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N L Chabot - 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, N 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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sulfur contents of the parental metallic cores of magmatic Iron Meteorites
Geochimica et Cosmochimica Acta, 2004Co-Authors: N L ChabotAbstract:Abstract Magmatic Iron Meteorites are thought to be samples of the central metallic cores of asteroid-sized parent bodies. Sulfur is believed to have been an important constituent of these parental cores, but due to the low solubility of S in solid metal, initial S-contents for the magmatic groups cannot be determined through direct measurements of the Iron Meteorites. However, experimental solid metal-liquid metal partition coefficients show a strong dependence on the S-content of the metallic liquid. Thus, by using the experimental partition coefficients to model the fractional crystallization trends within magmatic Iron meteorite groups, the S-contents of the parental cores can be indirectly estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of the largest magmatic Iron meteorite groups leads to best estimates for the S-contents of the parental cores of 12 ± 1.5 wt% for the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1 ± 1 wt% for the IVB group. The IVA elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial S-content. These S-content estimates are much higher than those recently inferred from crystallization models involving trapped melt. The discrepancy is due largely to the different partition coefficients that are used by the two models. When only partition coefficients that are consistent with the experimental data are used, the trapped melt model, and the low S-contents it advocates, cannot match the Ge and Ir fractionations that are observed in IIIAB Iron Meteorites.
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an experimental test of henry s law in solid metal liquid metal systems with implications for Iron Meteorites
Meteoritics & Planetary Science, 2003Co-Authors: N L Chabot, M Humayun, Andrew J Campbell, J H Jones, C B AgeeAbstract:Experimental solid metal-liquid metal partition coefficients have been used to model the crystallization of magmatic Iron Meteorites and understand the evolution of asteroid cores. However, the majority of the partitioning experiments have been conducted with trace elements doped at levels that are orders of magnitude higher than measured in Iron Meteorites. Concern about Henry' s Law and the unnatural doping levels have been cited as one reason that two recent Iron meteorite studies have dismissed the experimental partition coefficients in their modeling. Using laser ablation ICP-MS analysis, this study reports experimentally determined solid metal-liquid metal trace element partition coefficients from runs doped down to the levels occurring in Iron Meteorites. The analyses for 12 trace elements (As, Co, Cr, Cu, Ga, Ge, Ir, Os, Pd, Pt, Re, and W) show no deviations from Henry' s Law, and these results support decades of experimental work in which the partition coefficients were assumed to be independent of trace element concentration. Further, since our experiments are doped with natural levels of trace elements, the partitioning results are directly applicable to Iron Meteorites and should be used when modeling their crystallization. In contrast, our new Ag data are inconsistent with previous studies, suggesting the high Ag-content in previous studies may have influenced the measured Ag partitioning behavior.
Naoji Sugiura - One of the best experts on this subject based on the ideXlab platform.
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Mn‐Cr chronology of five IIIAB Iron Meteorites
Meteoritics & Planetary Science, 2020Co-Authors: Naoji Sugiura, H. HoshinoAbstract:Mn-Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB Iron Meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in 53Cr are found and 53Cr is well correlated with Mn/Cr ratios, suggesting that 53Mn was alive at the time of IIIAB Iron formation. The inferred Mn-Cr "ages" are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB Iron Meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB Iron Meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in 53Cr during the closure process. Apparently, old Mn-Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB Iron Meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB Iron Meteorites that satisfy the Mn-Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (
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Nitrogen‐isotopic compositions of IIIE Iron Meteorites
Meteoritics & Planetary Science, 2020Co-Authors: Naoji Sugiura, Y. Ikeda, Shigeo Zashu, John T WassonAbstract:Abstract-The (compositionally) closely related Iron meteorite groups IIIE and IIIAB were originally separated based on differences in kamacite bandwidth, the presence of carbides only in the IIIE group, and marginally resolvable differences on the Ga-Ni and Ge-Ni diagrams. A total of six IIIE Iron Meteorites have been analyzed for C and N using secondary ion mass spectrometry, and three of these have also been analyzed for N, Ne, and Ar by stepped combustion. We show that these groups cannot be resolved on the basis of N abundances or isotopic compositions but that they are marginally different in C-isotopic composition and nitride occurrence. Cosmic-ray exposure age distributions of the IIIE and IIIAB Iron Meteorites seem to be significantly different. There is a significant N-isotopic range among the IIIE Iron Meteorites. A negative correlation between 615N and N concentration suggests that the increase in 615N resulted from diffusional loss of N. INTRODUCTION
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mn cr chronology of five iiiab Iron Meteorites
Meteoritics & Planetary Science, 2003Co-Authors: Naoji Sugiura, H. HoshinoAbstract:Mn-Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB Iron Meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in 53Cr are found and 53Cr is well correlated with Mn/Cr ratios, suggesting that 53Mn was alive at the time of IIIAB Iron formation. The inferred Mn-Cr "ages" are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB Iron Meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB Iron Meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in 53Cr during the closure process. Apparently, old Mn-Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB Iron Meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB Iron Meteorites that satisfy the Mn-Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model-dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB Iron Meteorites, which seems to be mainly controlled by redox conditions.
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Ion probe measurements of carbon and nitrogen in Iron Meteorites
Meteoritics & Planetary Science, 1998Co-Authors: Naoji SugiuraAbstract:— Carbon and nitrogen distributions in Iron Meteorites, their concentrations in various phases, and their isotopic compositions in certain phases were measured by secondary ion mass spectrometry (SIMS). Taenite (and its decomposition products) is the main carrier of C, except for IAB Iron Meteorites, where graphite and/or carbide (cohenite) may be the main carrier. Taenite is also the main carrier of N in most Iron Meteorites unless nitrides (carlsbergite CrN or roaldite (Fe, Ni)4N) are present. Carbon and N distributions in taenite are well correlated unless carbides and/or nitrides are exsolved. There seem to be three types of C and N distributions within taenite. (1) These elements are enriched at the center of taenite (convex type). (2) They are enriched at the edge of taenite (concave type). (3) They are enriched near but some distance away from the edge of taenite (complex type). The first case (1) is explained as equilibrium distribution of C and N in Fe-Ni alloy with M-shape Ni concentration profile. The second case (2) seems to be best explained as diffusion controlled C and N distributions. In the third case (3), the interior of taenite has been transformed to the α phase (kamacite or martensite). Carbon and N were expelled from the α phase and enriched near the inner border of the remaining γ phase. Such differences in the C and N distributions in taenite may reflect different cooling rates of Iron Meteorites. Nitrogen concentrations in taenite are quite high approaching 1 wt% in some Iron Meteorites. Nitride (carlsbergite and roaldite) is present in Meteorites with high N concentrations in taenite, which suggests that the nitride was formed due to supersaturation of the metallic phases with N. The same tendency is generally observed for C (i.e., high C concentrations in taenite correlate with the presence of carbide and/or graphite). Concentrations of C and N in kamacite are generally below detection limits. Isotopic compositions of C and N in taenite can be measured with a precision of several permil. Isotopic analysis in kamacite in most Iron Meteorites is not possible because of the low concentrations. The C isotopic compositions seem to be somewhat fractionated among various phases, reflecting closure of C transport at low temperatures. A remarkable isotopic anomaly was observed for the Mundrabilla (IIICD anomalous) meteorite. Nitrogen isotopic compositions of taenite measured by SIMS agree very well with those of the bulk samples measured by conventional mass spectrometry.
Takafumi Hirata - One of the best experts on this subject based on the ideXlab platform.
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distribution of platinum group elements and rhenium between metallic phases of Iron Meteorites
Earth and Planetary Science Letters, 1997Co-Authors: Takafumi Hirata, Robert W NesbittAbstract:In situ measurement of the platinum group elements (PGE) and Re by laser ablation-ICPMS allows fractionation patterns to be established between host (kamacite) and Ni-rich exsolved phases (taenite and plessite). Measurements on two IAB Iron Meteorites (Canyon Diablo and Odessa) show that, for the Ni-rich phases: (a) there is no significant fractionation for Ru and Rh; (b) Pd is enriched; and (c) Ir, Os and Pt are strongly depleted. We suggest that, in the case of kamacite, taenite and plessite, the controlling mechanism is ionic size with the order of ions (small to large) being Ir-Os-Pt-(Re)-Rh-Ru-Pd. This progression plots as a smooth curve on a diagram of relative abundance distribution vs. ionic size. A comparison of PGE and Re data on Iron Meteorites with published data from CI chondrites indicate that there is little or no relative fractionation between the elements. The exception is Re that is clearly enriched in kamacite relative to chondrites. This confirms earlier observations on the fractionation of Re/Os between bulk IAB Irons and CI values [1]. Metallic veins cutting graphite inclusions within the Canyon Diablo IAB meteorite are interpreted as a melt fraction from the Iron meteorite. Abundance ratios for PGE and Re between kamacite and the metallic vein are similar to abundance ratios between kamacite and Ni-rich phases. The fact that those elements with large ionic radii (Rh, Ru and Pd) are concentrated in the metallic veins adds weight to the view that they represent a melt fraction. Our in situ measurements of the PGE and Re demonstrate that elemental fractionation takes place both during melt segregation and solid state diffusion (exsolution). The data have important implications for the Re/Os geochronometer and indicate that measurements of187Os/186Os ratios on individual phases within Iron Meteorites may provide a potential mineral isochron.
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Rhenium and osmium systematics on Iron and stony Iron Meteorites
Meteoritics, 1992Co-Authors: Takafumi Hirata, Akimasa MasudaAbstract:Abstract— Re and Os abundances and 187Os/186Os isotopic ratios in 12 Iron Meteorites of various groups and five stony Iron Meteorites have been determined by an inductively coupled plasma mass spectrometry (ICP-MS). The series of Iron Meteorites studied have Re and Os concentrations ranging from 0.004 to 3.3 ppm and 0.03 to 41 ppm, respectively. The 187Re/186Os ratios in these Meteorites fall between 3.0 and 6.1 and the 187Os/186Os between 1.0 and 1.2, giving an initial 187Os/186Os isotopic ratio of 0.790 and a Re-Os age of Iron Meteorites of 4.30 ± 0.28 Ga when employing the decay constant of 1.64 × 10−11 yr−1. The observed Re-Os age for Iron Meteorites appears somewhat younger than that for chondrites. The resultant younger age might be due either to a very slow cooling of the parental planetesimals or due to a secondary “shock” event. However, for definite conclusions about the Re-Os age, higher precisions of the Re and Os isotopic measurements and of the decay constant of 187Re are required. Furthermore, the clear elucidation of the mechanisms for the fractionation of the Re/Os abundance ratios are related to the understanding of the meaning of the Re-Os age. The Re and Os abundances in pallasite stony Iron Meteorites are extremely low compared with those for most Iron Meteorites. On the other hand, the Re and Os abundances in mesosiderite stony Iron Meteorites show values comparable with those observed in most Iron Meteorites. The difference in Re and Os abundances in pallasite and mesosiderite stony Iron Meteorites strongly suggests that these stony Iron Meteorites are different in origin or history of chemical evolution. Re and Os abundances in the series of Iron and stony Iron Meteorites were found to have a wide variation covering nearly four orders of magnitude, with a very high correlation coefficient (0.996), and a slope very slightly less than unity. The regression line observed here covers various groups of Iron Meteorites, stony Iron Meteorites and also chondrites. Masuda and Hirata (1991) suggested the possible direct mixing process of particles of most refractory metallic elements with gaseous clouds of less refractory matrix elements, since the Re and Os were predicted theoretically to be the first elements to condense as a solid phase from the high temperature solar nebula. The aims of this paper are to present a reliable technique for the Re-Os chronology and to study the cosmochemical sequences of the meteoritic metals.
T E Bunch - One of the best experts on this subject based on the ideXlab platform.
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the phosphates of iiiab Iron Meteorites
Meteoritics & Planetary Science, 1999Co-Authors: Edward J Olsen, Alfred Kracher, A M Davis, Ian M Steele, I D Hutcheon, T E BunchAbstract:Thirteen phosphate minerals are found in IIIAB Iron Meteorites. Four of these (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute the majority of occurrences. The IIIB Iron Meteorites are confined to occurrences of only these four phosphates. The IIIA Iron Meteorites may contain one or more of these four phases; they may also contain other rarer phosphates, and silica (in two instances) and a silicate rock (in one instance). Thus, the IIIA lithophile chemistry is more varied than that of the IIIB Meteorites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleated as separate crystals. The IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements (and 0) were concentrated in a residual, S-rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of P as the core solidified and excluded 0, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIAB Meteorites with low-Ir contents in the metal, the phosphates are mainly Fe-Mn phases; at intermediate Ir values, more Na-bearing phosphates appear; at the highest Ir values, the rarer Na-, K-, Mg-, Cr-, and Pb- bearing phosphates appear. The absence of significant amounts of Mg, Si, Al, and Ti suggest depletion of these elements in the core by the overlying mantle.
J. I. Goldstein - One of the best experts on this subject based on the ideXlab platform.
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Nucleation of the Widmanstatten Pattern in Iron Meteorites
2020Co-Authors: J. Yang, J. I. GoldsteinAbstract:Introduction: The Widmanstatten pattern develops at low temperatures during the evolution of the asteroids. We have studied the origin of the Widmanstatten pattern in order to obtain metallographic cooling rates in the temperature range (~ 700 to 300 deg C). This paper summarizes our recent evaluation of the various mechanisms for the formation of the Widmanstatten pattern. All chemical groups of the Iron Meteorites are considered [1, 2]. We also propose a new mechanism for the formation of the Widmanstatten pattern in the low P metal phase of Iron, stony-Iron and stony Meteorites. The results of this evaluation enables us to more accurately determine metallographic cooling rates particularly when incorporated with other recent advances in Fe-Ni and Fe-Ni (P saturated) phase diagrams and interdiffusion coefficients. Origin of the Widmanstatten Pattern in Meteorites: The formation of the Widmanstatten pattern depends on the bulk Ni and bulk P in a given meteorite. Five mechanisms have been proposed: (i) γ α+γ (mechanism I): This is the traditional mechanism which is based on the binary Fe-Ni phase diagram. This mechanism assumes that α can nucleate directly from γ phase within the α+γ two phase region when cooling from high temperature to low temperature. However, several experimental studies have shown that it is not possible to nucleate α phase directly within single crystal taenite, γ phase, in synthetic Fe-Ni alloys during cooling and before forming martensite, α2 phase [3, 4]. (ii) γ γ+Ph (phosphide, (FeNi)3P) α+γ+Ph (mechanism II): The important role of P in the formation of the Widmanstatten pattern in Iron Meteorites has been recognized, Goldstein et al. [5]. These authors showed, experimentally, that this mechanism can produce a Widmanstatten pattern in high P containing Fe-Ni-P alloys. (iii) γ α+γ α+γ+Ph (mechanism III): This mechanism was proposed by Moren et al. [6] in their cooling rate simulation of low P Iron Meteorites. However, under-cooling of 120-190 C below the equilibrium γ/(α + γ) phase boundary is necessary in many low P IVA Iron Meteorites. Narayan et al. [4] investigated, experimentally, the nucleation of intragranular α phase from γ phase in Fe-Ni-P alloys. They found that, in low P Fe-Ni alloys, α phase cannot nucleate when the alloy is cooled into the two-phase α+γ field. In fact, the α phase only nucleates when γ phase is saturated in P and enters the α+γ+Ph field. (iv) γ α2 α+γ (mechanism IV): This mechanism was initially proposed as an alternate to mechanism I, γ α+γ, for Iron Meteorites by Owen [7] and Buchwald [8]. Recently mechanism IV was used to determine the cooling rate of mesosiderites and low P IVA Iron Meteorites [9, 10]. However, the Widmanstatten pattern cannot form by this mechanism in low P Meteorites [2], even though this mechanism is applicable to the formation of the plessite structure in taenite lamellae. (v) (γ α2+γ α+γ+(Ph)) (mechanism V): This mechanism was recently proposed by Yang et al. [1, 2] and can be used to explain the origin of the Widmanstatten pattern in low P Meteorites. In this case, α2 forms once the martensite start temperature is crossed during cooling and before P saturation occurs. The kamacite phase nucleates near the Ms temperature by the reaction α2 α+γ.
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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, N 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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the metallographic cooling rate method revised application to Iron Meteorites and mesosiderites
Meteoritics & Planetary Science, 2001Co-Authors: W D Hopfe, J. I. GoldsteinAbstract: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 ala + y phase boundary and to the yla + y 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 "ClMa, about lox higher than the most recent measured cooling rates. The cooling rate curves Erom the current model do not accurately predict the central nickel content of taenite halfwidths smaller than -10 pm. 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.
