first iron meteorite found on Mars. It was discovered in 2005 by the Mars Exploration Rover Opportunity (MER‐B). Mössbauer spectra (MS) of the unbrushed and brushed meteorite species were acquired in 10 degrees temperature windows in the range of 210–260 K. Earlier examinations of these MS have led to the conclusion that the meteorite, which contains ～～7 wt% Ni, belongs to the IAB meteorite group. Here, making use of a recently developed calibration/folding procedure for MER MS, we report the results of the MS analyses for the single temperature windows m5 (210–220 K), m6 (220–230 K), m7 (230–240 K), and m89 (240–260 K). All spectra consist of a sextet and a ferric doublet. The hyperfine field of the sextet, extrapolated to room temperature, is ～～34.5 T, which is, based on Mössbauer studies of meteorites found on Earth, indeed consistent with the presence of kamacite. The fractional spectral area of the sextet is ～～0.96 of the total spectrum. The ferric doublet has an average quadrupole splitting of 0.70 mm/s and is not diagnostic of any specific Fe mineral.  相似文献   

11.
A meteorite fall was heard and collected on July 13, 2010 at about 18:00 (local time) in the Shibanjing village of the Huaxi district of Guiyang, Guizhou province, China. The total mass of the fall is estimated to be at least 1.6 kg; some fragments are missing. The meteorite consists mainly of olivine, low‐Ca pyroxene, high‐Ca pyroxene, plagioclase, kamacite, taenite, and troilite. Minor phases include chromite and apatite. Various textural types of chondrules exist in this meteorite: most chondrule textures can be easily defined. The grain sizes of secondary plagioclase in this meteorite range from 2 to 50 μm. The chemical composition of olivine and low‐Ca pyroxene are uniform; Fa in olivine and Fs in low‐Ca pyroxene are, respectively, 19.6 ± 0.2 and 17.0 ± 0.3 (mole%). Huaxi has been classified as an H5 ordinary chondrite, with a shock grade S2, and weathering W0. The weak shock features, rare fractures, and the high porosity (17.6%) indicates that Huaxi is a less compacted meteorite. The preatmospheric radius of Huaxi is ~11 cm, corresponding to ~21 kg. The meteorite experienced a relatively short cosmic‐ray exposure of about 1.6 ± 0.1 Ma. The 4He and 40Ar retention ages are older than 4.6 Ga implying that Huaxi did not degas after thermal metamorphism on its parent body.  相似文献   

12.
Abstract— The Tuxtuac meteorite fell in Zacatecas state, Mexico, on 16 October 1975, at 1820 hours. Two partly crusted masses, weighing 1924 g and 2340 g, were recovered. The stone is an ordinary chondrite, LL5, with olivine Fa30 and 19.22 weight % total iron. The silicates contain numerous voids and a froth-like mesostasis is present within some chondrules. Metal phases present are kamacite (5.7–6.4% Ni, 6–7% Co) and high nickel metal (taenite 37–41% Ni, 1.7 ± 0.3% Co; tetrataenite 47–52% Ni, 0.8–1.4% Co). The stone is unusual for an LL-group chondrite in that it exhibits neither large-scale brecciation features nor dark veins.  相似文献   

13.
NWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal‐saturated Fe‐Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni‐poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr‐bearing (up to 1 wt%), coexists with micrometer‐sized taenite/tetrataenite‐type native Ni‐Fe alloys (Ni/Fe = 1) and Fe‐Os‐Ir‐(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame‐like pentlandite (Fe/Fe + Ni = 0.5–0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni‐Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal‐deficient monoclinic‐type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO2 conditions inferred from chromites compositions and accepted for Chassignites (FMQ‐1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock‐related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites.  相似文献   

14.
Abstract— The Ulasitai iron was recently found about 130 km southeast to the find site of the Armanty (Xinjiang, IIIE) meteorite. It is a coarse octahedrite with a kamacite bandwidth of 1.2 ± 0.2 (0.9–1.8) mm. Plessite is abundant, as is taenite, kamacite, cohenite, and schreibersite with various microstructures. Schreibersite is Ni‐rich (30.5–55.5 wt%) in plessite or coexisting with troilite and daubreelite, in comparison with the coarse laths (20.6–21.2 wt%) between the Widmanstätten pattern plates. The correlation between the center Ni content and the half bandwidth of taenite suggest a cooling rate of ?20 °C/Myr based on simulations. The petrography and mineral chemistry of Ulasitai are similar to Armanty. The bulk samples of Ulasitai were measured, together with Armanty, Nandan (IIICD), and Mundrabilla (IIICD), by inductively coupled plasma atomic emission spectrometry (ICP‐AES) and mass spectrometry (ICP‐MS). The results agree with literature data of the same meteorites, and our analyses of four samples of Armanty (L1, L12, L16, L17) confirm a homogeneous composition (Wasson et al. 1988). The bulk composition of Ulasitai is identical to that of Armanty, both plotting within the IIIE field. We classify Ulasitai as a new IIIE iron and suggest that it pairs with Armanty.  相似文献   

15.
A 435 kg piece of the Mont Dieu iron meteorite (MD) contains cm‐sized silicate inclusions. Based on the concentration of Ni, Ga, Ge, and Ir (8.59 ± 0.32 wt%, 25.4 ± 0.9 ppm, 61 ± 2 ppm, 7.1 ± 0.4 ppm, respectively) in the metal host, this piece can be classified as a IIE nonmagmatic iron. The silicate inclusions possess a chondritic mineralogy and relict chondrules occur throughout the inclusions. Major element analysis, oxygen isotopic analysis (Δ17O = 0.71 ± 0.02‰), and mean Fa and Fs molar contents (Fa15.7 ± 0.4 and Fs14.4 ± 0.5) indicate that MD originated as an H chondrite. Because of strong similarities with Netschaëvo IIE, MD can be classified in the most primitive subgroup of the IIE sequence. 40Ar/39Ar ages of 4536 ± 59 Ma and 4494 ± 95 Ma obtained on pyroxene and plagioclase inclusions show that MD belongs to the old (~4.5 Ga) group of IIE iron meteorites and that it has not been perturbed by any subsequent heating event following its formation. The primitive character of MD sheds light on the nature of its formation process, its thermal history, and the evolution of its parent body.  相似文献   

16.
Magnetic 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.  相似文献   

17.
Abstract— Cooling rate experiments were performed on P‐free Fe‐Ni alloys that are compositionally similar to ordinary chondrite metal to study the taenite ? taenite + kamacite reaction. The role of taenite grain boundaries and the effect of adding Co and S to Fe‐Ni alloys were investigated. In P‐free alloys, kamacite nucleates at taenite/taenite grain boundaries, taenite triple junctions, and taenite grain corners. Grain boundary diffusion enables growth of kamacite grain boundary precipitates into one of the parent taenite grains. Likely, grain boundary nucleation and grain boundary diffusion are the applicable mechanisms for the development of the microstructure of much of the metal in ordinary chondrites. No intragranular (matrix) kamacite precipitates are observed in P‐free Fe‐Ni alloys. The absence of intragranular kamacite indicates that P‐free, monocrystalline taenite particles will transform to martensite upon cooling. This transformation process could explain the metallography of zoneless plessite particles observed in H and L chondrites. In P‐bearing Fe‐Ni alloys and iron meteorites, kamacite precipitates can nucleate both on taenite grain boundaries and intragranularly as Widmanstätten kamacite plates. Therefore, P‐free chondritic metal and P‐bearing iron meteorite/pallasite metal are controlled by different chemical systems and different types of taenite transformation processes.  相似文献   

18.
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19.
On August 19, 2020, at 13:18—UTC, a meteor event ended as a meteorite shower in Santa Filomena, a city in the Pernambuco State, northeast Brazil. The heliocentric orbital parameters resulting from images by cameras of the weather broadcasting system were semimajor axis a = 2.1 ± 0.1 au, eccentricity e = 0.55 ± 0.03, and inclination i = 0.15o ± 0.05. The data identified the body as an Apollo object, an Earth-crossing object with a pericenter interior to the Earth's orbit. The chemical, mineralogical, and petrological evaluations, as well as the physical analysis, followed several traditional techniques. The meteorite was identified as a H5-6 S4 W0 ordinary chondrite genomict breccia. The large amount of metal in the meteorite made a metallographic evaluation based on the opaque phases possible. The monocrystalline kamacite crystals suggest a higher petrological type and the distorted Neumann lines imply at least two different shock events. The absence of the plessite phase shows that the meteorite did not reach the highest shock levels S5 and S6. The well-defined polycrystalline taenite is indicative of petrologic types 4 and 5 due to the conserved internal tetrataenite rim at the boundaries. The presence of polycrystalline taenites and the characteristics of the Agrell Effect suggest that the Santa Filomena meteorite did not reheat above 700°C. The absence of martensite confirms reheating temperatures <800°C and a slow cooling rate. The Ni contents and sizes of the zoned taenite particles indicate a slow cooling rate ranging from 1 to 10 K Myr−1.  相似文献   

20.
Abstract— The (compositionally) closely related iron meteorite groups IIIE and IIIAB were originally separated based on differences in kamacite bandwidth, the presence of carbides only in the IIIE group, and marginally resolvable differences on the Ga‐Ni and Ge‐Ni diagrams. A total of six IIIE iron meteorites have been analyzed for C and N using secondary ion mass spectrometry, and three of these have also been analyzed for N, Ne, and Ar by stepped combustion. We show that these groups cannot be resolved on the basis of N abundances or isotopic compositions but that they are marginally different in C‐isotopic composition and nitride occurrence. Cosmic‐ray exposure age distributions of the IIIE and IIIAB iron meteorites seem to be significantly different. There is a significant N‐isotopic range among the IIIE iron meteorites. A negative correlation between δ15N and N concentration suggests that the increase in s?15N resulted from diffusional loss of N.  相似文献   

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