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1.
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   

2.
Abstract— We report data on three new iron meteorites from Northern Chile and propose names. All are unnamed iron meteorites from the meteorite collection at the Universidad de La Serena. For two, the provenance is unknown; for the third, the presumed discovery site is in the countryside east of Iquique. The three meteorites have been analyzed by instrumental neutron activation analysis (INAA) and their structures examined with a binocular microscope. La Serena is a complete 663 g iron, a new member of group IIICD; it is not paired with any other iron. Elqui has a mass of 260 g; two faces are fractures, possibly produced by human actions, but fusion crust appears to be present on some of the remaining surface. It is a hexahedrite and a member of group IIAB, but its composition differs from that of all other Chilean hexahedrites. The third iron, Pozo Almonte, is a medium octahedrite member of group IIIAB, one of the most common meteorite groups. To find out whether it is paired, we assembled a full set of IIIAB iron meteorites from Northern Chile. Our compositional data show that Pozo Almonte is not paired with any other IIIAB iron, and that there are no pairings within the full set with the possible exception of Joel's Iron and Sierra Sandon, which differ only in their contents of Ir, 0.39 and 0.34 μg/g, respectively. However, Buchwald's (1975) structural observations rule out this possible pairing. We find appreciable differences in Cu, As and Au between the previously paired IIIAB irons Chañaral and Ilimaës and conclude that these should not be paired.  相似文献   

3.
The Whitecourt meteorite impact crater, Alberta, Canada is a rare example of a well‐preserved small impact structure, with which thousands of meteorite fragments are associated. As such, this crater represents a unique opportunity to investigate the effect of a low‐energy impact event on an impacting iron bolide. Excellent documentation of meteorite fragment locations and characteristics has generated a detailed distribution map of both shrapnel and regmaglypted meteorite types. The meteorites' distribution, and internal and external characteristics support a low‐altitude breakup of the impactor which caused atmospherically ablated (regmaglypted) meteorites to fall close to the crater and avoid impact‐related deformation. In contrast, shrapnel fragments sustained deformation at macro‐ and microscales resulting from the catastrophic disruption of the impactor. The impactor was significantly fragmented along pre‐existing planes of weakness, including kamacite lamellae and inclusions, resulting in a bias toward low‐mass (<100 g) fragments. Meteorite mineralogy was investigated and the accessory minerals were found to be dominated by sulfides and phosphides with rare carlsbergite, consistent with other low‐Ni IIIAB iron meteorites. Considerations of the total mass of meteoritic material recovered at the site relative to the probable fraction of the impactor that was preserved based on modeling suggests that the crater was formed by a higher velocity, lower mass impactor than previously inferred.  相似文献   

4.
Abstract— The measured Cu and Cr contents in magmatic iron meteorites appear to contradict the behavior predicted by experimental fractional crystallization studies currently available. To investigate the origin of Cu and Cr concentrations observed in these meteorites, a thorough set of solid metal/liquid metal experiments were conducted in the Fe‐Ni‐S system. In addition to Cu and Cr, partitioning values were also determined for As, Au, Bi, Co, Mo, Ni, Pb, Rh, Ru, Sb, Sn, V, and Zn from the experiments. Experimental results for Cu and Cr showed similar chalcophile partitioning behavior, whereas these elements have differently sloped trends within magmatic iron meteorite groups. Thus, fractional crystallization alone cannot control both the Cu and Cr concentrations in these iron meteorite groups. A simple fractional crystallization model based on our experimental Cu partitioning results was able to match the Cu versus Au trend observed in the S‐poor IVB iron meteorite group but not the decreasing Cu versus Au trends in the IIAB and IIIAB groups or the unique S‐shaped Cu versus Au trend in the IVA group. However, the crystallization model calculations were found to be very sensitive to the specific choice for the mathematical expression of D(Cu), suggesting that any future refinement of the parameterization of D(Cu) should include a reassessment of the Cu fractional crystallization trends. The Cr versus Au trends in magmatic iron meteorite groups are steeper than those of Cu and not explained by fractional crystallization. Other influences, such as the removal of chromite from the crystallizing system or sampling biases during iron meteorite compositional analyses, are likely responsible for the Cr trends in magmatic iron meteorite groups.  相似文献   

5.
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6.
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   

7.
Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ17O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.  相似文献   

8.
Abstract– High‐precision Cu isotopic compositions have been measured for the metal phase of 29 iron meteorites from various groups and for four terrestrial standards. The data are reported as the δ65Cu permil deviation of the 65Cu/63Cu ratio relative to the NIST SRM 976 standard. Terrestrial mantle rocks have a very narrow range of variations and scatter around zero. In contrast, iron meteorites show δ65Cu approximately 2.3‰ variations. Different groups of iron meteorites have distinct δ65Cu values. Nonmagmatic IAB‐IIICD iron meteorites have similar δ65Cu (0.03 ± 0.08 and 0.12 ± 0.10, respectively), close to terrestrial values (approximately 0). The other group of nonmagmatic irons, IIE, is isotopically distinct (?0.69 ± 0.15). IVB is the iron meteorite group with the strongest elemental depletion in Cu and samples in this group are enriched in the lighter isotope (δ65Cu down to ?2.26‰). Evaporation should have produced an enrichment in 65Cu over 63Cu (δ65Cu >0) and can therefore be ruled out as a mechanism for volatile loss in IVB meteorites. In silicate‐bearing iron meteorites, Δ17O correlates with δ65Cu. This correlation between nonmass‐dependent and mass‐dependent parameters suggests that the Cu isotopic composition of iron meteorites has not been modified by planetary differentiation to a large extent. Therefore, Cu isotopic ratios can be used to confirm genetic links. Cu isotopes thus confirm genetic relationships between groups of iron meteorites (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H chondrites). Several genetic connections between iron meteorites groups are confirmed by Cu isotopes, (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H chondrites).  相似文献   

9.
We present high precision, low‐ and high‐resolution tungsten isotope measurements of iron meteorites Cape York (IIIAB), Rhine Villa (IIIE), Bendego (IC), and the IVB iron meteorites Tlacotepec, Skookum, and Weaver Mountains, as well as CI chondrite Ivuna, a CV3 chondrite refractory inclusion (CAI BE), and terrestrial standards. Our high precision tungsten isotope data show that the distribution of the rare p‐process nuclide 180W is homogeneous among chondrites, iron meteorites, and the refractory inclusion. One exception to this pattern is the IVB iron meteorite group, which displays variable excesses relative to the terrestrial standard, possibly related to decay of rare 184Os. Such anomalies are not the result of analytical artifacts and cannot be caused by sampling of a protoplanetary disk characterized by p‐process isotope heterogeneity. In contrast, we find that 183W is variable due to a nucleosynthetic s‐process deficit/r‐process excess among chondrites and iron meteorites. This variability supports the widespread nucleosynthetic s/r‐process heterogeneity in the protoplanetary disk inferred from other isotope systems and we show that W and Ni isotope variability is correlated. Correlated isotope heterogeneity for elements of distinct nucleosynthetic origin (183W and 58Ni) is best explained by thermal processing in the protoplanetary disk during which thermally labile carrier phases are unmixed by vaporization thereby imparting isotope anomalies on the residual processed reservoir.  相似文献   

10.
Abstract— We have evaluated various mechanisms proposed for the formation of the Widmanstätten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony‐iron meteorites. The Widmanstätten pattern in high P iron meteorites forms when meteorites enter the three‐phase field α + γ + Ph via cooling from the γ + Ph field. The Widmanstätten pattern in low P iron meteorites forms either at a temperature below the (α + γ)/(α + γ + Ph) boundary or by the decomposition of martensite below the martensite start temperature. The reaction γ → α + γ, which is normally assumed to control the formation of the Widmanstätten pattern, is not applicable to the metal in meteorites. The formation of the Widmanstätten pattern in the vast majority of low P iron meteorites (which belong to chemical groups IAB‐IIICD, IIIAB, and IVA) is controlled by mechanisms involving the formation of martensite α2. We propose that the Widmanstätten structure in these meteorites forms by the reaction γ → α2 + γ → α + γ, in which α2 decomposes to the equilibrium α and γ phases during the cooling process. To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the Widmanstätten pattern must first be established. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (γ + Ph)/(α + γ + Ph) boundary, the (α + γ)/(α + γ + Ph) boundary, or the Ms temperature. With the introduction of these three mechanisms and the specific phase boundaries and the temperatures where transformations occur, it is no longer necessary to invoke arbitrary amounts of under‐cooling in the calculation of the cooling rate. We conclude that martensite decomposition via the reactions γ → α2 → α + γ and γ → α2 + γ → α + γ are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction γ → αm → α at relatively high temperature in the two‐phase α + γ region of the Fe‐Ni‐P phase diagram near the α/(α + γ) phase boundary.  相似文献   

11.
Abstract– The fall of meteorites has been interpreted as divine messages by multitudinous cultures since prehistoric times, and meteorites are still adored as heavenly bodies. Stony meteorites were used to carve birds and other works of art; jewelry and knifes were produced of meteoritic iron for instance by the Inuit society. We here present an approximately 10.6 kg Buddhist sculpture (the “iron man”) made of an iron meteorite, which represents a particularity in religious art and meteorite science. The specific contents of the crucial main (Fe, Ni, Co) and trace (Cr, Ga, Ge) elements indicate an ataxitic iron meteorite with high Ni contents (approximately 16 wt%) and Co (approximately 0.6 wt%) that was used to produce the artifact. In addition, the platinum group elements (PGEs), as well as the internal PGE ratios, exhibit a meteoritic signature. The geochemical data of the meteorite generally match the element values known from fragments of the Chinga ataxite (ungrouped iron) meteorite strewn field discovered in 1913. The provenance of the meteorite as well as of the piece of art strongly points to the border region of eastern Siberia and Mongolia, accordingly. The sculpture possibly portrays the Buddhist god Vai?ravana and might originate in the Bon culture of the eleventh century. However, the ethnological and art historical details of the “iron man” sculpture, as well as the timing of the sculpturing, currently remain speculative.  相似文献   

12.
We report the first combined atom‐probe tomography (APT) and transmission electron microscopy (TEM) study of a kamacite–tetrataenite (K–T) interface region within an iron meteorite, Bristol (IVA). Ten APT nanotips were prepared from the K–T interface with focused ion beam scanning electron microscopy (FIB‐SEM) and then studied using TEM followed by APT. Near the K‐T interface, we found 3.8 ± 0.5 wt% Ni in kamacite and 53.4 ± 0.5 wt% Ni in tetrataenite. High‐Ni precipitate regions of the cloudy zone (CZ) have 50.4 ± 0.8 wt% Ni. A region near the CZ and martensite interface has <10 nm sized Ni‐rich precipitates with 38.4 ± 0.7 wt% Ni present within a low‐Ni matrix having 25.5 ± 0.6 wt% Ni. We found that Cu is predominantly concentrated in tetrataenite, whereas Co, P, and Cr are concentrated in kamacite. Phosphorus is preferentially concentrated along the K‐T interface. This study is the first precise measurement of the phase composition at high spatial resolution and in 3‐D of the K‐T interface region in a IVA iron meteorite and furthers our knowledge of the phase composition changes in a fast‐cooled iron meteorite below 400 °C. We demonstrate that APT in conjunction with TEM is a useful approach to study the major, minor, and trace elemental composition of nanoscale features within fast‐cooled iron meteorites.  相似文献   

13.
The mineralogy and bulk chemical compositions of three iron meteorites (Zhaoping, Xifu and Hami) recently found in China are reported here and are classified on the basis of their bulk chemical compositions. Zhaoping contains 93.4 mg/g Ni, 85.9 μg/g Ga, 418 μg/g Ge, 5.24 mg/g Co, 1.94 μg/g Ir, 0.774 μg/g W, and 1.62 μg/g Au and belongs to the low-Ni, low-Au subgroup of IAB. It is a coarse octahedrite and consists of kamacite, taenite, troilite, schreibersite and cohenite. The cohenite has entirely decomposed to graphite and low-Ni kamacite in our samples. Zhaoping contains some inclusions of Mn-free sarcopside which were rarely reported in IAB iron meteorites. Xifu has 74.1 mg/g Ni, 58.8gμg/g Ga, 150 μg/g Ge, and 0.913 μg/g W. Xifu is a member of group IIICD iron meteorite. Like most of IIICD irons, Xifu is a coarsest octahedrite with kamacite bandwidth larger than 3mm, and contains kamacite, taenite and schreibersite. Carbides and graphite are not found in the sample because of its being heterogeneous. Hami has 106 mg/g Ni, 5.36 mg/g Co and 0.922 μg/g Ir. We did not obtain the Ga and Ge contents in Hami because of their low concentrations and the limited precision of the INAA technique. Hami is an unclassified iron meteorite on the basis of the contents of other trace elements, structure and mineralogy. On mineralogy and structure, Hami resembles Rafruti, another unclassified iron meteorite.  相似文献   

14.
On February 24, 1979, a deeply oxidized mass of iron meteorite was excavated from bauxite at an open cut mine on the Gove Peninsula, Northern Territory, Australia. The meteorite, measuring 0.75–1 m in diameter and of unknown total weight, was found at coordinates 12°15.8′S, 136°50.3′E. On removal from the ground, the meteorite is reported to have disintegrated rapidly. A preliminary analysis at the mine laboratory reportedly gave 8.5 wt% Ni. A modern analysis of oxidized material gave Ni = 32.9, Co = 3.67 (both mg g?1), Cr = 168, Cu = 195, Ga = 22.5, Ge = <70, As = 4.16, W = 1.35, Ir = 10.5, Pt = 21.2, Au = 0.672 (all μg g?1), Sb = <150, and Re = 844 (both ng g?1). Competent fragments of oxidized material retain a fine to medium Widmanstätten pattern with an apparent average bandwidth of 0.5 mm (range 0.2–0.9 mm in plane section). Primary mineralogy includes rare γ–taenite and daubréelite, and secondary minerals produced by weathering include awaruite (with up to 78.5 wt% Ni) and an, as yet, unnamed Cu‐Cr‐bearing sulfide with the ideal formula CuCrS2 that is hitherto unknown in nature. Deep weathering has masked many of the features of the meteorite; however, the analysis normalized to the analyses of fresh iron meteorites favors chemical group IIIAB. The terrestrial age of the meteorite is unknown, although it is likely to be in the Neogene (2.5–23 Ma), which is widely accepted as the major period of bauxite formation in the Northern Territory of Australia. Gove is the second authenticated relict meteorite found in Australia.  相似文献   

15.
Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ～0.4‰ amu?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (～0.0 to ～0.3‰ amu?1) and chondrites (～0.0 to ～0.2‰ amu?1) are similar, whereas the range in pallasite metal (～–0.1 to 0.0‰ amu?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (～0.0 to ～0.3‰ amu?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ～0.4‰ amu?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern.  相似文献   

16.
Two new iron meteorites from Western Australia are described: Cosmo Newberry — a 2.156 kg meteorite of unusual spiky shape, and Gnowangerup — a 33.6 kg pear-shaped meteorite. X-ray fluorescence spectrometry shows that Cosmo Newberry can be classified in Group IIA, whilst Gnowangerup is a member of Group IIIAB. Neither iron can be associated with any other Western Australian meteorite.  相似文献   

17.
Abstract— Mn‐Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB iron meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in 53Cr are found and δ53Cr is well correlated with Mn/Cr ratios, suggesting that 53Mn was alive at the time of IIIAB iron formation. The inferred Mn‐Cr “ages” are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB iron meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB iron meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in 53Cr during the closure process. Apparently, old Mn‐Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn‐Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model‐dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.  相似文献   

18.
Abstract– We report Si concentrations in the metal phases of iron meteorites. Analyses were performed by secondary ion mass spectrometry using a CAMECA 1270 ion probe. The Si concentrations are low (0.09–0.46 μg g?1), with no apparent difference in concentration between magmatic and nonmagmatic iron meteorites. Coexisting kamacite and Ni‐rich metal phases have similar Si contents. Thermodynamic calculations show that Fe,Ni‐metal in equilibrium with silicate melts at temperatures where metal crystallizes should contain approximately 100 times more Si than found in iron meteorites in this work. The missing Si may either occur as tiny silicate inclusions in metal or it may have diffused as Si‐metal into surrounding silicates at low temperatures. In both cases, extensive low‐temperature diffusion of Si in metal is required. It is therefore concluded that low Si in iron meteorites is a result of subsolidus reactions during slow cooling.  相似文献   

19.
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   

20.
Abstract– The isotope fractionation of Zn in meteorites has been measured for the first time using thermal ionization mass spectrometry and a double spiking technique. The magnitude of δZn ranged from ?0.29 to +0.38‰ amu?1 for five stone meteorites whereas the iron meteorite Canyon Diablo displays δZn of 1.11 ± 0.11‰ amu?1. The results for chondrites in this work can be divided into positive and negative δZn, supporting a previous proposal that chondrites are a mixture of materials from two different temperature sources. The Zn isotope fractionation present in meteorites may represent a primordial heterogeneity formed in the early solar system. An anomalous isotopic composition of Zn obtained for the Redfields iron meteorite suggests large‐scale inherited isotope heterogeneity of the protosolar nebula, or the presence of a parent body that has formed within its own isotopically anomalous reservoir. These anomalies are in the same direction but smaller than nuclear field shift effects observed in chemical exchange reactions. The isotope dilution mass spectrometry (IDMS) technique was used to measure Zn concentration, yielding a range from 20.1 μg g?1 to 302 μg g?1 in five stone meteorites and from 0.019 to 26 μg g?1 in seven iron meteorites. The IDMS‐measured abundance of Zn in Orgueil is 302 ± 14 μg g?1 and should be considered for future compilations of the abundance of Zn in the solar system.  相似文献   

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