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John T. Wasson - One of the best experts on this subject based on the ideXlab platform.
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Author's personal copy Relationship between iron-meteorite composition and size: Compositional distribution of irons from North Africa
2016Co-Authors: John T. WassonAbstract:During the past three decades many iron meteorites have been collected from the deserts of North Africa. Almost all are now characterized, and the distribution among classes is found to be very different from those that were in museums prior to the collection of meteorites from hot and cold (Antarctica) deserts. Similar to the iron meteorites from Antarctica, the irons from Northwest Africa include a high fraction of ungrouped irons and of minor subgroups of group IAB. The different dis-tribution is attributed to the small median size of the desert meteorites (1.3 kg in North African irons, 30 kg in non-desert irons). It appears that a sizable fraction of these small (several centimeter) masses constitute melt pockets produced by impacts in chondritic regoliths; they were never part of a large (meter-to-kilometer) magma bodies. As a result, a meter-size fragment ejected from the regolith of the asteroid may contain several of these small metallic masses. It may be that such stochastic sampling effects enhanced the fraction of IAB-sHL irons among the irons from Northwest Africa. The variety observed in small meteoroids is also enhanced because (relative to large) small fragments are more efficiently ejected from asteroids and because the orbital parameters of small meteoroids are more strongly affected by collisions and drag effects, they evolve to have Earth-crossing perihelia more rapidly than large meteoroids; as a result, the set of small mete-oroids tends to sample a larger number of parent asteroids than does the set of larger meteoroids. 2010 Elsevier Ltd. All rights reserved. 1
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relationship between iron meteorite composition and size compositional distribution of irons from north africa
Geochimica et Cosmochimica Acta, 2011Co-Authors: John T. WassonAbstract:Abstract During the past three decades many iron meteorites have been collected from the deserts of North Africa. Almost all are now characterized, and the distribution among classes is found to be very different from those that were in museums prior to the collection of meteorites from hot and cold (Antarctica) deserts. Similar to the iron meteorites from Antarctica, the irons from Northwest Africa include a high fraction of ungrouped irons and of minor subgroups of group IAB. The different distribution is attributed to the small median size of the desert meteorites (∼1.3 kg in North African irons, ∼30 kg in non-desert irons). It appears that a sizable fraction of these small (several centimeter) masses constitute melt pockets produced by impacts in chondritic regoliths; they were never part of a large (meter-to-kilometer) magma bodies. As a result, a meter-size fragment ejected from the regolith of the asteroid may contain several of these small metallic masses. It may be that such stochastic sampling effects enhanced the fraction of IAB-sHL irons among the irons from Northwest Africa. The variety observed in small meteoroids is also enhanced because (relative to large) small fragments are more efficiently ejected from asteroids and because the orbital parameters of small meteoroids are more strongly affected by collisions and drag effects, they evolve to have Earth-crossing perihelia more rapidly than large meteoroids; as a result, the set of small meteoroids tends to sample a larger number of parent asteroids than does the set of larger meteoroids.
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chemical classification of iron meteorites xii new members of the magmatic groups
Geochimica et Cosmochimica Acta, 1998Co-Authors: John T. Wasson, Eric A Jerde, Byeongak Choi, Finn UlffmollerAbstract:Abstract Data are reported for thirty iron meteorites that are members of the magmatic groups, for three main group pallasites, one anomalous mesosiderite, and for three ungrouped irons and an ungrouped pallasite that are similar to IIIAB irons in their Ni, Ga, and Ge contents. The set includes four observed falls (11% of iron falls) Ban Rong Du, Chisenga, Nyaung and Sterlitamak, and Zaisho, one of two known pallasite falls. Two of the ungrouped irons (Ban Rong Du and Mount Howe 88403) and the ungrouped pallasite Yamato 8451, although having Ni, Ga, and Ge contents in the same general range as IIIAB, have very different contents of Co and exhibit significant differences for several other elements; they are clearly not related to IIIAB or to its little sister, group IIIE. A fourth ungrouped iron, Tres Castillos, chiefly differs from IIIAB in terms of its low Ga and high Ge contents; its Ga/Ge ratio is 35% higher than that of any other IIIAB iron. We report data on four new IIAB irons, all falling within established fields; the Bilibino iron is somewhat unusual, having a low Ir content (0.12 μg/g) and a structure altered by reheating. The IVA irons are also typical. One, Albion, may be a mislabeled specimen of Gibeon; another, Page City, exhibits large cracks (up to 3 cm). The Chaunskij anomalous mesosiderite has exceptionally high Ni and very low Ir concentrations. Two of three new main group pallasites are anomalous; Pecora Escarpment 91004 has an Ir content above the normal range, and Zaisho has an exceptionally high Fa content in the olivine.
Torranin Chairuangsri - One of the best experts on this subject based on the ideXlab platform.
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relationship between microstructure hardness and corrosion resistance in 20 wt cr 27 wt cr and 36 wt cr high chromium cast irons
Materials Chemistry and Physics, 2011Co-Authors: Amporn Wiengmoon, J T H Pearce, Torranin ChairuangsriAbstract:Abstract The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M 7 C 3 with stoichiometry as (Cr 3.37 , Fe 3.63 )C 3 , (Cr 4.75 , Fe 2.25 )C 3 and (Cr 5.55 , Fe 1.45 )C 3 , respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M 7 C 3 in 20 wt.%Cr iron, whereas both M 23 C 6 and M 7 C 3 secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.
Edward R D Scott - One of the best experts on this subject based on the ideXlab platform.
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iron meteorites crystallization thermal history parent bodies and origin
Chemie Der Erde-geochemistry, 2009Co-Authors: J. I. Goldstein, Edward R D Scott, Nancy L. ChabotAbstract:We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstatten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–7001C are about 100–10,0001C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 3001C/Myr for the IIIAB group and 100–66001C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150750 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, o1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets. r 2009 Elsevier GmbH. All rights reserved.
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thermal histories of iva stony iron and iron meteorites evidence for asteroid fragmentation and reaccretion
Geochimica et Cosmochimica Acta, 1996Co-Authors: Henning Haack, Edward R D Scott, Stanley G Love, Adrian J Brearley, T J MccoyAbstract:We have investigated the thermal history of the IVA iron and stony-iron meteorites to help resolve the apparent conflict between their metallographic cooling rates, which are highly diverse, and their chemical trends, which favor crystallization in a single core. Transmission electron microscopy of the disordered clinobronzite in the stony-iron, Steinbach, using electron diffraction and high resolution imaging techniques indicates that this meteorite was rapidly cooled at ≈ 100°C/hr through 1200°C. The IVA irons cooled much slower in the range 1200–1000°C: absence of dendrites in large troilite nodules indicate cooling rates of <300°C/y. We infer that the parent asteroid was catastrophically fragmented and reaccreted when the core had cooled to 1200°C and was 95% crystallized. We argue that radiative heat losses from the debris cloud would have been minor due to its high opacity, small size (only a few asteroid diameters), and short reaccretion times (∼ a few hours). We calculate that global heating effects were also minor (ΔT < 300°C for a body with a diameter of < 400 km) and that the mean temperature of the IVA parent body before and after the impact was 450–700°C. We infer that Steinbach cooled rapidly from 1200°C at the edge of a core fragment by thermal equilibration with cooler silicates during and after reaccretion. Metallographic cooling rates of IVA irons and stony-irons for the temperature range 600–350°C (Rasmussen et al., 1995) strongly support this model and indicate that the IVA meteorites are derived from only a few core fragments. The large range of these cooling rates (20–3000°C/My) and the decrease in the metallographic cooling rates of high-Ni IVA irons with falling temperature probably reflect the diversity of thermal environments in the reaccreted asteroid, the low thermal conductivity of fragmental silicates, and the limited sintering of this fragmental material.
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CORE CRYSTALLIZATION AND SILICATE-METAL MIXING IN THE PARENT BODY OF THE IVA IRON AND STONY-IRON METEORITES
Geochimica et Cosmochimica Acta, 1996Co-Authors: Edward R D Scott, Henning Haack, Timothy J. MccoyAbstract:Abstract We have analyzed metallic and silicate phases in the IVA iron meteorites and two related stony irons, Steinbach and Sao Joao Nepomuceno. Analyses of bulk metal phases in the two stony irons using INAA show that they plot as close to the chemical trends within group IVA as most IVA irons, indicating a common source. Our fractional crystallization models for the IVA chemical trends suggest that the irons crystallized from a metallic melt that initially contained 2.5 ± 1 wt% S. After S concentrations in the liquid reached 6 wt%, liquid trapping during crystallization increased the apparent distribution coefficient for S, as in group IIIAB. Compositions of the metal fractions in Steinbach and Sao Joao Nepomuceno match the calculated solid compositions after 50 ± 10% and 80 ± 10%, respectively, of the metallic melt had crystallized. We confidently conclude that the IVA irons and metal in the two stony irons were derived from the core of a single asteroid that fractionally crystallized. The wide range of metallographic cooling rates of IVA irons cannot result from crystallization in isolated pools in one or more bodies, as some authors have argued. Large depletions of Ga, Ge, and other moderately volatile elements in group IVA are unlikely to result from planetary processes; they may have been inherited from chondritic precursor material. The two IVA stony irons contain up to 60 vol% of a unique, coarse-grained mixture of tridymite, orthobronzite, and clinobronzite. Silicate-metal textures resemble those in rounded-olivine pallasites; both may result from the depression of cumulate silicates into underlying molten S-rich metal. Two IVA irons contain rare plate-like, silica crystals up to 10 mm long, but these occurrences seem unrelated to the stony-iron silicates. Because of the difficulty in forming the stony irons in an isolated, slowly cooling asteroid, we infer that they may have formed during the breakup and reassembly event invoked by Haack et al. (1995) to account for the fast cooling of Steinbach from 1200°C.
Jeffrey Williams - One of the best experts on this subject based on the ideXlab platform.
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Cosmogenic effects on Cu isotopes in IVB iron meteorites
Geochimica et Cosmochimica Acta, 2016Co-Authors: Heng Chen, Frédéric Moynier, Munir Humayun, M. Cole Bishop, Jeffrey WilliamsAbstract:We measured Cu isotope compositions of 12 out of the 14 known IVB iron meteorites. Our results show that IVB iron meteorites display a very large range of δ65Cu values (−5.84‰ < δ65Cu < −0.24‰; defined as per mil deviation of the 65Cu/63Cu ratio from the NIST-976 standard). These Cu isotopic data display clear correlations with W, Pt, and Os isotope ratios, which are very sensitive to secondary neutron capture due to galactic cosmic ray (GCR) irradiation. This demonstrates that δ65Cu in IVB irons is majorly modified by neutron capture by the reaction 62Ni(n,γ)63Ni followed by beta decay to 63Cu. Using correlations with Pt and Os neutron dosimeters, we calculated a pre-exposure δ65Cu of −0.3 ± 0.8‰ (95% conf.) of IVB irons that agrees well with the Cu isotopic compositions of other iron meteorite groups and falls within the range of chondrites. This shows that the volatile depletion of the IVB parent body is not due to evaporation that should have enriched IVB irons in the heavy Cu isotopes.
Henning Haack - One of the best experts on this subject based on the ideXlab platform.
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thermal histories of iva stony iron and iron meteorites evidence for asteroid fragmentation and reaccretion
Geochimica et Cosmochimica Acta, 1996Co-Authors: Henning Haack, Edward R D Scott, Stanley G Love, Adrian J Brearley, T J MccoyAbstract:We have investigated the thermal history of the IVA iron and stony-iron meteorites to help resolve the apparent conflict between their metallographic cooling rates, which are highly diverse, and their chemical trends, which favor crystallization in a single core. Transmission electron microscopy of the disordered clinobronzite in the stony-iron, Steinbach, using electron diffraction and high resolution imaging techniques indicates that this meteorite was rapidly cooled at ≈ 100°C/hr through 1200°C. The IVA irons cooled much slower in the range 1200–1000°C: absence of dendrites in large troilite nodules indicate cooling rates of <300°C/y. We infer that the parent asteroid was catastrophically fragmented and reaccreted when the core had cooled to 1200°C and was 95% crystallized. We argue that radiative heat losses from the debris cloud would have been minor due to its high opacity, small size (only a few asteroid diameters), and short reaccretion times (∼ a few hours). We calculate that global heating effects were also minor (ΔT < 300°C for a body with a diameter of < 400 km) and that the mean temperature of the IVA parent body before and after the impact was 450–700°C. We infer that Steinbach cooled rapidly from 1200°C at the edge of a core fragment by thermal equilibration with cooler silicates during and after reaccretion. Metallographic cooling rates of IVA irons and stony-irons for the temperature range 600–350°C (Rasmussen et al., 1995) strongly support this model and indicate that the IVA meteorites are derived from only a few core fragments. The large range of these cooling rates (20–3000°C/My) and the decrease in the metallographic cooling rates of high-Ni IVA irons with falling temperature probably reflect the diversity of thermal environments in the reaccreted asteroid, the low thermal conductivity of fragmental silicates, and the limited sintering of this fragmental material.
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CORE CRYSTALLIZATION AND SILICATE-METAL MIXING IN THE PARENT BODY OF THE IVA IRON AND STONY-IRON METEORITES
Geochimica et Cosmochimica Acta, 1996Co-Authors: Edward R D Scott, Henning Haack, Timothy J. MccoyAbstract:Abstract We have analyzed metallic and silicate phases in the IVA iron meteorites and two related stony irons, Steinbach and Sao Joao Nepomuceno. Analyses of bulk metal phases in the two stony irons using INAA show that they plot as close to the chemical trends within group IVA as most IVA irons, indicating a common source. Our fractional crystallization models for the IVA chemical trends suggest that the irons crystallized from a metallic melt that initially contained 2.5 ± 1 wt% S. After S concentrations in the liquid reached 6 wt%, liquid trapping during crystallization increased the apparent distribution coefficient for S, as in group IIIAB. Compositions of the metal fractions in Steinbach and Sao Joao Nepomuceno match the calculated solid compositions after 50 ± 10% and 80 ± 10%, respectively, of the metallic melt had crystallized. We confidently conclude that the IVA irons and metal in the two stony irons were derived from the core of a single asteroid that fractionally crystallized. The wide range of metallographic cooling rates of IVA irons cannot result from crystallization in isolated pools in one or more bodies, as some authors have argued. Large depletions of Ga, Ge, and other moderately volatile elements in group IVA are unlikely to result from planetary processes; they may have been inherited from chondritic precursor material. The two IVA stony irons contain up to 60 vol% of a unique, coarse-grained mixture of tridymite, orthobronzite, and clinobronzite. Silicate-metal textures resemble those in rounded-olivine pallasites; both may result from the depression of cumulate silicates into underlying molten S-rich metal. Two IVA irons contain rare plate-like, silica crystals up to 10 mm long, but these occurrences seem unrelated to the stony-iron silicates. Because of the difficulty in forming the stony irons in an isolated, slowly cooling asteroid, we infer that they may have formed during the breakup and reassembly event invoked by Haack et al. (1995) to account for the fast cooling of Steinbach from 1200°C.
