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J I Goldstein - One of the best experts on this subject based on the ideXlab platform.
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Thermal and collisional history of Tishomingo iron meteorite: More evidence for early disruption of differentiated planetesimals
Geochimica et Cosmochimica Acta, 2020Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, Ansgar Grimberg, Ingo LeyaAbstract:Abstract Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual Taenite and 80% martensite plates, which formed on cooling to between −75 and −200 °C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), X-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of Taenite and decomposed during reheating forming 10–100 nm Taenite particles with ∼50 wt.% Ni, kamacite with ∼4 wt.%Ni, along with martensite or Taenite with 32 wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320–400 °C for about a year transforming some martensite to kamacite and to Taenite particles and some martensite directly to Taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7 wt.% Ni and 2.6 wt.% Co, and Taenite with 56 wt.% Ni and 0.15 wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28 wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25–36 wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy Taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of ∼50–85 cm.
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Ion microprobe analyses of carbon in Fe–Ni metal in iron meteorites and mesosiderites
Geochimica et Cosmochimica Acta, 2017Co-Authors: J I Goldstein, Gary R. Huss, Edward R D ScottAbstract:Abstract Carbon concentrations in kamacite, Taenite, and plessite (kamacite-Taenite intergrowths) were measured in 18 iron meteorites and 2 mesosiderites using the Cameca ims 1280 ion microprobe at the University of Hawai‘i with a 5–7 μm beam and a detection limit of Grains of Taenite and fine-grained plessite in carbon-rich meteorites, which all have normal M-shaped nickel profiles due to slow cooling, have diverse carbon contents and zoning profiles. This is because Taenite decomposed by diverse mechanisms over a range of temperatures, when nickel could only diffuse over sub-μm distances. Carbon diffusion through Taenite to growing carbides was rapid at the upper end of this temperature range, but was very limited at the lower end of the temperature range. In mesosiderites, carbon increases from 12 ppm in tetraTaenite to 40–115 ppm in cloudy Taenite as nickel decreases from 50% to 35%. Low carbon levels in tetraTaenite may reflect ordering of iron and nickel; higher carbon in cloudy Taenite is attributed to metastable bcc phase, possibly martensite, with ∼300 ppm carbon intergrown with tetraTaenite. Pearlitic plessite, which only forms in carbon-rich irons, contains much less carbon than martensitic plessite: 10–20 ppm and 300–500, respectively, in IAB irons. Pearlitic plessite consists of μm-scale intergrowths of low-nickel kamacite and tetraTaenite, which formed during cooling from ∼450 to 300 °C when haxonite was forming. Martensitic plessite decomposed to tetraTaenite and metastable high-nickel kamacite at temperatures below 300 °C, which depended on nickel content. Carbon accumulated in untransformed Taenite when haxonite growth ceased, producing M-shaped carbon profiles. Bulk carbon concentrations inferred from our ion probe data are 3–4 ppm in IVA, IVB, and Tishomingo, which has IVB-like depletions of moderately volatile siderophiles. Published bulk carbon contents of IVA and IVB irons are >10 times higher suggesting contamination problems. Our ion probe analyses and observations of carbide and graphite show that bulk carbon decreases with decreasing germanium and other moderately volatile elements from group IAB, through IIAB and IIIAB, to group IVA and IVB. These trends may have been inherited from fractionated chondritic precursors, or may have been produced by impacts that caused volatile loss, separation of mantle from core material, and relatively rapid cooling of irons poor in volatiles and carbon.
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Thermal and impact histories of reheated group IVA, IVB, and ungrouped iron meteorites and their parent asteroids
Meteoritics & Planetary Science, 2011Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, T. Pham, Timothy J. MccoyAbstract:The microstructures of six reheated iron meteoritesotwo 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)owere 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 WidmanstItten pattern. The other four, which show no trace of their original microstructure, were heated above 600-700 � C and recrystallized to form 10-20 lm wide homogeneous Taenite grains. On cooling, kamacite formed on Taenite grain boundaries with their close-packed planes aligned. Formation of homogeneous 20 lm 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 lm 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 (a2) in Taeniteothe first occurrence in ironsoappears 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).
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Olivine zoning and retrograde olivine-orthopyroxene-metal equilibration in H5 and H6 chondrites
Meteoritics & Planetary Science, 2006Co-Authors: R J Reisener, J I Goldstein, M. I. PetaevAbstract:Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned Taenite + kamacite particles (formed after relatively small amounts of Taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after ~200 °C of Taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of Taenite undercooling at temperatures
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The formation of plessite in meteoritic metal
Meteoritics & Planetary Science, 2006Co-Authors: J I Goldstein, Joseph R. MichaelAbstract:Plessite is a mixture of body-centered cubic (bcc) kamacite (α), face-centered cubic (fcc) Taenite (γ), and/or ordered FeNi-tetraTaenite (γ") phases and is observed in the metal of iron, stony- iron, and chondritic meteorites. The formation of plessite was studied by measuring the orientation of the bcc and fcc phases over large regions of plessite using electron backscatter diffraction (EBSD) analysis in five ataxites, the Carlton IAB-IIICD iron, and zoneless plessite metal in the Kernouve H6 chondrite. The EBSD results show that there are a number of different orientations of the bcc kamacite phase in the plessite microstructure. These orientations reflect the reaction path γ (fcc) → α2 (bcc) in which the α2 phase forms during cooling below the martensite start temperature, Ms, on the close- packed planes of the parent fcc phase according to one or more of the established orientation relationships (Kurdjumov-Sachs, Nishiyama-Wasserman, and Greninger-Troiano) for the fcc to bcc transformation. The EBSD results also show that the orientation of the Taenite and/or tetraTaenite regions at the interfaces of prior α2 (martensite) laths, is the same as that of the single crystal parent Taenite γ phase of the meteorite. Therefore, the parent Taenite γ was retained at the interfaces of martensite laths during cooling after the formation of martensite. The formation of plessite is described by the reaction γ → α2 + γ → α + γ. This reaction is inconsistent with the decomposition of martensite laths to form γ phase as described by the reaction γ → α2 → α + γ, which is the classical mechanism proposed by previous investigators. The varying orientations of the fine exsolved Taenite and/or tetraTaenite within decomposed martensite laths, however, are a response to the decomposition of α2 (martensite) laths at low temperature and are formed by the reaction α2 → α + γ.
Edward R D Scott - One of the best experts on this subject based on the ideXlab platform.
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Thermal and collisional history of Tishomingo iron meteorite: More evidence for early disruption of differentiated planetesimals
Geochimica et Cosmochimica Acta, 2020Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, Ansgar Grimberg, Ingo LeyaAbstract:Abstract Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual Taenite and 80% martensite plates, which formed on cooling to between −75 and −200 °C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), X-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of Taenite and decomposed during reheating forming 10–100 nm Taenite particles with ∼50 wt.% Ni, kamacite with ∼4 wt.%Ni, along with martensite or Taenite with 32 wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320–400 °C for about a year transforming some martensite to kamacite and to Taenite particles and some martensite directly to Taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7 wt.% Ni and 2.6 wt.% Co, and Taenite with 56 wt.% Ni and 0.15 wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28 wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25–36 wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy Taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of ∼50–85 cm.
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Ion microprobe analyses of carbon in Fe–Ni metal in iron meteorites and mesosiderites
Geochimica et Cosmochimica Acta, 2017Co-Authors: J I Goldstein, Gary R. Huss, Edward R D ScottAbstract:Abstract Carbon concentrations in kamacite, Taenite, and plessite (kamacite-Taenite intergrowths) were measured in 18 iron meteorites and 2 mesosiderites using the Cameca ims 1280 ion microprobe at the University of Hawai‘i with a 5–7 μm beam and a detection limit of Grains of Taenite and fine-grained plessite in carbon-rich meteorites, which all have normal M-shaped nickel profiles due to slow cooling, have diverse carbon contents and zoning profiles. This is because Taenite decomposed by diverse mechanisms over a range of temperatures, when nickel could only diffuse over sub-μm distances. Carbon diffusion through Taenite to growing carbides was rapid at the upper end of this temperature range, but was very limited at the lower end of the temperature range. In mesosiderites, carbon increases from 12 ppm in tetraTaenite to 40–115 ppm in cloudy Taenite as nickel decreases from 50% to 35%. Low carbon levels in tetraTaenite may reflect ordering of iron and nickel; higher carbon in cloudy Taenite is attributed to metastable bcc phase, possibly martensite, with ∼300 ppm carbon intergrown with tetraTaenite. Pearlitic plessite, which only forms in carbon-rich irons, contains much less carbon than martensitic plessite: 10–20 ppm and 300–500, respectively, in IAB irons. Pearlitic plessite consists of μm-scale intergrowths of low-nickel kamacite and tetraTaenite, which formed during cooling from ∼450 to 300 °C when haxonite was forming. Martensitic plessite decomposed to tetraTaenite and metastable high-nickel kamacite at temperatures below 300 °C, which depended on nickel content. Carbon accumulated in untransformed Taenite when haxonite growth ceased, producing M-shaped carbon profiles. Bulk carbon concentrations inferred from our ion probe data are 3–4 ppm in IVA, IVB, and Tishomingo, which has IVB-like depletions of moderately volatile siderophiles. Published bulk carbon contents of IVA and IVB irons are >10 times higher suggesting contamination problems. Our ion probe analyses and observations of carbide and graphite show that bulk carbon decreases with decreasing germanium and other moderately volatile elements from group IAB, through IIAB and IIIAB, to group IVA and IVB. These trends may have been inherited from fractionated chondritic precursors, or may have been produced by impacts that caused volatile loss, separation of mantle from core material, and relatively rapid cooling of irons poor in volatiles and carbon.
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determining cooling rates of iron and stony iron meteorites from measurements of ni and co at kamacite Taenite interfaces
Geochimica et Cosmochimica Acta, 2014Co-Authors: J. I. Goldstein, J. Yang, Edward R D ScottAbstract:Abstract Analyses and modeling of Ni zoning in Taenite in differentiated meteorites provide metallographic cooling rates at ∼500 °C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100–6600 °C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite–Taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite–Taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in Taenite (Niγ) and kamacite (Niα) at kamacite–Taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52 wt.% while Niα decreases from 7 to 4 wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and Taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetraTaenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe–Ni–P phase diagram and Ni diffusion rates in Taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using Niγ/Niα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion of Wasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe–Ni melt and crustal rocks, cooled at uniquely slow rates.
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Thermal and impact histories of reheated group IVA, IVB, and ungrouped iron meteorites and their parent asteroids
Meteoritics & Planetary Science, 2011Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, T. Pham, Timothy J. MccoyAbstract:The microstructures of six reheated iron meteoritesotwo 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)owere 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 WidmanstItten pattern. The other four, which show no trace of their original microstructure, were heated above 600-700 � C and recrystallized to form 10-20 lm wide homogeneous Taenite grains. On cooling, kamacite formed on Taenite grain boundaries with their close-packed planes aligned. Formation of homogeneous 20 lm 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 lm 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 (a2) in Taeniteothe first occurrence in ironsoappears 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).
P Hoppe - One of the best experts on this subject based on the ideXlab platform.
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co ni ratios at Taenite kamacite interfaces and relative cooling rates in iron meteorites
Geochimica et Cosmochimica Acta, 2012Co-Authors: John T Wasson, P HoppeAbstract:Abstract We report a pilot study of a new technique to use the distribution of Co between kamacite and Taenite to infer relative cooling rates of iron meteorites; data of Widge and Goldstein (1977) showed that the distribution is temperature dependent. A plot of the logarithm of the double ratio [(Co/Ni) kamacite /(Co/Ni) Taenite ] (abbreviated Rαγ) against inverse temperature yields a linear equation showing that the ratio ranges from ∼2.5 at 1080 K to ∼30 at 710 K. Thus, a measurement of Rαγ in the kamacite and Taenite near the interface offers information about relative cooling rates; the higher Rαγ, the lower the cooling rate. A major advantage of this technique is that it is mainly affected by the final (low-temperature) cooling rate, just before the sample cooled to the blocking temperature where diffusion became insignificant. To test this method we used the NanoSIMS ion probe to measure Rαγ in two IVA and two IIIAB irons; members of each pair differ by large factors in elemental composition and in published metallographic cooling rates ( Yang and Goldstein, 2006; Yang et al., 2008 ). Despite differing by a factor of 25 in estimated metallographic cooling rate, the two IVA irons showed similar Rαγ values of ∼22. If experimental uncertainties are considered this implies that, at low temperatures, their cooling rates differ by less than a factor of 5 with 95% confidence, i.e., significantly less than the range in metallographic cooling rates. In contrast, the IIIAB irons have different ratios; Rαγ in Haig is 29 whereas that in Cumpas, with a reported cooling rate 4.5 times lower, is 22, the opposite of that expected from the published cooling rates. A reevaluation of the Yang–Goldstein IIIAB data set shows that Haig has anomalous metallographic properties. We suggest that both the high Rαγ in Haig and the systematically low Taenite central Ni contents are the result of impact-produced fractures in the Taenite that allowed equilibration with kamacite down to lower temperatures but shut down Ni transport to the interiors of Taenite lamellae. Our observations of similar Rαγ values in IVA irons differing by a factor of 25 in metallographic cooling rates implies that there was, in fact, only a comparatively small difference in low-temperature cooling rates in IVA irons; because we studied only two IVA irons, this conclusion will remain tentative until further studies can be completed.
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Co/Ni ratios at Taenite/kamacite interfaces and relative cooling rates in iron meteorites
Geochimica et Cosmochimica Acta, 2012Co-Authors: John T Wasson, P HoppeAbstract:Abstract We report a pilot study of a new technique to use the distribution of Co between kamacite and Taenite to infer relative cooling rates of iron meteorites; data of Widge and Goldstein (1977) showed that the distribution is temperature dependent. A plot of the logarithm of the double ratio [(Co/Ni) kamacite /(Co/Ni) Taenite ] (abbreviated Rαγ) against inverse temperature yields a linear equation showing that the ratio ranges from ∼2.5 at 1080 K to ∼30 at 710 K. Thus, a measurement of Rαγ in the kamacite and Taenite near the interface offers information about relative cooling rates; the higher Rαγ, the lower the cooling rate. A major advantage of this technique is that it is mainly affected by the final (low-temperature) cooling rate, just before the sample cooled to the blocking temperature where diffusion became insignificant. To test this method we used the NanoSIMS ion probe to measure Rαγ in two IVA and two IIIAB irons; members of each pair differ by large factors in elemental composition and in published metallographic cooling rates ( Yang and Goldstein, 2006; Yang et al., 2008 ). Despite differing by a factor of 25 in estimated metallographic cooling rate, the two IVA irons showed similar Rαγ values of ∼22. If experimental uncertainties are considered this implies that, at low temperatures, their cooling rates differ by less than a factor of 5 with 95% confidence, i.e., significantly less than the range in metallographic cooling rates. In contrast, the IIIAB irons have different ratios; Rαγ in Haig is 29 whereas that in Cumpas, with a reported cooling rate 4.5 times lower, is 22, the opposite of that expected from the published cooling rates. A reevaluation of the Yang–Goldstein IIIAB data set shows that Haig has anomalous metallographic properties. We suggest that both the high Rαγ in Haig and the systematically low Taenite central Ni contents are the result of impact-produced fractures in the Taenite that allowed equilibration with kamacite down to lower temperatures but shut down Ni transport to the interiors of Taenite lamellae. Our observations of similar Rαγ values in IVA irons differing by a factor of 25 in metallographic cooling rates implies that there was, in fact, only a comparatively small difference in low-temperature cooling rates in IVA irons; because we studied only two IVA irons, this conclusion will remain tentative until further studies can be completed.
J. Yang - One of the best experts on this subject based on the ideXlab platform.
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Thermal and collisional history of Tishomingo iron meteorite: More evidence for early disruption of differentiated planetesimals
Geochimica et Cosmochimica Acta, 2020Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, Ansgar Grimberg, Ingo LeyaAbstract:Abstract Tishomingo is a chemically and structurally unique iron with 32.5 wt.% Ni that contains 20% residual Taenite and 80% martensite plates, which formed on cooling to between −75 and −200 °C, probably the lowest temperature recorded by any meteorite. Our studies using transmission (TEM) and scanning electron microscopy (SEM), X-ray microanalysis (AEM) and electron backscatter diffraction (EBSD) show that martensite plates in Tishomingo formed in a single crystal of Taenite and decomposed during reheating forming 10–100 nm Taenite particles with ∼50 wt.% Ni, kamacite with ∼4 wt.%Ni, along with martensite or Taenite with 32 wt.% Ni. EBSD data and experimental constraints show that Tishomingo was reheated to 320–400 °C for about a year transforming some martensite to kamacite and to Taenite particles and some martensite directly to Taenite without composition change. Fizzy-textured intergrowths of troilite, kamacite with 2.7 wt.% Ni and 2.6 wt.% Co, and Taenite with 56 wt.% Ni and 0.15 wt.% Co formed by localized shock melting. A single impact probably melted the sub-mm sulfides, formed stishovite, and reheated and decomposed the martensite plates. Tishomingo and its near-twin Willow Grove, which has 28 wt.% Ni, differ from IAB-related irons like Santa Catharina and San Cristobal that contain 25–36 wt.% Ni, as they are highly depleted in moderately volatile siderophiles and enriched in Ir and other refractory elements. Tishomingo and Willow Grove therefore resemble IVB irons but are chemically distinct. The absence of cloudy Taenite in these two irons shows that they cooled through 250 °C abnormally fast at >0.01 °C/yr. Thus this grouplet, like the IVA and IVB irons, suffered an early impact that disrupted their parent body when it was still hot. Our noble gas data show that Tishomingo was excavated from its parent body about 100 to 200 Myr ago and exposed to cosmic rays as a meteoroid with a radius of ∼50–85 cm.
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THERMAL HISTORY AND ORIGIN OF THE MAIN GROUP PALLASITES
2020Co-Authors: J. Yang, J. I. Goldstein, E. R. D. ScottAbstract:Introduction: The origin of the pallasites, mixtures of Fe-Ni metal and olivine, and their parent bodies is not well established. The main group of pallasites which are thought to come from one parent body may have formed 1) near the surface [1,2], 2) close to the center [3,4], 3) at the metal-olivine contact zones of isolated metal pods [5], or 4) at core-mantle boundaries [6, 7]. In the latter case, the IIIAB irons have been proposed as the metal associated with such a boundary. Our metallographic studies of the cloudy zone in the main group pallasites suggest a new origin, viz., from a range of depths within a metal-poor parent body, and that the main group pallasites and IIIAB irons are from separate parent bodies [8]. Here we report new data based on an investigation of the cloudy zone and tetraTaenite rim microstructures and preliminary measurements of metallographic cooling rates which give insight into the origin of the main group pallasites. Techniques: Metal regions of 20 main group pallasites were prepared by standard metallographic procedures for various analyses such as compositional analysis using the electron probe microanalyser (EPMA), crystal orientation measurements using electron backscatter diffraction techniques (EBSD), and microstructure analysis using the light optical microscope (LOM) and scanning electron microscope (SEM). Results: Metallic microstructure of pallasites. A variety of microstructures are observed in the metallic regions of the Taenite after etching with nital (2 vol % nitric acid in ethyl alcohol), including a characteristic Widmanstatten pattern in > 2cm wide metal regions and plessitic metal regions which are surrounded by kamacite bordering olivine. TetraTaenite and cloudy zone microstructures are observed at high magnification in the Taenite regions bordering kamacite (Fig. 1). Polycrystals. EBSD measurements of metal in pallasites show that the Taenite is polycrystalline and each crystal may not solidify from the same metal pool or reservoir. Olivine-free regions of Brenham have meter-sized Taenite crystals. On the other hand, the presence of cooler olivine probably promoted nucleation of Taenite crystals. TetraTaenite and cloudy zone measurements. We measured the width of the tetraTaenite zone and the high-Ni particle size of the cloudy zone (Fig. 1) in 20 main group pallasites. The width of the tetraTaenite zone was measured and corrected for orientation with respect to the growing kamacite. The orientation correction is obtained by measuring the crystal orientation of the Taenite and kamacite phases using EBSD. After correction, the tetraTaenite zone bandwidth in 20 main group pallasites varies from 1370 nm to 2550 nm, with a 1 standard deviation of 5-20% of the bandwidth measurement. The high-Ni particle size in the cloudy zone varies from 91 nm to 188 nm. The tetraTaenite width and the size of the high-Ni particles in the cloudy zone are well correlated and increase with decreasing cooling rate. However, both cooling rate parameters are not correlated with the bulk metal concentration (e.g. Au, Ni, Ga, and Ir) of the pallasite.
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determining cooling rates of iron and stony iron meteorites from measurements of ni and co at kamacite Taenite interfaces
Geochimica et Cosmochimica Acta, 2014Co-Authors: J. I. Goldstein, J. Yang, Edward R D ScottAbstract:Abstract Analyses and modeling of Ni zoning in Taenite in differentiated meteorites provide metallographic cooling rates at ∼500 °C that are inconsistent with conventional formation models. Group IVA iron meteorites have very diverse cooling rates of 100–6600 °C/Myr indicating that they cooled inside a large metallic body with little or no silicate mantle (Yang et al., 2007). Wasson and Hoppe (2012) have questioned these diverse cooling rates on the basis of their ion probe measurements of Ni/Co ratios at the kamacite–Taenite interface in two group IVA and in two group IIIAB iron meteorites. To investigate their claims and to assess methods for determining relative cooling rates from kamacite–Taenite interface compositions, we have analyzed 38 meteorites—13 IVA, 14 IIIAB irons, 4 IAB complex irons, 6 pallasites and a mesosiderite—using the electron probe microanalyzer (EPMA). Ni concentrations in Taenite (Niγ) and kamacite (Niα) at kamacite–Taenite interfaces are well correlated with metallographic cooling rates: Niγ values increase from 30 to 52 wt.% while Niα decreases from 7 to 4 wt.% as cooling rates decrease. EPMA measurements of Niγ, Niα, and Niα/Niγ, can therefore be used to provide order-of-magnitude estimates of relative cooling rates. Concentrations of Co in kamacite and Taenite at their interface (Coα, Coγ) are controlled by bulk Ni and Co composition, as well as cooling rate. The ratios Coα/Coγ and (Co/Ni)α/(Co/Ni)γ are correlated with cooling rate, but because of significant scatter, these parameters should not be used to estimate cooling rates. Our analyses of 13 group IVA irons provide robust support for diverse cooling rates that decrease with increasing bulk Ni, consistent with measurements of cloudy zone size and tetraTaenite width. Apparent equilibration temperatures, which are inferred from Niγ values and the Fe–Ni–P phase diagram and Ni diffusion rates in Taenite, show that cooling rates of IVA irons vary by a factor of ≈100, in excellent agreement with the metallographic cooling rates. Similar calculations using Niγ/Niα and Coα/Coγ ratios and phase diagram data give factors that are an order of magnitude lower but have larger uncertainties. Thus we strongly disagree with the conclusion of Wasson and Hoppe (2012) that interface concentrations of Ni and Co are in any way in conflict with the cooling rates of Yang et al. (2008). Our measurements confirm that the IVA irons could not have cooled in an asteroidal core surrounded by a silicate mantle, and also that main-group pallasites cooled slower than IIIAB irons and did not cool at the boundary between the mantle and core from which the IIIAB irons originated. Our data provide additional evidence that mesosiderites, which formed by impact mixing of Fe–Ni melt and crustal rocks, cooled at uniquely slow rates.
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Thermal and impact histories of reheated group IVA, IVB, and ungrouped iron meteorites and their parent asteroids
Meteoritics & Planetary Science, 2011Co-Authors: J. Yang, J I Goldstein, Edward R D Scott, Joseph R. Michael, Paul Gabriel Kotula, T. Pham, Timothy J. MccoyAbstract:The microstructures of six reheated iron meteoritesotwo 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)owere 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 WidmanstItten pattern. The other four, which show no trace of their original microstructure, were heated above 600-700 � C and recrystallized to form 10-20 lm wide homogeneous Taenite grains. On cooling, kamacite formed on Taenite grain boundaries with their close-packed planes aligned. Formation of homogeneous 20 lm 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 lm 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 (a2) in Taeniteothe first occurrence in ironsoappears 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).
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Olivine zoning and retrograde olivine-orthopyroxene-metal equilibration in H5 and H6 chondrites
Meteoritics & Planetary Science, 2006Co-Authors: R J Reisener, J I Goldstein, M. I. PetaevAbstract:Electron microprobe studies of several H5 and H6 chondrites reveal that olivine crystals exhibit systematic Fe-Mg zoning near olivine-metal interfaces. Olivine Fa concentrations decrease by up to 2 mol% toward zoned Taenite + kamacite particles (formed after relatively small amounts of Taenite undercooling) and increase by up to 2 mol% toward zoneless plessite particles (formed after ~200 °C of Taenite undercooling). The olivine zoning can be understood in terms of localized olivine-orthopyroxene-metal reactions during cooling from the peak metamorphic temperature. The silicate-metal reactions were influenced by solid-state metal phase transformations, and the two types of olivine zoning profiles resulted from variable amounts of Taenite undercooling at temperatures
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ordinary chondrite metallography part 1 fe ni Taenite cooling experiments
Meteoritics & Planetary Science, 2003Co-Authors: R J Reisener, J I GoldsteinAbstract: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 Widmanstatten 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.
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Ordinary chondrite metallography: Part 2. Formation of zoned and unzoned metal particles in relatively unshocked H, L, and LL chondrites
Meteoritics & Planetary Science, 2003Co-Authors: R J Reisener, J I GoldsteinAbstract:We studied the metallography of Fe-Ni metal particles in 17 relatively unshocked ordinary chondrites and interpreted their microstructures using the results of P-free, Fe-Ni alloy cooling experiments (described in Reisener and Goldstein 2003). Two types of Fe-Ni metal particles were observed in the chondrites: zoned Taenite + kamacite particles and zoneless plessite particles, which lack systematic Ni zoning and consist of tetraTaenite in a kamacite matrix. Both types of metal particles formed during metamorphism in a parent body from homogeneous, P-poor Taenite grains. The phase transformations during cooling from peak metamorphic temperatures were controlled by the presence or absence of grain boundaries in the Taenite particles. Polycrystalline Taenite particles transformed to zoned Taenite + kamacite particles by kamacite nucleation at Taenite/Taenite grain boundaries during cooling. Monocrystalline Taenite particles transformed to zoneless plessite particles by martensite formation and subsequent martensite decomposition to tetraTaenite and kamacite during the same cooling process. The varying proportions of zoned Taenite + kamacite particles and zoneless plessite particles in types 46 ordinary chondrites can be attributed to the conversion of polycrystalline Taenite to monocrystalline Taenite during metamorphism. Type 4 chondrites have no zoneless plessite particles because metamorphism was not intense enough to form monocrystalline Taenite particles. Type 6 chondrites have larger and more abundant zoneless plessite particles than type 5 chondrites because intense metamorphism in type 6 chondrites generated more monocrystalline Taenite particles. The distribution of zoneless plessite particles in ordinary chondrites is entirely consistent with our understanding of Fe-Ni alloy phase transformations during cooling. The distribution cannot be explained by hot accretion-autometamorphism, post-metamorphic brecciation, or shock processing.
