Ordinary chondrite metallography: Part 1. Fe‐Ni taenite cooling experiments

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

Olivine zoning and retrograde olivine‐orthopyroxene‐metal equilibration in H5 and H6 chondrites

Abstract— 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 <700 °C. The relevant silicate‐metal reactions are modeled using chemical thermodynamics. Systematic olivine Fe‐Mg zoning adjacent to metal is an expected consequence of retrograde silicate‐metal reactions, and the presence of such zoning provides strong evidence that the silicate and metallic minerals evolved in situ during cooling from the peak metamorphic temperature.     

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single‐domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650–450 °C interval for higher petrographic types (consistent with an onion‐shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock‐related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.     

The formation of plessite in meteoritic metal

Abstract— 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, M_s, 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.     

Fe‐Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

Abstract— We report the results of our petrological and mineralogical study of Fe‐Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe‐Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni‐rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni‐rich metal in type 3.15–3.9 chondrites always contains less Co than does kamacite. Fe‐Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni‐rich regions. Metal in other type 3 chondrites is composed of fine‐ to coarse‐grained aggregates of kamacite and Ni‐rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni‐rich grains in metal (number of Ni‐rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe‐Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe‐Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01.     

Minor and trace element concentrations in adjacent kamacite and taenite in the Krymka chondrite

We report in situ NanoSIMS siderophile minor and trace element abundances in individual Fe‐Ni metal grains in the unequilibrated chondrite Krymka (LL3.2). Associated kamacite and taenite of 10 metal grains in four chondrules and one matrix metal were analyzed for elemental concentrations of Fe, Ni, Co, Cu, Rh, Ir, and Pt. The results show large elemental variations among the metal grains. However, complementary and correlative variations exist between adjacent kamacite‐taenite. This is consistent with the unequilibrated character of the chondrite and corroborates an attainment of chemical equilibrium between the metal phases. The calculated equilibrium temperature is 446 ± 9 °C. This is concordant with the range given by Kimura et al. (2008) for the Krymka postaccretion thermal metamorphism. Based on Ni diffusivity in taenite, a slow cooling rate is estimated of the Krymka parent body that does not exceed ~1K Myr^?1, which is consistent with cooling rates inferred by other workers for unequilibrated ordinary chondrites. Elemental ionic radii might have played a role in controlling elemental partitioning between kamacite and taenite. The bulk compositions of the Krymka metal grains have nonsolar (mostly subsolar) element/Ni ratios suggesting the Fe‐Ni grains could have formed from distinct precursors of nonsolar compositions or had their compositions modified subsequent to chondrule formation events.     

What metal‐troilite textures can tell us about post‐impact metamorphism in chondrite meteorites

Abstract— Metal‐troilite textures are examined in metamorphosed and impact‐affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal‐troilite textures are shown to form in response to three different impact‐related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re‐equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ～850 ° C, this post‐impact metamorphism results in melting of conjoined metal‐troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe‐Ni‐S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite‐rich margins and taenite‐rich cores. (3) At lower temperatures, post‐impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe‐Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post‐impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10⁶ years). The fast kinetics of metal‐sulfide reactions allows widespread textural changes in conjoined metal‐troilite grains during post‐impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post‐impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.     

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

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

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

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.     

In situ analysis of platinum group elements in equilibrated ordinary chondrite kamacite and taenite

Platinum group element (PGE) concentrations have been determined in situ in ordinary chondrite kamacite and taenite grains via laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). Results demonstrate that PGE concentrations in ordinary chondrite metal (kamacite and taenite) are similar among the three ordinary chondrite groups, in contrast to previous bulk metal studies in which PGE concentrations vary in the order H < L < LL. PGE concentrations are higher in taenite than kamacite, consistent with preferential PGE partitioning into taenite. PGE concentrations vary between and within metal grains, although average concentrations in kamacite broadly agree with results from bulk studies. The variability of PGE concentrations in metal decreases with increasing petrologic type; however, variability is still evident in most type six ordinary chondrites, suggesting that equilibration of PGEs does not occur between metal grains, but rather within individual metal grains via self‐diffusion during metamorphism. The constant average PGE concentrations within metal grains across different ordinary chondrite groups are consistent with the formation of metal via nebular condensation prior to the accretion of ordinary chondrite parent bodies. Post‐condensation effects, including heating during chondrule‐formation events, may have affected some element ratios, but have not significantly affected average metal PGE concentrations.     

The Santa Filomena meteorite shower: Trajectory,classification, and opaque phases as indicators of metamorphic conditions

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

Thermal and impact histories of reheated group IVA,IVB, and ungrouped iron meteorites and their parent asteroids

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

The Leedey,Oklahoma, chondrite: Fall,petrology, chemistry and an unusual Fe,Ni-FeS inclusion

Abstract— The Leedey, Oklahoma, meteorite shower fell on 1943 November 25, following a fireball which was visible across much of southwestern Oklahoma and northcentral Texas. The shower produced 24 stones with a total mass of ～51.5 kg. The stones formed a strewnfield ～18 km in length in the same direction as the observed path of the meteor (N50°W). Leedey is classified as an L6(S3) ordinary chondrite. We report bulk major element chemical analyses from four separate laboratories. Leedey contains an unusual 6 by 8 mm composite Fe,Ni-FeS grain, which is composed of a 3 mm kamacite grain adjacent to a 5 mm troilite grain. A 50–100 μm rim of high-Ni (45–55 wt%) taenite (tetrataenite) occurs at the boundary between kamacite and troilite. A single, zoned pyrophanite grain is observed at the boundary between the inclusion troilite and host silicates. An origin as a foreign particle incorporated after metamorphism or during impact melting appears unlikely. This particle likely formed by a complex set of processes, including melting in the nebula, parent body metamorphism and reheating by later shock, mirroring the history of the host chondrite.     

Fe‐Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history

Abstract– CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe‐Ni metal and sulfide phases in 13 CM and 2 CM‐related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe‐Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni‐Co‐rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites.     

Unmelted cosmic metal particles in the Indian Ocean

Fe‐Ni metal is a common constituent of most meteorites and is an indicator of the thermal history of the respective meteorites, it is a diagnostic tool to distinguish between groups/subgroups of meteorites. In spite of over a million micrometeorites collected from various domains, reports of pure metallic particles among micrometeorites have been extremely rare. We report here the finding of a variety of cosmic metal particles such as kamacite, plessite, taenite, and Fe‐Ni beads from deep‐sea sediments of the Indian Ocean, a majority of which have entered the Earth unaffected by frictional heating during atmospheric entry. Such particles are known as components of meteorites but have never been found as individual entities. Their compositions suggest precursors from a variety of meteorite groups, thus providing an insight into the metal fluxes on the Earth. Some particles have undergone heating and oxidation to different levels during entry developing features similar to I‐type cosmic spherules, suggesting atmospheric processing of individual kamacites/taenite grains as another hitherto unknown source for the I‐type spherules. The particles have undergone postdepositional aqueous alteration transforming finally into the serpentine mineral cronstedtite. Aqueous alteration products of kamacite reflect the local microenvironment, therefore they have the potential to provide information on the composition of water in the solar nebula, on the parent bodies or on surfaces of planetary bodies. Our observations suggest it would take sustained burial in water for tens of thousands of years under cold conditions for kamacites to alter to cronstedtite.     

The metallographic cooling rate method revised: Application to iron meteorites and mesosiderites

Abstract— A major revision of the current Saikumar and Goldstein (1988) cooling rate computer model for kamacite growth is presented. This revision incorporates a better fit to the α/α + γ phase boundary and to the γ/α + γ phase boundary particularly below the monotectoid temperature of 400 °C. A reevaluation of the latest diffusivities for the Fe‐Ni system as a function of Ni and P content and temperature is made, particularly for kamacite diffusivity below the paramagnetic to ferromagnetic transition. The revised simulation model is applied to several iron meteorites and several mesosiderites. For the mesosiderites we obtain a cooling rate of 0.2 °C/Ma, about 10x higher than the most recent measured cooling rates. The cooling rate curves from the current model do not accurately predict the central nickel content of taenite halfwidths smaller than ～10 μm. This result calls into question the use of conventional kamacite growth models to explain the microstructure of the mesosiderites. Kamacite regions in mesosiderites may have formed by the same process as decomposed duplex plessite in iron meteorites.     

Thermal histories of IVA iron meteorites from transmission electron microscopy of the cloudy zone microstructure

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

The formation of the Widmanstätten structure in meteorites

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 M_s 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 → α_m → 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.     

Reevaluation of formation of metal nodules in ordinary chondrites

Abstract— Compositions of four metal nodules from two ordinary chondrites, WIS91627 (H3.7) and Juin (H5), were determined by instrumental and radiochemical neutron activation analyses. Compared with bulk metal fractions, the metal nodules are characterized by strong and variable depletion of the refractory siderophile elements Re, Os, Ir, Ru, Pt and Rh but normal W and Mo, some fractionation of Co from Ni, and low Cu concentrations. These characteristics are difficult to explain by shock-induced vaporization followed by fractional condensation, a mechanism suggested by Widom et al. (1986). We propose formation of metal nodules during metamorphism in the parent body. Refractory siderophile elements, such as Ir, Os, Rh, etc., are partly locked up in noble metal nuggets and cannot participate in kamacite formation. The occurrence of metal nodules in both equilibrated and unequilibrated ordinary chondrites suggests that diffusion along grain boundaries was important in the development of kamacite and taenite in ordinary chondrites.     

Shock features in iron-nickel metal and troilite of L-group ordinary chondrites

Abstract— Shock metamorphic features in opaque minerals (FeNi metal and troilite) of 22 L chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate-based shock classification scheme (Stöffler et al., 1991) and the peak-shock and postshock thermal history recorded in these minerals. Unshocked to weakly shocked (S1–S3) L chondrites contain FeNi metal and troilite that display textures related to normal, slow cooling. They may also contain rare disequilibrium shock features, which suggest localized departures from equilibrium shock conditions. Above shock stage S3, selected melting of FeNi metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample. At these higher shock levels, the abundance of other shock-induced features, such as polycrystalline kamacite, sheared and fizzed troilite, coarse-grained pearlitic plessite, polycrystalline troilite, and polymineralic melt veins serve as textural criteria that can be used to establish peak-shock conditions. Minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage, thereby providing additional information about the postshock thermal histories of L chondrites. At the highest shock levels recorded in L chondrites (S6 and above), melting and chemical homogenization of FeNi metal produces flattened Ni profiles that may partially to completely obscure any evidence for an earlier, slow-cooling history. All of these features serve as aids for shock classifying L chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism.