Thermal Histories of IVA Iron Meteorites from Transmission Electron Microscopy of the Cloudy Zone Microstructure

Meteoritics & Planetary Science 44, Nr 3, 343–358 (2009) Abstract available online at http://meteoritics.org

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

J. I. GOLDSTEIN1*, J. YANG1, P. G. KOTULA2, J. R. MICHAEL2, and E. R. D. SCOTT3

1Department of Mechanical and Industrial Engineering, 313 Engineering Lab, University of Massachusetts, 160 Governors Drive, Amherst, Massachusetts 01003, USA 2Materials Characterization Department, Sandia National Laboratories, P.O. Box 5800, MS 0886, Albuquerque, New Mexico 87185, USA 3Hawai’i Institute of Geophysics and Planetology, University of Hawai’i, Honolulu, Hawai’i 96822, USA *Corresponding author. E-mail: [email protected] (Received 30 June 2008; revision accepted 06 November 2008)

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.

INTRODUCTION zone microstructure borders the outer taenite rim and formed by a spinodal phase transformation around 300 °C during Constraints on the thermal histories of meteorites are cooling of meteoritic parent bodies. A distinct boundary is invaluable for elucidating the sizes of their parent bodies and clearly observed between the OTR and CZ in the metal of the their isotopic ages and for understanding their origin (Ganguly 3 meteorites shown in Fig. 1. The cloudy zone (CZ) has a and Stimpfl 2000; Trieloff et al. 2003). Although most studies structure in which high-Ni particles (“island phase”) are of the thermal history of metallic Fe-Ni grains have focused enclosed by a low-Ni phase (“honeycomb phase”). The on the metallographic cooling rates obtained from Ni zoning in formation of the spinodal is an uphill diffusion process. After taenite that developed during kamacite growth at ~650–400 °C, initial formation, the phases grow during cooling and the cooling rates at lower temperatures can be determined from cloudy zone structure coarsens by a diffusion-controlled the dimensions of the cloudy zone microstructure observed in process. Some of the high-Ni particles grow together and residual taenite rims (Yang et al. 1997a). some of the high-Ni particles disappear in the coarsening Figure 1 shows scanning electron microscope (SEM) process (See Figs. 1a and 1b). photomicrographs of the microstructure of taenite rims in the Yang et al. (1997a) measured the size of the high-Ni metal of several meteorites. At the high-Ni periphery of the particles at the outer edge of the cloudy zone adjacent to the taenite boundary with kamacite, sulfide or silicate, there is a tetratenite rim in various irons, stony-iron, and stony narrow region that appears relatively clear on etching called meteorites by high-resolution SEM. They showed that the the outer taenite rim (OTR) or tetrataenite region. The cloudy size of the high-Ni particles increased with decreasing

343 © The Meteoritical Society, 2009. Printed in USA. 344 J. I. Goldstein et al.

cooling rate. The sizes of the high-Ni particles range from 400–450 nm in the mesosiderites, which cooled very slowly (Fig. 1a) to ~20–40 nm (Fig. 1c) in the rapidly cooled IVA iron meteorites (Yang et al. 1997a). The metallographic cooling rate measured from the growth of the Widmanstätten pattern at temperatures from about 650 to 400 °C ranges from 0.2 °C/Myr for the mesosiderites (Hopfe and Goldstein 2001) to greater than 6,000 °C/Myr for the IVA irons (Yang et al. 2007, 2008). Since the inverse relationship between high-Ni particle size and metallographic cooling rate is applicable for metal in irons, stony-irons, and stony meteorites, the size of the high-Ni particles in the cloudy zone microstructure provides valuable information on the cooling rates of metal- bearing meteorites at 350 to 200 °C, for example, the anomalous Portales Valley chondrite (Sepp et al. 2001). Measurements of the size of the high-Ni particles in the cloudy zone of fast-cooled meteorites such as the IVA irons are more difficult to obtain with the SEM as the size of the high-Ni particles approaches the resolution of the SEM (≤10 nm). Specimen preparation becomes more critical. The amount of chemical attack or etching used to reveal the cloudy zone microstructure can affect the measured size of the high-Ni particles and also the visibility of the microstructure in a high-resolution SEM. Too little etching will not produce enough contrast between the high-Ni particles and the low-Ni honeycomb phase to make the microstructure visible in the SEM. Too much etching will dissolve some of the high-Ni particles as well as the surrounding honeycomb phase. Because of their small size, the high-Ni particles in the cloudy zones of IVA irons are especially sensitive to shock. The thermal spike associated with the shock process can cause the high-Ni particles to dissolve rapidly with the formation of single phase taenite and the dissolution of the boundary between tetrataenite and the prior cloudy zone. Slow cooling after the reheating process may allow formation of a new generation of cloudy zone by a spinodal reaction or perhaps precipitation of a new phase from the newly formed taenite. It may be difficult to tell which generation of cloudy zone is being observed in the SEM without microchemical data which can be measured using the transmission electron microscope (TEM) equipped with an X-ray (EDS) detector. The current study was stimulated by the need to understand the low-temperature thermal history of the IVA irons. The metallographic cooling rates of the IVA irons determined from kamacite growth modeling vary directly with Ni content by a factor of ~60 indicating that the IVA irons formed in a 150 ± 50 km radius metallic body (Yang Fig. 1. SEM micrographs of the cloudy zone (CZ) microstructure of et al. 2007, 2008). The highest cooling rate of 6,600 °C/Myr metal in 3 meteorites. a) Patwar mesosiderite, b) Carbo IID iron, and allows for the formation of some of the smallest high-Ni c) Steinbach IVA iron. OTR represents the outer taenite rim or particles, ~10 nm in size, in the cloudy zone of any group of tetrataenite region. The high-Ni particle size or island phase size varies in size from a) 469 nm to b) 134 nm to c) 29 nm. K indicates meteorites. However, several members of the IVA chemical kamacite surrounding the OTR in Carbo. All SEM photmicrographs group have suffered shock reheating (Buchwald 1975), which were taken at 5 kV. may have affected the cloudy zone microstructure. In order to Thermal histories of IVA iron meteorites from transmission electron microscopy 345 measure the size of small high-Ni particles <30 nm in size, to using a Zeiss Supra 55VP field emission SEM at Sandia avoid potential artifacts which may be introduced during National Laboratories and a Hitachi 4300 field emission SEM specimen preparation, and to determine the microchemistry at Lehigh University. Most of the SEM data were obtained at of the cloudy zone region in reheated IVA irons, we use the 5kV in order to maximize the measurement of surface detail. TEM, equipped with an X-ray (EDS) detector, to examine the Each sample was also examined for shock effects evidenced cloudy zone structure. in the microstructure by optical microscopy and SEM and the The purpose of this study is three fold: 1) to measure the shock level was estimated. If shock-hatched kamacite was size of the high-Ni particles of the cloudy zone as well as the present, the meteorite was shocked above 13 GPa (either size of the surrounding outer taenite rim or tetrataenite region moderate or heavy shock); if only Neumann bands were (Fig. 1) where present in a representative group of IVA irons observed, the shock level was low, <13 GPa (Jain and using the TEM; 2) to use these size measurements to infer Lipschutz 1970). Of the 16 IVA irons studied, 8 suffered low cooling rates for the IVA irons at low temperatures and to shock and 8 were moderately to heavily shocked (Table 1). explain how the IVA irons formed in their parent asteroidal For three of the meteorites, we observed different shock levels body; and 3) to determine the effect of shock and shock than those listed by Jain and Lipschutz (1970) (Table 1). We reheating on several moderately to heavily shocked IVA irons adopted the shock levels we observed since the TEM samples by examining the tetrataenite and cloudy zone microstructure were taken directly from the polished sections that we using the TEM. prepared. We will show that the widths of the tetrataenite regions Thin sections of all 16 IVA irons were examined using correlate directly with cloudy zone particle sizes and indirectly the TEM (Table 1). The IVA iron samples were prepared for with the cooling rate of the meteorite providing another TEM using dual beam FEI DB-235 focused ion beam (FIB)/ measurement of low-temperature metal cooling rates. We also SEM instruments at Sandia National Laboratories and at document, for the first time, examples of shock reheating in Lehigh University. The TEM samples had a thickness of 50 to IVA irons that erased the cloudy zone microstructures which 100 nm and were approximately 10 µm long and 5 µm in formed during initial cooling of taenite. Most critically, our width. These TEM sections were cut perpendicular to studies of the cloudy zone microstructures in the IVA irons kamacite-taenite interfaces of the Widmanstätten pattern from show that IVA cooling rates at 350–200 °C vary by a factor of the polished and etched meteorite sample surfaces. Using this ~15 and are inversely correlated with bulk Ni concentration. methodology, the kamacite-taenite interface is oriented This cooling rate variation is incompatible with cooling in a perpendicular to the surface of the thin section. Figure 2 metallic core that was insulated with a silicate mantle, but is shows the region of interest and the FIB section formed for compatible with cooling in a metallic body of radius 150 ± 50 km the TEM analysis of cloudy zone in a taenite region of the (Yang et al. 2007, 2008). Bristol IVA iron. Three FIB samples were made of kamacite/ taenite interfaces for Jamestown, Obernkirchen, and Seneca METHOD Township, all moderately to heavily shocked IVA irons that lacked a cloudy zone microstructure. Two FIB samples were We prepared and examined the microstructure of 16 IVA made of kamacite/taenite interfaces for New Westville and irons by light optical microscopy and 12 IVA irons by SEM, Chinautla and one FIB sample was made for the remaining 11 as described by Yang et al. (1997a), covering the Ni meteorites. composition range of the IVA chemical group (See Table 1). Selected kamacite/taenite interface regions in the The samples listed in Table 1 include the 13 IVA irons whose thinnest areas of each FIB section were analyzed using a FEI metallographic cooling rates were measured by Yang et al. Tecnai F30ST field emission transmission–analytical electron (2008) and the 6 IVA irons whose cloudy zone was studied by microscope (TEM-AEM) at Sandia National Laboratories or Yang et al (1997a). The meteorite samples were epoxy- a VG 603 scanning transmission electron microscope mounted, ground with SiC paper, and polished with 6 µm (STEM) instrument at Lehigh University, both operated at diamond paste, 1 µm alumina powder, and 0.05 µm silica. To 300 kV. X-ray scans were taken in micron and sub-micron minimize contamination, the samples were put in a glass sized regions of the thin foil. For iron meteorites with a container with ethyl alcohol or methyl alcohol and cloudy zone, the size of the high-Ni particles in the cloudy ultrasonically cleaned after each grinding or polishing step. zone next to the cloudy zone/tetrataenite boundary (Yang et Each sample was etched with 2% nital for various times so al. 1997a) was measured directly from the STEM image or the that the cloudy zone, if present, was visible in the SEM. In the Ni X-ray scan obtained from the thin foil. The measurement etching process, the low-Ni phase (honeycomb phase) in the of the size of the high-Ni particles in the cloudy zone was cloudy zone is preferentially etched away during the chemical made next to the cloudy zone/tetrataenite boundary since the treatment developing a surface topology that gives the local Ni content at that position is constant at 40 to 42 wt% Ni necessary contrast so that the island phase can be observed in (Yang et al. 1997a). High-Ni particles farther away from the the SEM (Note Fig. 1). The SEM studies were accomplished cloudy zone/tetrataenite boundary were not measured as the 346 J. I. Goldstein et al.

Table 1. The 16 IVA irons studied by TEM. The width of the outer taenite rim or tetrataenite region Meteorite Nia Pc Shock level (Fig. 1) was also measured at several points along the kamacite/ name Source (wt%) (wt%) (GPa) taenite interface in each FIB section. The widths are measured Jamestown AMNH88 7.45 0.02 13–401 directly from STEM images or Ni X-ray scans in each La Grange USNM55 7.57 0.03 13–751 meteorite. No correction for orientation effects was necessary Obernkirchen AMNH189 7.64 0.02 401 for samples examined in the TEM since the kamacite-taenite 2 1 Bishop Canyon USNM770 7.71 0.05 <13 (13–40 ) interface is normal to the thin section (See Fig. 2c). San Francisco Mtn USNM1270a 7.73 0.05 <132 Bristol USNM1324 7.92 0.06 <131 São João Nepomuceno USNM6881a 8.02 – <132 RESULTS AND DISCUSSION Gibeon UMass 8.04 0.04b >132 (<131) Altonah UMass 8.36 0.09 13–751 High-Ni Particle Sizes of the Cloudy Zone Seneca Township AMNH3756 8.41 0.08 13–751 Bushman Land USNM2515 8.76 0.1 <132 Duchesne UMass 9.28 0.18 <131 SEM Data Steinbach USNM3086 9.40 0.15d <131 Figure 4 shows SEM pictures of 5 low shocked IVA New Westville USNM1412 9.42 0.14 13–751 irons. All low shocked irons have clear boundaries between Chinautla USNM742 9.54 0.17 <133 (13–401) the outer taenite rim (OTR) and the cloudy zone (Fig. 4). The 3 Duel Hill (1854) USNM2748 10.45 0.15 >13 cloudy zone microstructures are similar but not identical to AMNH: American Museum of Natural History. USNM: National Museum of Natural History. UMass: University of Massachusetts. the typical CZ microstructures shown in Fig. 1. Measurement aWasson and Richardson (2001). of cloudy zone sizes are difficult to make in some of the SEM bMoren and Goldstein (1979). cBuchwald (1975). pictures and no attempt was made to obtain high-Ni particle dRasmussen et al. (1995) sizes. Examination of the original SEM pictures of Yang et al. 1Jain and Lipschutz (1970). 2Yang et al. (2007). (1997a) show similar microstructures for the 5 low shocked 3This study, optical and SEM. IVA irons although the samples were usually heavily attacked Except for La Grange, Obernkirchen, Altonah, and Duel Hill, all the IVA irons listed above were studied using the scanning electron microscope. by the nital etchant. Figure 5 shows SEM pictures of 3 medium to high shocked IVA irons. In these meteorites, the high-Ni particle size and Ni content of taenite decrease with boundary between the outer taenite rim and the cloudy zone increasing distance from the cloudy zone/tetrataenite microstructure is no longer present. Starting about 100 nm boundary (Yang et al. 1997a). from the kamacite-taenite boundary, there appears to be a The measurement of the size of the high-Ni particles is microstructure that resembles the normal cloudy zone dependent on the geometry of the particles. Figure 1c shows microstructure where cloudy zone was once present before the typical microstructure of the cloudy zone in the IVA irons. reheating. This microstructure is, however, more irregular Most of the particles in these fast cooled meteorites are than that observed in the low shocked IVA irons and does not spherical, potato shaped, or elliptical. Figure 3 shows a decrease in size systematically with increasing distance from variety of particle shapes that are observed in the IVA irons the kamacite interface, as in the cloudy zone. In addition, as (compare to the microstructure of Steinbach IVA, Fig 1c). For discussed below, there is no evidence for separation of high the near-spherical particles (Fig. 3a), the size of the particle is and low-Ni phases as in the cloudy zone. Examination of the measured across the diameter. For the potato shaped particles original SEM pictures of Yang et al. (1997a) for the Gibeon (Fig. 3b), the size of the particle is the width along the minor meteorite show similar microstructures for the medium to axis. For the egg, banana, and raindrop shapes (Figs. 3c–e), high shocked IVA irons. The origin of this microstructure, the width is measured along the minor axis which splits the which appears to have formed after shock heating, is unclear. major axis of the particle in two. For amoeba-shaped particles (Fig. 3f ), the size of the particle is the shortest distance from TEM Data one corner to the other side. This approach was employed for Eleven of the 16 IVA irons that were studied by TEM the measurement of the size of irregularly shaped high-Ni have a cloudy zone microstructure. Figure 6a shows a TEM particles from the X-ray scans as well as the bright and dark bright field image of the cloudy zone, outer taenite rim and field TEM images. kamacite at a kamacite/taenite interface of the Steinbach IVA For some X-ray images, a low pass filter was used to iron. This microstructure can be compared with the SEM better delineate the boundaries of the particles. Only 5 to 9 image of a similar region of Steinbach, Fig. 1c. Figure 6b high-Ni particles are located next to the cloudy zone/ displays an annular dark field STEM image of the cloudy tetrataenite boundary in each FIB section since the region zone, outer taenite rim and kamacite at a kamacite/taenite along the boundary analyzed from the X-ray area scan is interface in the Chinautla IVA iron. The cloudy zone is clearly usually 500 nm or less. The lack of a large sample of high-Ni observed and the sizes of 13 individual high-Ni particles, particles limits the accuracy of the measurement of the mean which were mostly spherical, potato-shaped, or elliptical in size of the high-Ni particles. shape, were measured in the cloudy zone at the cloudy zone/ Thermal histories of IVA iron meteorites from transmission electron microscopy 347

Fig. 3. Shows a variety of particle shapes that are observed in the IVA irons. For the near spherical particles (Fig. 3a), the size of the particle is measured across the diameter. For the potato-shaped particles (Fig. 3b), the size of the particle is the width along the minor axis. For the egg, banana, and raindrop shapes (Figs. 3c–e), the width is measured along the minor axis which splits the major axis of the particle in two. For amoeba-shaped particles (Fig. 3f ), the size of the particle is the shortest distance from one corner to the other side.

outer taenite rim interface. Figure 7 shows Ni Kα X-ray area scans of the cloudy zone/outer taenite rim/kamacite interfaces at various magnifications from four low shocked IVA irons. The high-Ni particles in the cloudy zone vary in size from one IVA iron to another. Figure 7e gives a Ni composition profile across the kamacite/taenite interface in Fig. 7d. The Ni variation from ~50 to ~40 wt% Ni in the OTR is typical of the boundary region observed between the cloudy zone and kamacite for most low shocked iron meteorites (Yang et al. 1997b). The Ni content of the kamacite, which decreases below 5 wt% within 100 nm of this boundary, is also typical of Widmanstätten pattern formation. In addition, a measurable decrease in Ni content is observed at the boundary between the OTR and the CZ region for low shocked metal. The Ni content in the cloudy zone at this boundary is about 40 wt%. Figure 8 shows Ni Kα X-ray area scans at various magnifications of the cloudy zone microstructure which is present in two moderately to heavily shocked high Ni IVAs, New Westville, and Duel Hill. The mean sizes of high-Ni particles in IVA irons vary from 10 nm to 32 nm as the Ni content of the meteorites increases from 7.7 to 10.45 wt%. In this Ni composition range, the measured metallographic cooling rate decreases from about 6,600 to 100 °C/Myr. The mean sizes of the high- Ni particles at the OTR/CZ boundary, the number of measured high-Ni particles, and the accompanying one σ Fig. 2. FIB section of the Bristol kamacite/taenite interface. a) SEM photo of kamacite/taenite region of interest. The square indicates the measurement error for each of the IVA irons are given in region from which the FIB sample was extracted. b) Higher Table 2. Since the typical thickness of a TEM thin section magnification SEM photo of kamacite/taenite region showing outer prepared with the FIB instrument is generally 50–100 nm, and taenite rim, decomposed martensite, and plessite in the center of the the high-Ni particle size in IVA irons is 10 to 32 nm (Table 2), region The crosses indicate the position from which the FIB sample was several high-Ni particles are intercepted by the focused extracted. c) SEM photo of FIB section which is normal to the sample surface. The sample is a wafer about to be cut out of the sample. electron beam as it passes through the TEM thin section. The kamacite/taenite boundary which intersects the sample surface can When the high-Ni particles overlap through the thin section, be observed in the thin FIB sample. the contrast in the STEM and X-ray images is degraded, and 348 J. I. Goldstein et al.

Fig. 4. SEM photomicrographs of 5 low shocked IVA irons. The kamacite/taenite and OTR/cloudy zone boundaries can be observed. K is kamacite, CZ is cloudy zone, OTR is outer taenite rim or tetrataenite region. The OTR/CZ interface is indicated by an arrow. it is more difficult to measure the island phase size (see variations between regions. However, for New Westville and Figs. 7a and 7b, and 8a, 8b). Figures 8c and 8d illustrate how Chinautla more high-Ni particles were measured since two the size measurement of the high-Ni phase was made for the FIB sections were made and several cloudy zone regions were Duel Hill IVA iron. Figure 8c shows the low pass filtered available for measurement. For New Westville the mean high- image of Fig. 8b, which in this example helps better establish Ni particle size was 17.6 nm and 17.9 nm for two different the interfaces of the high-Ni particles. Figure 8d shows the cloudy zone regions measured with the FEI instrument and outline of the 8 high-Ni particles that were measured and the 21.2 nm for one cloudy zone region measured with the VG positions where the size measurements were made. We instrument. The mean high-Ni particle size using all the estimate that the measurement errors in the mean high-Ni measurements was 20 ± 3 nm (Table 2), and the differences particle size of each IVA iron are 10 to 20% relative, Table 2. between mean particle sizes measured for 3 separate cloudy For most IVA irons, only a single cloudy taenite region zones were within the 1σ measurement error. For Chinautla, was studied, and we cannot exclude the possibility of the mean particle size was 30 ± 4.5 nm for one cloudy zone Thermal histories of IVA iron meteorites from transmission electron microscopy 349

Fig. 6. a) Bright field TEM image of the Steinbach (low shock) IVA iron. b) Annular dark field STEM image of the Chinautla (low shock) IVA iron. The CZ, OTR, and K regions of both samples are observed in the images.

Figure 9a shows that the high-Ni particle size of the Fig. 5. SEM photomicrographs of Gibeon, Jamestown, and Seneca eleven IVA irons with a cloudy zone microstructure is Township IVA irons. All 3 meteorites have been shock reheated. The kamacite/taenite boundary is clearly observed; however, the OTR/CZ positively correlated with the bulk Ni content. Since the boundary has been removed during reheating. K is kamacite. metallographic cooling rate determined from kamacite growth at temperatures between 650 and 400 °C decreases region measured with the FEI instrument. In this one case, we with increasing Ni content (Yang et al. 2008, Table 1), the were also able to measure a mean high-Ni particle size of high-Ni particle size also decreases with increasing 31.2 ± 3.8 nm using the annular dark field STEM image metallographic cooling rate, Fig. 9b. (Fig. 3b). The mean high-Ni particle size in the second FIB The mean width of the tetrataenite region for one or more section of Chinautla was 33.5 ± 5.4 nm measured with the VG kamacite/tetrataenite interfaces of each of the eleven IVA instrument. The mean high-Ni particle size using all the irons that contain a cloudy taenite microstructure is given in measurements was 32 ± 4.5 nm (Table 2) and the differences Table 2. The error in the width of the tetrataenite region, in mean particle size measured for 3 separate cloudy zones Table 2, represents the range of tetratenite widths measured were well within the 1σ measurement error. for each meteorite but is limited to the length of kamacite/ 350 J. I. Goldstein et al.

Fig. 7. Ni X-ray area scans of the cloudy zone (CZ), outer taenite rim (OTR), and kamacite (K) at the kamacite/taenite interface of 4 low shock IVA irons. a) San Francisco Mountains 100 × 100 nm scan. 128 pixels, ~0.78 nm/pixel. High-Ni particle size in the CZ is 13 ± 2 nm. b) Bristol 500 × 500 nm scan. 128 pixels, ~3.9 nm/pixel. High-Ni particle size in the CZ is 17 ± 3 nm. c) Steinbach 500 × 500 nm scan. 128 pixels, ~3.9 nm/pixel. High-Ni particle size in the CZ is 25 ± 3 nm. d) Chinautla 500 × 1000 nm scan. 128 × 256 pixels, ~3.9 nm/pixel. High-Ni particle size in the CZ is 32 ± 4.5 nm. e) A Ni composition profile across the kamacite/taenite interface in (d). The Ni content in the OTR decreases from 50 wt% at the kamacite/taenite interface to about 40 wt% at the OTR/CZ interface. taenite interface (~1 to 3 microns) available for analysis. the trend defined by the unshocked IVA irons, Fig. 10. Figure 10 shows that the width of the tetrataenite region is Considering the errors in the measurement of high-Ni particle positively correlated with the high-Ni particle size in the sizes, it is not clear if this observation is significant. cloudy taenite. Therefore, the tetrataenite width also increases with increasing Ni content and decreases with increasing The Effect of Shock and Shock Reheating metallographic cooling rate. The measured sizes of the high- Ni particles in the three moderately to heavily shocked IVA The cloudy zone microstructure was not present in 5 irons which contain a cloudy zone microstructure fall below moderately to heavily shocked IVA irons (Tables 1 and 2). Figure 11a Thermal histories of IVA iron meteorites from transmission electron microscopy 351

Fig. 8. Ni X-ray area scans of the cloudy zone (CZ), outer taenite rim (OTR), and kamacite (K-black) in two moderately to highly shocked IVA irons. a) New Westville 250 × 250 nm scan. 128 pixels, ~2 nm/pixel. High-Ni particle size in the CZ is 20 ± 3 nm. b) Duel Hill 300 × 300 nm scan. 150 pixels, ~2 nm/pixel. High-Ni particle size in the CZ is 26.5 + 4 nm. c) Low pass filtered image of (b) that helps delineate the interfaces of the high-Ni particles. d) Outline of the 8 high-Ni particles that we measured and the positions where the size measurements were made. shows a STEM image of the kamacite/taenite interface in a allow a rapid diffusion path and local removal of the Ni-rich thin section of Jamestown, the lowest Ni (7.45 wt%) IVA iron rim. Figure 11d shows two Ni versus distance profiles across that was studied. The kamacite is recrystallized with grain the kamacite/taenite interface. The position of the two profiles sizes of ~200 to 500 nm and the kamacite/tetrataenite 1 and 2 are shown in Fig. 11c. Profile 1 shows the Ni interface is wavy. No cloudy zone or tetrataenite/cloudy zone distribution well away from the newly formed grain boundary is observed. The SEM photomicrograph of boundaries while profile 2 crosses a kamacite/taenite Jamestown (Fig. 5) also shows the absence of the outer taenite boundary between two of the newly formed grain boundaries. rim (OTR)/cloudy zone interface. Figure 11b shows a 2 × Diffusion across the kamacite/taenite interface varies from as 2 µm Ni X-ray scan along the kamacite/taenite interface little as 30 nm (profile 1) to 200 nm (profile 2). In profile 2 shown in Fig. 11a. Although no cloudy zone is observed, the we observe that the Ni in the taenite next to the original high Ni taenite region (OTR) can still be observed in most kamacite/tetrataenite boundary has decreased from about places. Several grain boundaries containing about 20 wt% Ni 50 wt% to almost 30 wt% (compare with Fig. 7e) due to have formed across the tetrataenite region. The Ni content in diffusion through taenite and along newly formed taenite kamacite rises somewhat to about 5 wt% (compare to Fig. 7e) grain boundaries which form at high shock temperatures. As during reheating. In addition, a Ni-enriched diffusion zone up shown in Fig. 11d, regions beyond about 200 nm from the to 200 nm in width is observed in the kamacite next to the kamacite/taenite boundary have Ni contents less than about kamacite/taenite interface. The grain boundaries have formed 30 wt% Ni. During cooling after the shock reheating event, in the taenite normal to the kamacite/taenite boundary and new high Ni regions may form, as observed in Figs. 11c, 11d. 352 J. I. Goldstein et al.

Table 2. High-Ni particle size, tetrataenite width, and cooling rate data for IVA irons. Mean high-Ni particle size Metallog. (nm) cooling rate Yang Tetrataenite width Meteorite name Nia (°C/Myr)b et al. (1997) This study Number of particles (nm) Jamestown 7.45 1900 No CZc – La Grange 7.57 6600 12 ± 2c 6 45 ± 6 Obernkirchen 7.64 2900 No CZc – Bishop Canyon 7.71 2500 23 ± 4 12 ± 3 9 80 ± 4 San Francisco Mtn 7.73 – 13 ± 2 6 102 ± 4 Bristol 7.92 – 23 ± 2 17 ± 3 5 120 ± 5 São João Nepomuceno 8.02 – 10 ± 1 5 44 ± 2 Gibeon 8.04 1500 33 ± 5No CZc Altonah 8.36 420 No CZc Seneca Township 8.41 580 No CZc – Bushman Land 8.76 260 15 ± 3 8 80 ± 2 Duchesne 9.28 100 33 ± 3 25 ± 3 6 200 ± 2 Steinbach 9.40 150 29 ± 3 7 195 ± 5 New Westville 9.42 120 18 ± 3 20 ± 3c 28 105 ± 5 Chinautla 9.54 110 39 ± 3 32 ± 4.5 35 215 ± 25 Duel Hill (1854) 10.45 220 26.5 ± 4c 8 133 ± 5 aWasson and Richardson (2001). bYang et al. (2007, 2008). cModerately to heavily shocked. The rest of the samples were shocked to <13 GPa.

This secondary microstructure, developed on cooling after Figure 13 shows that the effect of shock heating is less in diffusion takes place, is also observed in the SEM images of Seneca Township (8.41 wt% Ni) than in Jamestown, Gibeon, Fig. 5. This microstructure is probably decomposed Obernkirchen, or Altonah. There is a relatively small decrease martensite in Jamestown. in the Ni content in the high Ni taenite at the kamacite/taenite Figure 12a shows a STEM image of the kamacite/taenite interface from about 50 to 45 wt% and limited diffusion interface in the Obernkirchen IVA meteorite, the third lowest across the tetrataenite/kamacite boundary of about 20 nm Ni (7.64 wt%) IVA iron that was studied. Shock heating is (See Fig. 13c; compare to Fig. 7e). The Ni content in more limited in this meteorite. Figure 12b shows a 1 × 1 µm kamacite is below 5 wt% consistent with limited diffusion Ni X-ray scan along the kamacite/taenite interface shown in across the kamacite/taenite interface during shock reheating. Fig. 12a. Although no cloudy zone microstructure is present, There is, however, no abrupt decrease in Ni content as the Ni X-ray map shows that there is an intermittent Ni-rich normally observed at the tetrataenite/cloudy zone boundary zone remaining with up to ~40 wt%. Profile 1 (Figs. 12c, 12d) (compare to the Ni area scans, Figs. 7 and 10, or the Ni shows diffusion across the kamacite/taenite interface composition profile in Fig. 7e.). The lack of an interface extending about 30 nm into kamacite with a decrease in Ni between tetrataenite and the original cloudy zone is also content in taenite at the OTR/kamacite boundary from about shown in the SEM image of Seneca Township, Fig. 5. 50 to 40 wt%. The Ni content in kamacite rises somewhat to Regions beyond about 150 nm from the kamacite/taenite over 5 wt%. In addition, there is no abrupt decrease in Ni boundary have Ni contents ~35 to 30 wt%, Fig. 13c. In this content as normally observed at the tetrataenite/cloudy zone region, a microstructure of <5 to 10 nm particles was boundary (See Fig. 4e). Profile 2 (Figs. 12c, 12d) shows a observed in the SEM image of Seneca Township, Fig. 5. more extensive diffusion zone of about 150 nm with a However, no Ni variation was observed with a X-ray spatial decrease in Ni content in the outer taenite region from about resolution of ~4 nm. It is possible that a fine-scale 50 to 30 wt%. As in Jamestown, the Ni content in kamacite microstructure may have formed during cooling after the rises somewhat to about 5 wt%. Altonah (8.36 wt% Ni) is also shock reheating event. shock reheated at about the same level as Obernkirchen. The effect of shock heating in Gibeon (8.04 wt% Ni) is Although no grain boundaries are observed, the kamacite/ similar to that in Seneca Township. No grain boundaries are taenite interface is no longer straight with extensive diffusion present in the outer taenite rim, the decrease in the Ni content of up to 400 nm. The Ni content in the outer taenite region in the high Ni taenite is from about 50 to 43 wt% Ni, limited decreases from about 50 to about 35 wt%. No cloudy zone is diffusion of about 50 nm is observed across the tetrataenite/ present, and there is no abrupt decrease in Ni content as kamacite boundary, and there is no abrupt decrease in Ni normally observed at the tetrataenite/cloudy zone boundary content which is observed at the tetrataenite/cloudy zone (See Fig. 7e). boundary in IVA irons containing cloudy zone, Figs. 6–8. As Thermal histories of IVA iron meteorites from transmission electron microscopy 353

Fig. 10. Relationship between tetrataenite width and high-Ni particle size for IVA irons subjected to low shock (<13 GPa) and to medium to high shock (>13 GPa).

X (Dt)1/2 (1) We calculate that reheating to 1000 °C for two seconds will remove 20 nm sized high-Ni particles and reheating for 40 seconds will cause diffusion zones up to 100 nm in width to develop across kamacite/taenite boundaries. Longer times at lower temperatures are required for the removal of 20 nm sized high-Ni particles in the cloudy zone, for example ~20 s at 900 °C, ~7 min. at 800 °C, and ~4.7 h at 700 °C. Reheating at low temperatures, <550 °C requires times exceeding a year. Grain boundary formation in the taenite will aid the diffusion process decreasing the time necessary for Ni diffusion between kamacite and taenite. Although Fig. 9. Relationship between high-Ni particle size and a) Ni content, mild impact heating to low temperatures may require long and b) metallographic cooling rate for IVA irons that experience low times to dissolve the cloudy zone, even a few seconds shock (<13 GPa) or medium to high shock (>13 GPa). at these low temperatures should be sufficient to disorder the tetrataenite in the high-Ni particles to form disordered in Seneca Township, no Ni variation at an X-ray spatial taenite since no diffusion is involved. However, we did not resolution of 2 nm is observed. The fine-scale microstructure check whether tetrataenite was still ordered. Impact heating in the SEM image, Fig. 5, may have been identified as the that erases the cloudy zone microstructure and allows cloudy zone by Yang et al. (1997a). diffusion over distances up to 100 nm will not affect the Ni We have made some relatively simple diffusion gradient that is measured with the electron microanalyzer calculations to put limits on time and temperature during since these X-ray data are obtained with micron-level shock reheating which causes the disappearance of the resolution. Therefore the reheating process considered for cloudy zone and produces the diffusion profiles across removal of the cloudy zone in the IVA irons will not affect the kamacite/taenite interface (Figs. 11–13). To calculate the Ni gradient and the central Ni content of the taenite potential time-temperature scenarios, we used the which are used to obtain metallographic cooling rates. interdiffusion coefficients D for Fe-Ni in taenite (γ−fcc) as a It is curious that two high-Ni slow cooled IVA irons, function of temperature (Goldstein et al. 1965). To calculate New Westville and Duel Hill, and the fastest cooled low-Ni diffusion distances, X, we used the approximation that the IVA iron, La Grange, all contain a cloudy zone microstructure distance over which diffusion takes place is roughly equal to even though they suffered moderate to high shock. In the five the square root of the product of the inter-diffusion other moderately to highly shocked IVA irons, features such coefficient D at a given temperature times the reheating as grain boundary formation and diffusion at kamacite-taenite time t: boundaries were observed along with the loss of the cloudy 354 J. I. Goldstein et al.

Fig. 11. Jamestown IVA iron. a) STEM image of the kamacite/taenite interface, 2 µm × 2 µm scan area, 256 pixels, ~7.7 nm/pixel. The kamacite/taenite interface is vertical in the image. b) Ni X-ray area scan of 2 µm × 2 µm, color scale gives the Ni content from 5 to 50 wt%. c) Ni X-ray scan of 2 × 2 µm area. The position of the Ni profiles is shown by the dashed rectangles. Two grain boundaries are noted by the heads of the arrows. d) Ni composition profiles across the kamacite/taenite interface 1) well away from newly formed grain boundaries and 2) between two grain boundaries. zone microstructure. Apparently, the shock process allowed Comparison with Previous High-Ni Particle Size for local heating of these meteorites but not for New Measurements Westville, Duel Hill, and La Grange. It appears that the shock event which removed the cloudy zone in some but not all Except for New Westville, the high-Ni particle sizes moderate to highly shocked IVA irons takes place after the determined by Yang et al. (1997a) using SEM techniques are IVA irons cool below 300 °C, probably in the event that 20 to 100% larger than the values we obtained using TEM produced the meter-sized bodies that find their way to Earth. measurements (Table 2). The difference between the SEM Since shock effects are highly dependent on the direction of and TEM measurements is larger for the IVA irons with the the shock wave and the orientation of the sample with respect smaller high-Ni particles. It is probable that the effect of the to the shock wave, it is quite possible that other samples of heavy etching employed in the specimen preparation process the same meteorite may have been subjected to a thermal by Yang et al. (1997a) to enhance contrast for SEM cycle which removed the cloudy zone structure. Another examination of the high-Ni particles is responsible for these possible explanation for heterogeneous shock heating in IVA differences in high-Ni particle size. In the etching process, the irons is that some irons like Gibeon, which contain troilite low-Ni phase in the cloudy zone is preferentially etched away nodules 2–20 mm in size, were shock melted. The post-shock during chemical treatment. The secondary electron signal thermal histories of taenite grains were probably dependent which is recorded is obtained not only from the high-Ni on the distance from the nearest shock-melted troilite nodule particles on the flat polished surface but also from high-Ni and its size. particles of the cloudy zone which are exposed when the low- Thermal histories of IVA iron meteorites from transmission electron microscopy 355

Fig. 12. Obernkirchen IVA. a) STEM image of the kamacite/taenite interface 1 µm × 1 µm scan area, 256 pixels, ~3.9 nm/pixel. The kamacite/ tetrataenite boundary is vertical in the image. b) Ni X-ray scan of 1 µm × 1 µm. Color scale gives the Ni content from 5 to 50 wt%. c) Ni X-ray scan of a 1 µm × 1 µm area. The position of the Ni profiles across the taenite is shown by the dashed rectangles, profiles 1 and 2. d) Ni composition profiles across the kamacite/taenite interface in two positions along the boundary.

Ni phase is etched away. The low kV electron beam employed Unfortunately, the original Yang et al. (1997a) data for low to view the high-Ni particles is particularly sensitive to Ni IVAs were probably inaccurate because of the effect of surface details and the size of the cloudy zone high-Ni the etching treatment in the preparation of the samples for particles may not be accurately represented. This effect is SEM observation with high-Ni particles <30 nm in size as greater as the size of the high-Ni particles decreases since the discussed above. As shown in Fig. 5, measurements of cloudy electron beam interacts with more of these high-Ni particles zone sizes are difficult to make using SEM in 3 dimensions. We did not make any thorough examination photomicrographs, for example some of the IVA irons that of the effect of etching although from Fig. 4 it is clear that the were investigated in this study. Another possible problem ability to measure small-sized high-Ni particles is very with the data of Yang et al. (1997a) is that the microstructure dependent on preparing a sample with just the right etching in Gibeon that we observed with the SEM (Fig. 5) was treatment. formed during cooling after the major shock event. This Wasson and Richardson (2001) and Wasson et al. (2006) shock event removed the original cloudy zone microstructure argue that the 6 high-Ni particle sizes measured by Yang which developed during Widmanstätten pattern formation. et al. (1997a) indicate constant cooling rates for the IVA We infer that our measurements of the high-Ni particle size irons and interpret this result as evidence that the diverse in IVA irons using X-ray images and bright and dark field cooling rates determined from kamacite growth models are images from the TEM are more accurate than the SEM data flawed. However, except for the New Westville since the high-Ni particles are observed directly without the measurement, the Yang et al. (1997) data show a direct need to enhance contrast by special specimen preparation variation of high-Ni particle size with Ni content and techniques and the microchemical effects of shock-induced decreasing metallographic cooling rate (Table 2). reheating can be directly observed. 356 J. I. Goldstein et al.

Cooling rates and the variation of cooling rate with high-Ni particle size cannot be derived directly because there is no effective model available for spinodal growth (Yang et al. 2007). However, the relative cooling rates of two meteorites, (CR1) and (CR2), can be estimated empirically from the ratio of their respective high-Ni particle sizes (PS2/PS1): ()⁄ ()⁄ n CR1 CR2 = PS2 PS1 (2) The parameter n obtained from Fig. 14 equals 2.4 ± 0.4. Given the threefold variation in IVA high-Ni particle size (2.9 ± 0.5; Table 2), Yang et al. (2007) estimate that cooling rates at 300–100 °C varied by a factor of ~15, decreasing with increasing bulk Ni. We have shown that the width of the outer taenite rim (OTR) or tetrataenite region, which separates the kamacite from the cloudy zone, increases in size with increasing high- Ni particle size (Fig. 10). The measurement of the width of the OTR is a new method that can be applied to obtain relative cooling rates of meteorites. Since high-Ni particle size increases with decreasing metallographic cooling rate, the tetrataenite width also correlates inversely with metallographic cooling rate. The increase in the width of the outer taenite rim and increasing cooling rate is consistent with the fact that the tetrataenite phase nucleates below 350 °C and grows by diffusion control (Yang et al. 1997b). The measurement of the width of the OTR can be made directly on a TEM thin section. However, only one kamacite/taenite interface is usually available for measurement from each FIB prepared TEM thin section. Ultimately, a more thorough investigation is needed using a number of FIB samples for each meteorite to establish the constancy of the width of the OTR. It should be possible, however, to measure the width of the tetrataenite with the SEM on a variety of kamacite/taenite interfaces as long as the orientation of the interface with the polished surface is known.

IVA Cooling Rates and Impact Histories

Fig. 13. Seneca Township. a) STEM image of the kamacite/taenite The results of this study of cloudy zone microstructures interface 1 µm × 1 µm scan area, 256 pixels, ~3.9 nm/pixel. The kamacite/taenite interface is vertical in the image. b) Ni X-ray scan of for group IVA combined with the cooling rates for these iron a 1 µm × 1 µm area. c) Ni composition profile across the kamacite/ meteorites based on kamacite growth models show that the taenite interface. The dashed box in (b) shows the position of the Ni IVA irons cooled with very diverse cooling rates that correlate profile. inversely with bulk Ni compositions. Cooling rates varied by a factor of over 50 from ~650 to ~400 °C and most likely by Sizes of High-Ni Particles and Tetrataenite Rims as a factor of 15 during cloudy zone formation below 350 °C— Cooling Rate Indicators ranges that are totally incompatible with cooling in a mantled core. The cooling rates of low-Ni IVA irons from ~650 to The inverse relationship between cooling rate and high- ~400 °C are also incompatible with conventional models for Ni particle size for mesosiderites, pallasites, IIIAB and IVA igneous differentiation of chondritic asteroids melted by 26Al. irons is shown in Fig. 14. This diagram resembles that shown To cool a mantled core at 6,600 °C/Myr would have required by Yang et al. (1997a), but the new data show less scatter for a body with a radius of only ~2.5 km, according to thermal the faster-cooled meteorites. High-Ni particle sizes decrease models of differentiated asteroids (Haack et al. 1990). by a factor of ~30 as the metallographic cooling rate increases However, only bodies 10–20 km or more in radius could have by 4 orders of magnitude (0.2 °C/Myr to 6,600 °C/Myr). been melted appreciably by 26Al (Hevey and Sanders 2006). Thermal histories of IVA iron meteorites from transmission electron microscopy 357

Applicability of the Cloudy Zone Method to Other Meteorites

The cloudy zone measurement scheme is a new independent measurement that can be used to obtain relative cooling rates at ~350 to 200 °C and to provide important information for understanding the origin of asteroidal bodies. For slow-cooled iron meteorites with Widmanstätten patterns or meteorites which lack a Widmanstätten pattern such as pallasites, H chondrites, mesosiderites, etc., the sizes of high-Ni particles in the cloudy zone greater than 50 nm can be measured using the SEM. For fast-cooled meteorites such as the IVA or IVB irons, and acapulcoites, which supposedly cooled at 103−105 °C/Myr (McCoy et al. 1996), specimen preparation becomes more critical and high-Ni particle sizes are best Fig. 14. Variation of high-Ni particle size versus metallographic measured using TEM techniques on thin sections of the high Ni cooling rate in mesosiderites, pallasites, IIIAB, and IVA irons. Data regions at the kamacite-taenite boundaries of the metal. sources: 1) high Ni particle size—mesosiderites and pallasites (Yang et al. 1997a), IIIAB irons—Cumpus and Spearman using SEM The nanometer-scale cloudy zone intergrowth is much measurements from (Yang et al. 1997a), Urachic (46 nm), Point of more susceptible to shock heating than the micrometer-scale Rocks (44 nm), Chupaderos (58 nm), Casas Grandes (42 nm), and Ni zoning across the taenite grain that develops during Bella Roca (53 nm) using SEM measurements, this study, IVA kamacite growth. The presence of a cloudy zone is therefore irons—this paper. 2) Metallographic cooling rates: mesosiderites excellent evidence that brief impact heating and rapid cooling (Hopfe and Goldstein 2001), pallasites (Yang et al. 1997a), IIIAB irons (Yang et al. 2006), and IVA irons (Yang et al. 2008). The have not compromised the inferred metallographic cooling rate. Thus for chondrites with well-preserved cloudy zones, relative cooling rates of two meteorites, CR1 and CR2, can be estimated from the ratio of their respective high-Ni particle sizes, PS1 metallographic cooling rates determined from taenite zoning = 2.4 and PS2 by the equation (CR1/CR2) (PS2/PS1) . profiles should be immune to errors due to shock heating and rapid cooling. However, the presence of a cloudy zone does The only plausible solution to the IVA cooling rate enigma is not preclude the possibility that an impact once mixed large that the IVA irons cooled in a metallic body with virtually no volumes of hot and cold material allowing a cloudy zone to silicate mantle so that it had a significant thermal gradient when reform during slow, post-impact cooling. kamacite nucleated at ~700 °C (Yang et al. 2007). Yang et al. (2008) infer that the IVA irons are the product Acknowledgments—The financial support from NASA of at least three different impact events. The first was the through grant NNG05GK84G (J. I. Goldstein, P. I.) and creation of the metallic body with virtually no silicate mantle. NNX08AE08G (K. Keil, P. I.) is acknowledged. Sandia is a Yang et al. (2007, 2008) suggest that a grazing collision multiprogram laboratory operated by Sandia Corporation, a between protoplanets created a 150 km radius molten metal Lockheed Martin Company, for the United Stated Department body from a differentiated protoplanet 103 km in diameter. of Energy’s National Nuclear Security Administration under This body would develop a steep thermal gradient on cooling contract DE-AC0494AL85000. We thank Mr. Michael Rye that could explain the cooling rates of the IVA irons. Inwards and Ms. Bonnie McKensie (Sandia) for assistance with the crystallization would account for the correlation between Ni FIB samples and SEM analysis, respectively. We also thank content and cooling rate. The second impact occurred after H. Haack, H. Watson, and N. Chabot for their helpful reviews. the 150 km body had cooled below −100 °C, perhaps only ~20 Myr later, and produced a fragment >30 km across or, Editorial Handling—Dr. Nancy Chabot more likely, a smaller rubble pile body perhaps a few kilometers in size. The third impact 400 Myr ago destroyed REFERENCES this smaller metallic body generating a swarm of meter-sized Buchwald V. F. 1975. Handbook of iron meteorites. Their history, metallic fragments that are still drifting into a major distribution, composition and structure, vol. 1. Berkeley: resonance at the inner edge of the asteroid belt and University of California Press. 262 p. bombarding the Earth. The medium-to-high shock event that Ganguly J. and Stimpfl M. 2000. Cation ordering in orthopyroxenes affected many IVA irons probably occurred during the third from two stony-iron meteorites: Implications for cooling rates impact. The second impact is required because demolition of and metal-silicate mixing. 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