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Geochimica et Cosmochimica Acta 74 (2010) 4471–4492 www.elsevier.com/locate/gca

Main-group pallasites: Thermal history, relationship to IIIAB irons, and origin

Jijin Yang a,*, Joseph I. Goldstein a,**, Edward R.D. Scott b

a Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA b Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA

Received 14 December 2009; accepted in revised form 15 April 2010; available online 27 April 2010

Abstract

We have determined metallographic cooling rates below 975 K for eight main group (MG) pallasites from Ni profiles across taenite lamellae of known crystallographic orientation in metallic regions with Widmansta¨tten patterns. Comparison with profiles generated by modeling kamacite growth gave cooling rates ranging from 2.5 to 18 K/Myr. Relative cooling rates were also inferred from the sizes of cloudy zone particles in 28 MG pallasites (86–170 nm) and tetrataenite bandwidths in 20 MG pallasites (1050–2170 nm), as these parameters are positively correlated with each other and negatively correlated with the metallographic cooling rates. These three different techniques show that MG pallasites cooled below 975 K at significantly diverse rates. Since samples from the core–mantle boundary should have indistinguishable cooling rates, MG pallasites could not have cooled at this location. Group IIIAB irons, which were previously thought to be core samples from the MG pallasite body, have faster cooling rates (50–350 K/Myr) and smaller cloudy zone particle sizes and tetrataenite bandwidths. This shows that IIIAB irons cooled faster than MG pallasites and could not plausibly be from the same body. The absence of related iron meteorites and achondrites and our thermal constraints suggest that MG pallasites cooled at diverse depths in a pallasitic body consisting of well-mixed olivine and metallic Fe–Ni. Such a body may have formed during an impact on a differentiated asteroid or protoplanet that mixed olivine mantle fragments with residual Ir-poor molten metal from the out- ermost part of a core that chemically resembled the IIIAB core and was 80% fractionally crystallized. Separation of the solid core and most of the associated mantle may have resulted from a grazing hit-and-run impact with a larger protoplanet or asteroid. Thermal calculations suggest that the radius of the pallasitic body was 400 km but the likely presence of a regolith would reduce this estimate considerably. Ó 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Wasson and Choi, 2003; Haack and McCoy, 2003; Greenwood et al., 2006). However, formation at a core– Pallasites are stony-iron meteorites containing 65 vol% mantle boundary is difficult to reconcile with the tendency olivine, 30 vol% metallic Fe–Ni, and 5 vol% of chro- for olivine crystals to separate gravitationally from molten mite, phosphate, and troilite (Buseck, 1977; Ulff-Møller metal even in asteroids, and the relatively large number of et al., 1998), and are widely thought to have formed by mix- pallasites: 85 cf., 1000 iron meteorites (Meteoritical Bulletin ing of molten Fe–Ni from the core with olivine mantle in Database see http://tin.er.usgs.gov/meteor/metbull.php). one or more differentiated asteroids (e.g., Scott, 1977a,b; In addition, concentrations of rare-earth elements in some phosphates and minor elements in olivine appear to require a near-surface origin for pallasites (Davis and Olsen, 1991; * Corresponding author. Tel.: +1 413 545 2513. Hsu, 2003). ** Corresponding author. Large slices of most pallasites show millimeter to centi- E-mail addresses: [email protected] (J. Yang), jig0@ecs. meter-sized fragments of olivine crystals embedded in metal umass.edu (J.I. Goldstein). with polycrystalline olivine masses up to 10–20 cm in size,

0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.04.016 4472 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492 which appear to be disintegrating to form the olivine frag- rates from peak Ni concentrations in taenite are not accu- ments (Scott, 1977b; Ulff-Møller et al., 1998; Wasson and rate because the latter are sensitive to impingement effects Choi, 2003). This suggests that pallasites formed after im- and to the exact placement of the electron beam close to pacts crushed olivine mantle and mixed in molten metal the kamacite/taenite interface. Nevertheless, pallasite metal from the core. About 15–20% of pallasites show rather dif- appears to have cooled slower than IIIAB irons as the peak ferent textures: rounded olivines with equilibrated olivine– Ni concentrations measured in taenite in MG pallasites ex- olivine and olivine–metal grain boundaries. Since pallasites ceed those reported in IIIAB irons. In addition, high-Ni with angular olivine fragments commonly show micro- particle sizes in cloudy taenite, which are inversely corre- rounding textures like those observed on a larger scale in lated with cooling rate, are larger in MG pallasites than pallasites with rounded olivines, the latter probably experi- in IIIAB irons (Yang et al., 1997; Yang and Goldstein, enced more extensive annealing in the presence of residual 2006). S-rich Fe–Ni–S liquid (Scott, 1977b; Saiki et al., 2003). The cooling rates of pallasites clearly need to be reinves- Oxygen isotopic compositions of the olivine and trace tigated as metallographic cooling rate models have im- element concentrations of the metallic Fe–Ni in pallasites proved significantly in the last 40 years (Yang and show that they can be divided into several groups that Goldstein, 2005, 2006). In addition, the cooling rates of probably come from separate parent bodies, some of which Buseck and Goldstein (1969) are not consistent with the may also have supplied certain iron meteorites. Pallasites cooling rate of 20–40 K/Myr determined for the Omolon are divided into a main group (MG), which accounts for pallasite from the Mn–Cr age of the olivine (Ito and Gan- 90–95% of pallasites and has affinities with the largest guly, 2006) and the rates of 20–200 K/yr and 0.001– group of irons—IIIAB, the Eagle Station trio, and several 100 K/yr determined by Miyamoto (1997) and Tomiyama ungrouped pallasites including Milton (Lovering et al., and Huss (2006), respectively, from concentration profiles 1957; Scott, 1977a; Davis, 1977; Clayton and Mayeda, of Ca, Cr and other elements in olivine. 1996; Boesenberg et al., 2000; Wasson and Choi, 2003; We have investigated the thermal history of pallasites Jones et al., 2003). Fractional crystallization models for and IIIAB irons to constrain the origin of pallasites and the chemical variations within group IIIAB irons show that to test the possible relationship between MG pallasites the metallic Fe–Ni portions of most MG pallasites, which and IIIAB irons. We determined the cooling rates of the are especially depleted in Ir, are a good compositional pallasite metal below 975 K (700 °C) by applying the match for the liquid in the IIIAB body after 80% of the metallographic method. We also obtained relative cooling metal crystallized (Scott, 1977c; Davis, 1977; Wasson and rates below 675 K (400 °C) by measuring the tetrataenite Choi, 2003). bandwidth and the size of the high-Ni particles in the clou- A link between MG pallasites and IIIAB irons, if con- dy zone of the taenite in pallasites and IIIAB irons (Yang firmed, would clearly favor a core–mantle origin for these et al., 2008; Goldstein et al., 2009a). We also developed a pallasites. However, the oxygen isotopic compositions of thermal model of a pallasitic asteroid to obtain the size of the MG pallasites and the IIIAB irons are not definitive evi- MG pallasite parent body. Finally, we offer a schematic im- dence for a common source as they lie in a crowded region pact model to help explain how the MG pallasites may have of isotopic space where unrelated meteorites may be super- formed. imposed, e.g., angrites and the ungrouped eucrite, Ibitira (Scott et al., 2009a). The siderophile element data also fail 2. METEORITE SAMPLES AND ANALYTICAL to provide definitive evidence for a common source as the TECHNIQUES compositional range of the metal in MG pallasites is much broader than in iron meteorite groups (excluding IAB and Twenty-nine MG pallasites, three Eagle Station group IIICD), and their Ga and Ge concentrations are not unlike pallasites, and two ungrouped pallasites were selected for those in many other meteorites. A genetic link between study. In one MG pallasite, Phillips County (pallasite), IIIAB irons and MG pallasites is not favored by their shock the metallic Fe–Ni was too corroded to obtain any data. and reheating indicators: over 80% of IIIAB irons have The remaining 33 pallasites are listed in Table 1 with their shock-hatched or recrystallized kamacite and many have sources and catalog numbers. In addition, we studied clou- extensively shock-melted troilites (Buchwald, 1975) but dy taenite and tetrataenite in six IIIAB iron meteorites that none of the MG pallasites show these features. In addition, are listed in Table 2; five of the six have metallographic the cosmic-ray exposure ages of the pallasites (6200 Myr) cooling rates that were determined by Yang and Goldstein are much less than those of IIIAB irons, 600–700 Myr (2006). (Eugster et al., 2006). Samples of pallasites and iron meteorites were prepared Cooling rates of pallasites offer powerful constraints on for reflected light microscopy and scanning electron micros- their formation and possible associations with iron meteor- copy (SEM) using standard metallographic procedures: ite groups. Buseck and Goldstein (1968, 1969) used mea- mounting, grinding, polishing, and etching with 2% nital. surements of taenite edge compositions to infer that the SEM studies were performed with a Zeiss EVO 50 at the pallasites cooled at uniformly slow rates of 0.5–2 K/Myr University of Massachusetts (UMass), a Hitachi S4300 below 975 K, possibly near the core. These rates are much FE-SEM at Lehigh University, and a Zeiss Ultra-55 FE- lower than the values of 50–350 K/Myr measured in IIIAB SEM at Harvard University. A low accelerating voltage irons arguing against a close link with MG pallasites (Yang (1.8–10 kV) was used to reveal the surface character of and Goldstein, 2006). However, determinations of cooling the various metal phases. The tetrataenite bandwidth and Thermal history and origin of main-group pallasites 4473

Table 1 Sizes of cloudy zone high-Ni particles and tetrataenite bandwidths in 33 pallasites. Meteorite Source Class Ni in metal Cloudy zone particle size No. of Tetrataenite bandwidth No. of name (wt%) (±2SEM) (nm) particles (±2SEM) (nm) tetrataenite bands Admire USNM MG 11.76 130 ± 8 30 — — 2620a Ahumada ASU MG 9.36 124 ± 6 91 — — 354.1.1 Albin ASU MG 10.84 112 ± 5 42 1560 ± 70 9 (pallasite) 314.1a Argonia ASU MG 12.36 127 ± 6 30 1910 ± 70 5 458.2 Brahin USNM MG 11.16 106 ± 9 8 — — 7018a Brenham AMNH MG 11.73 123 ± 3 203 1550 ± 80 33 989-3 Dora AMNH MG 11.55 136 ± 6 83 1760 ± 80 24 (pallasite) 4394-2 Esquel AMNH MG 9.31 157 ± 11 23 2090 ± 250 3 4761-2 Finmarken USNM MG 10.28 86 ± 2 238 1050 ± 100 14 392c Fukang SML MG* 9.5* 129 ± 10 15 — — Giroux USNM MG 10.67 120 ± 7 35 1760 ± 300 2 1574a Glorieta UMass MG 11.67 170 ± 6 92 2170 ± 90 15 Mountain Hambleton OpenU MG* 152 ± 6 28 — — Huckitta USNM MG 8.5 127 ± 7 25 1770 ± 360 3 2273 Imilac AMNH MG 8.28 143 ± 4 133 1980 ± 120 15 896-2 Krasnojarsk ASU MG 8.3 145 ± 5 76 1870 ± 60 3 182.2a Marjalahti AMNH MG 8.75 118 ± 3 201 1530 ± 100 15 4088-2 Molong AMNH MG 9.99 142 ± 4 145 2020 ± 210 9 2463-2 Mount USNM MG* 118 ± 6 20 — — Dyrring unk Mount USNM MG 10.91 127 ± 5 44 1850 ± 420 3 Vernon 300e Newport AMNH MG 11.5 109 ± 3 138 — — 4407-3 Otinapa AMNH MG 10.86 114 ± 9 16 1550 ± 220 3 4600-2 Rawlinna ASU MG 13.53 158 ± 12 11 2090 ± 200 3 001 677.1 Santa ASU MG* 10.1 153 ± 7 18 — — Rosalia 599.1.2 Seymchan USNM MG* 9.51* 122 ± 5 77 1390 ± 100 11 7200a Somervell USNM MG* 11.3* 133 ± 11 20 1800 ± 70 4 County 1408a South Bend USNM MG 9.22 115 ± 5 51 1320 ± 70 6 2616a Springwater AMNH MG 12.18 132 ± 3 121 1630 ± 170 11 2641-4 Cold Bay USNM EST 20.6 90 ± 8 11 — — 636 Eagle USNM EST 15.3 a —— — Station 275b (continued on next page) 4474 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

Table 1 (continued) Meteorite Source Class Ni in metal Cloudy zone particle size No. of Tetrataenite bandwidth No. of name (wt%) (±2SEM) (nm) particles (±2SEM) (nm) tetrataenite bands Itzawisis USNM EST 14.0 82 ± 3 12 800 ± 50 5 UMass Milton USNM Ungr* 15.0* Absent Absent — 7186a Vermillion NHM Ungr 7.49 a —— — 1966.m2

Dashed line: tetrataenite bandwidth unavailable as the orientation relative to the polished surface was not known. Class abbreviations: MG, Main Group; EST, Eagle Station Grouplet; Ungr, Ungrouped. Errors are ±2 standard error of the mean, ±2SEM. Source abbreviations: USNM, US National Museum of Natural History; AMNH, American Museum of Natural History; OpenU, Open University, UK; ASU, Arizona State University; UMass, University of Massachusetts; SML, Southwest Meteorite Laboratory; NHM, Natural History Museum, London. * Classification and Ni content from Wasson and Choi (2003) except for the following: Fukang, classification from Greenwood et al. (2006), Ni content from Lauretta et al. (2006); Hambleton (Johnson et al., 2006); Mount Dyrring (Mason, 1963; Buseck and Goldstein, 1969); Santa Rosalia (Scott, 1977a); Seymchan (van Niekerk et al., 2007); Somervell Co. (Davis, 1977), and Milton (Jones et al., 2003). The classification of Mount Dyrring is uncertain as its metal and its oxygen isotopic composition have not been analyzed. a No cloudy zone particle size as SEM image of poor quality.

Table 2 Sizes of cloudy zone high-Ni particles, and widths of tetrataenite bands in six IIIAB irons. Meteorite name Source Ni# Cooling rate* Particle size No. of high- Tetrataenite No. of (wt%) (K/Myr) (±2SEM) (nm) Ni particles bandwidth (±2SEM) tetrataenite (nm) bands Bella Roca FMNH 9.91a 162 53 ± 3 43 360 ± 10 2 611 Buenaventura UCLA LC 9.96b — 51 ± 3 20 340 ± 30 3 947J Casas Grandes USNM 7.72a 102 42 ± 3 20 ++ 3696 Chupaderos FMNH 9.96b 82 58 ± 4 11 450 ± 40 4 ME1045 Point of Rocks UNM 8.4c 145 44 ± 4 11 290 ± 10 4 (iron) I23.4 Uruachic UNM 8.33d 72 46 ± 4 14 ++ I63.1 Errors are ±2 standard error of the mean, ±2SEM. Source abbreviations: UNM, University of New Mexico; FMNH, Field Museum of Natural History; UCLA, University of California at Los Angeles; USNM, US National Museum of Natural History. * Cooling rates from Yang and Goldstein (2006). + Tetrataenite bandwidth omitted as the orientation relative to the polished surface was not known. # Sources of Ni concentrations: a, Buchwald (1975);b,Wasson (1999);c,Scott et al. (1973);d,Wasson et al. (1998). the size of the high-Ni particles in the cloudy zone next to Pallasite samples were polished for electron backscatter the tetrataenite band were measured from SEM images diffraction (EBSD) analysis using standard metallographic (e.g., Fig. 1d) using the methodology developed by Gold- polishing procedures followed by vibratory polishing with stein et al. (2009a). 0.06 lm SiO2 for 2–4 h. This polishing procedure produces Samples were prepared for electron probe microanalysis very smooth surfaces free of damage induced by specimen using the same procedures except that the samples were not preparation. Electron backscatter diffraction and orienta- etched. Local bulk compositions of the metal phases in pall- tion maps of pallasite metal were used to determine crystal- asites were determined by analyzing kamacite, taenite, and lographic orientations of kamacite and taenite with respect phosphide for Fe, Ni, Co, and P. Nickel gradients across to the polished surface of the specimen (see Yang et al., taenite lamellae were analyzed in 1–2 lm steps for cooling 2008). The EBSD orientation studies were made using a rate measurements. Analyses were made with a Cameca Zeiss EVO 50 SEM, at Amherst College, outfitted with an SX-50 electron probe micro-analyzer (EPMA) at the Uni- EBSD unit from HKL Technology, with Channel 5 soft- versity of Massachusetts (see Yang et al., 2008). ware and a beam voltage of 20 kV. Thermal history and origin of main-group pallasites 4475

Fig. 1. Optical and SEM images of a polished section of the Springwater main group pallasite. (a) Optical image showing metal regions, indicated by arrows. The olivine crystals are indicated by Ol. The metal contains swathing kamacite, KSW, which surrounds the olivine, Ol, crystals. In the central metal particle, Widmansta¨tten plates (Wid) of kamacite have formed. The central regions of the taenite are composed of plessite, a composite of finely divided kamacite and taenite. (b and c) SEM images of the large metallic region in the center of (a) showing a Widmansta¨tten pattern of oriented kamacite and taenite plates. (d) SEM image showing from left to right, cloudy zone (CZ) microstructure, tetrataenite band (Tt) and kamacite (K).

3. COOLING RATE MODELING meteorites, viz., from single crystal, P-bearing taenite. Additional minor elements that could influence nucleation In pallasites, the nature of the kamacite that forms de- and diffusion-controlled growth must have been absent, pends on the dimensions of the metal regions between the and major shock events that could modify kamacite nucle- olivine grains. In rare cases, for example in Brenham, Glo- ation and diffusion could not have taken place during rietta Mountain, and Seymchan (Fig. 2a and b), in regions cooling. where the olivine-bearing regions are many centimeters The chemical composition of the metal in most MG pall- apart, the oriented kamacite plates form a Widmansta¨tten asites is very close to that of Ni-rich IIIAB irons although a pattern like those in iron meteorites from a single crystal few are somewhat richer in Ni, Co, Cu, Ir, Ga, As, or Au of taenite. However, in most pallasites, olivines are gener- and a couple have slightly lower concentrations of Co or ally separated by metal regions only a few millimeters or Ga (Wasson and Choi, 2003). The metallic portions of main less in thickness and only the largest metal regions typically group pallasites also contain comparable P concentrations larger than 5 mm across contain oriented kamacite plates to those in IIIAB irons: 0.3–0.6 wt% P, some in solid solu- (Fig. 1a). We have measured metallographic cooling rates tion within the metal and some in schreibersite and rhab- only from these large metal areas where the Widmansta¨tten dites (Buseck, 1977; Davis, 1982; Ulff-Møller et al., 1998; pattern formed from a single crystal of taenite and taenite Lauretta et al., 2006). As in Ni-rich IIIAB irons, carbides grain boundaries were absent. and graphite appear to be absent in MG pallasites and sig- To obtain metallographic cooling rates from the Wid- nificant amounts of C have not been reported. We conclude mansta¨tten pattern kamacite plates, we need to know how that the kamacite nucleation temperature in MG pallasites the kamacite plates nucleated, the nucleation temperature, was controlled by the bulk Ni and P contents, as in IIIAB the effect of impingement by adjacent kamacite plates, the irons, and the Ni diffusion rates in the metal are dependent Fe–Ni and Fe–Ni–P phase diagrams, and the interdiffusion on the Ni, Fe, and P content and are not measurably influ- coefficients which control the growth of the Widmansta¨tten enced by any metal impurity. pattern (Yang et al., 2008). In order to apply the metallo- Olivine crystals in pallasites are enclosed by swathing graphic cooling rate model developed for iron meteorite kamacite (Figs. 1a and 2b). For metal less than 1cm groups IIIAB, IVA, and IVB (Yang and Goldstein, 2006; across, the swathing kamacite commonly approaches a mil- Yang et al., 2010), we need to be sure that the Widmansta¨t- limeter in width and wider than the adjacent oriented ten pattern in pallasites formed in the same way as in iron kamacite plates (Fig. 1a). This observation implies that 4476 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

Fig. 2. (a) Polished and etched slice of the Seymchan main group pallasite showing three different types of material: 5–10 cm wide olivine-free metal regions with Widmansta¨tten patterns, 5–15 cm long angular regions with Brenham-like pallasitic material consisting of rounded olivine aggregates with interstitial metallic Fe–Ni (left and center), and a 10 cm wide zone on the right side with seemingly isolated, mostly angular olivines in metal. The isolated olivines may have been detached from Brenham-like angular regions or from large polycrystalline olivine masses (see Meteoritical Bulletin Database links at http://tin.er.usgs.gov/meteor/metbull.php). Longest dimension of the slice is 48 cm. (b) Oblique view of the region marked in (a) under different lighting conditions showing oriented kamacite plates, swathing kamacite (S) around olivine, and small dark inclusions of the phosphide, schreibersite (P) in metal. Photographs taken by R.A. Langheinrich.

on cooling, kamacite nucleates initially on taenite–olivine this case the nucleation temperature of the oriented kama- boundaries because of the lowering of surface energy be- cite plates (or plessite in small taenite regions) is lowered. tween taenite and olivine. Rapid diffusion of Fe and Ni Consequently, the nucleation temperature for the Wid- along the metal–olivine boundary as envisioned for chon- mansta¨tten kamacite plates can vary throughout the palla- drite metal (Reisener and Goldstein, 2003) may also play site according to the proximity to the olivine crystals. For a role in forming the swathing kamacite. At slightly lower large metal regions, greater than 1 cm across, the metal temperatures, kamacite nucleates homogeneously within regions are so large that the Ni build up in the taenite the remaining taenite to form either a Widmansta¨tten pat- and consequent depression of the nucleation temperature tern of oriented kamacite plates or, if the remaining taenite are negligible. In these regions, the swathing kamacite is is too small, plessite at lower temperatures (see Fig. 1). The comparable in width to the oriented kamacite plates prior growth of swathing kamacite in the metal regions that (Fig. 2b). are less than 1 cm across raises the Ni concentration in the We determined the local bulk Ni and P concentrations residual taenite above the bulk value of the Fe–Ni metal. In around oriented kamacite plates by electron microprobe Thermal history and origin of main-group pallasites 4477 analysis for Ni and P point by point from the middle of the compositionally homogeneous when the kamacite plates kamacite (a) band at one side of a taenite band (c) to the nucleate. We therefore used the Ni profile matching middle of the kamacite on the other side of taenite, follow- method (Goldstein and Ogilvie, 1965) taking into account ing the method of Rasmussen (1981). As we shall see in Sec- the local Ni content of the parent taenite. In this method, tion 4.1, the local bulk Ni concentrations increase modestly a unique cooling rate is obtained for each taenite plate up to 3 wt% Ni from the bulk Ni concentrations in the when the measured Ni concentration profile across the metallic Fe–Ni where kamacite plates form. In most palla- plate matches the calculated profile. Nickel profiles were sites, the bulk Ni concentration in metallic Fe–Ni is 8– calculated for the pallasites using the same set of diffu- 14 wt%; Cold Bay and a few others have higher bulk Ni sion coefficients and phase diagrams that were used in concentrations, possibly up to 20% Ni (Wasson and Choi, our studies of IVA and IVB irons (Yang et al., 2010). 2003; Table 1). Thus local bulk Ni concentrations in the Our study of IVA irons showed that results from the pallasite metal are almost all below 17 wt%. The effect of Ni profile matching method are entirely consistent with a higher local bulk Ni content, although relatively minor, those of the Wood method when taenite is chemically needs to be considered in measuring the metallographic homogeneous at the time that the Widmansta¨tten kama- cooling rates. cite plates nucleate. The local bulk P concentrations that we measured in MG pallasite metal, typically 0.04–0.08 wt%, are much 4. RESULTS smaller than the bulk P in metal since the measured local bulk P did not include the P content in any nearby phos- 4.1. Metallographic cooling rates of pallasites phides. The P is at the solubility limit in both kamacite and taenite as temperature decreases, and the metal compo- Eight samples of pallasites, which are all MG members, sition is in the three phase field (a + c + phosphide, Ph) in contain metal regions with Widmansta¨tten pattern kama- the Fe–Ni–P phase diagram. Based on the solubility of P in cite suitable for cooling rate analysis. The remaining 20 taenite and the measured local bulk Ni (<16 wt%), the local MG pallasite samples either did not have an observable bulk P at the time of the Widmansta¨tten kamacite nucle- Widmansta¨tten pattern in our sample or we could not ation is above 0.1 wt% (Yang and Goldstein, 2005). From determine the crystallographic orientation of the taenite the Fe–Ni–P phase diagram and the local bulk Ni and P bands. We measured the ‘‘M” shaped Ni profile across 7– values, we infer that phosphides form before the Wid- 10 taenite bands in each of these eight pallasites (Table mansta¨tten kamacite plates nucleate and their formation 3). Since accurate cooling rate determinations require that is controlled by mechanism II: c ? c +Ph?a + c +Ph the Ni profiles are measured perpendicular to the plane of (Yang and Goldstein, 2005). Kamacite nucleation tempera- the kamacite–taenite boundary, the measured taenite dis- tures range between 975 and 875 K (700 and 600 °C). tance on the polished sample surface was corrected for ori- Swathing kamacite also nucleated after the phosphides entation effects by determining the crystallographic formed. Note that in chondrites, swathing kamacite is not orientation of the kamacite and taenite plates (Yang and observed at metal/olivine interfaces since P is not present Goldstein, 2006). in the metal (Reisener and Goldstein, 2003). Fig. 3a shows a backscattered electron image of a micro- Aside from the effect of swathing kamacite in raising the probe trace in the Giroux pallasite across one of the taenite local bulk Ni in nearby metal that was described above, the bands. In Fig. 3a and some other cases, the trace of the Ni olivine grains do not appear to have affected the growth of profile was not oriented perpendicular to the taenite band the Widmansta¨tten pattern. For example, photos of olivine- in the plane of the polished section and an additional cor- poor Brenham and Seymchan (Fig. 2a and b) samples show rection was made to the distance measurements to derive that the Widmansta¨tten pattern has uniform kamacite plate the Ni profile perpendicular to the kamacite plate. Fig. 3b thickness right up to the swathing kamacite around the shows the measured “M” shaped Ni profile across the tae- olivine. We see no evidence for localized strain in the metal nite band in Fig. 3a after correction for the orientation of near the metal–olivine boundary that might locally enhance the plates relative to the polished surface and the orienta- diffusion rates—a possibility raised by one of the reviewers tion of the Ni profile to the taenite band in the polished of this paper. There is also no evidence for shock that might section. have affected the nucleation temperature of the Wid- In order to obtain the cooling rate for each pallasite, we mansta¨tten pattern and the Ni diffusion rates. Olivine crys- first plotted the orientation-corrected Ni profile for each tals show sharp optical extinction, aside from kink bands taenite band. We then measured the local bulk Ni content caused by annealing at high temperature after deformation of the kamacite–taenite bands. Finally, Ni profiles were cal- (Klosterman and Buseck, 1973), and olivine and metal have culated for a set of varying cooling rates using the measured low dislocation densities (Matsui et al., 1980; Desrousseaux local bulk Ni content until a match was obtained for the et al., 1997). Kamacite in pallasites lacks the shock-hatched measured profile (e.g., Fig. 3b). Fig. 4 shows the measured structure that is visible on etching in many IIIAB irons that and the calculated matching Ni profiles for 10 taenite bands were shocked above 13 GPa (Buchwald, 1975). in the Giroux pallasite. The local bulk Ni content and The Wood method, which is usually employed for the nucleation temperature for each band is also shown in measurement of the metallographic cooling rates (Wood, Fig. 4. For the 10 taenite bands analyzed in Giroux, the 1964), cannot be applied for cooling rate measurements cooling rate of the matching profile ranges from 5 to in the pallasites because it assumes that the taenite is 10 K/Myr. The mean cooling rate for Giroux and the other 4478 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

Table 3 Metallographic cooling rates for eight main group pallasites obtained by matching Ni profiles across 7–10 taenite bands in each pallasite. Meteorite name Source Ni in metal Wid. pattern cooling rate (±2SEM) 2r uncertainty No. of taenite (wt%) (K/Myr) factor bands Brenham AMNH 989-3 11.73 6.2 ± 0.9 1.4 8 Dora (pallasite) AMNH 4394-2 11.55 4.7 ± 1.3 1.9 7 Finmarken ASU 47 10.28 18.2 ± 8.7 2.4 7 Giroux USNM 1574a 10.67 6.8 ± 1.0 1.5 10 Glorieta UMass 11.67 2.5 ± 0.3 1.4 9 Mountain Seymchan USNM 7200a 9.51 7.1 ± 1.2 1.5 8 South Bend USNM 2616a 9.22 8.9 ± 1.2 1.4 7 Springwater AMNH 2641-4 12.18 5.1 ± 0.7 1.4 7 Errors are ±2 standard error of the mean, ±2SEM. Source abbreviations are given in Table 1 footnotes.

Fig. 3. (a) Backscattered electron image of an oriented taenite lamellae in a Widmansta¨tten pattern in the Giroux main group pallasite showing the position of the electron microprobe trace. The two arrows indicate the starting and ending points of the trace. (b) Ni composition profile (open circles) along the trace with distances measured normal to the plane of the kamacite–taenite interface. Three calculated cooling rate curves are plotted for the local bulk content of 12.2 wt% Ni and 0.3 wt% P, and a kamacite nucleation temperature of 905 K. The data closely match the Ni profile for a cooling rate of 6 K/Myr. pallasites was calculated using the logarithm of the mea- of the 10 cooling rates for Giroux is 0.83 equivalent to a sured cooling rates for all the taenite bands. For example, cooling rate of 6.8 K/Myr with a two standard deviation the logarithmic average, which equals the geometric mean, (2r) value of ±0.17, which is equivalent to an uncertainty Thermal history and origin of main-group pallasites 4479

Fig. 4. Ni concentration vs. distance profiles across 10 taenite bands in the Giroux main group pallasite measured at intervals of 1–2 lm together with matching calculated cooling rate curves (solid curves). The local bulk Ni concentration and kamacite nucleation temperature for each band are marked on the side of each profile. For the 10 taenite bands, the local bulk Ni concentration varies from 11.0–13.8 wt% Ni and the kamacite nucleation temperature varies from 885 to 930 K. The mean value of the matching cooling rate curves is 6.8 ± 1.0 K/Myr (±2 standard error of the mean). 4480 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492 factor in the cooling rate of 1.5 (i.e., 4.5–10.2 K/Myr). We to the polished surface, the thickness of the tetrataenite also list 2 the standard error of the arithmetic mean band normal to the plane of the kamacite–taenite interface (SEM) of the cooling rate measurements, which for Giroux was inferred from the crystallographic orientations of was 1.0 K/Myr (See Table 3). kamacite and taenite which were determined by EBSD. Fig. 5 shows the mean cooling rates vs. the bulk Ni con- The mean bandwidth and 2 the standard error of the tent for the 8 MG pallasites and error bars showing twice mean (2SEM) were obtained for each meteorite (Tables 1 the standard error of the mean (2 SEM in Table 3). These and 2). Fig. 6a shows a plot of tetrataenite bandwidth vs. cooling rates vary from 2.5 to 18 K/Myr and are much low- bulk Ni. The tetrataenite bandwidth varies from 1050 to er than the cooling rates for the IIIAB irons, which are 2170 nm for 20 MG pallasites, and is 800 nm for the Itzawi- 50–350 K/Myr (Yang and Goldstein, 2006). The negative sis pallasite, a member of Eagle Station group. The tetrata- correlation between bulk Ni and the logarithmic cooling enite bandwidth of the four IIIAB irons is much smaller rate for the 8 MG pallasites is significant at the 90% level. and ranges from 290 to 450 nm. There is no correlation be- However, cloudy zone particle size and tetrataenite band tween tetrataenite bandwidth and the bulk Ni of the metal width data for 28 and 20 MG pallasites, respectively (see in the MG pallasites. below), show that these parameters are uncorrelated. The sizes of between 8 and 238 high-Ni particles in the cloudy zones of several taenite bands were measured for 4.2. Tetrataenite bandwidth and cloudy zone particle size 28 MG pallasites, two Eagle Station group pallasites and six IIIAB irons. The average particle size and the corre- The tetrataenite and cloudy zone regions of nearly all sponding 2 the standard error of the mean (2SEM) are pallasites are well developed and preserved and there is listed in Tables 1 and 2. The mean sizes of the cloudy zone no evidence for shock heating. However, the structure in particles range from 86 to 170 nm for 28 MG pallasites, and Phillips County (MG pallasite) was corroded, and the from 82 to 90 nm in two Eagle Station group pallasites SEM images in both Eagle Station (Eagle Station group) (Fig. 6b). The high-Ni particle sizes in the cloudy zone of and Vermillion (ungrouped pallasite) were not of sufficient six IIIAB irons are much smaller with mean sizes ranging quality. Milton (an ungrouped pallasite) is unique as it con- from 42 to 58 nm. There is no correlation between the tains no cloudy zone structure as shown by SEM and by high-Ni particle size and the bulk Ni of the metal regions TEM techniques and shows no evidence of reheating which in the MG pallasites. In fact, tetrataenite width and cloudy might remove this low temperature microstructure. It ap- zone particle size in pallasites are closely correlated (Fig. 7), pears that Milton cooled rapidly. as observed for the IVA irons (Goldstein et al., 2009a). The width of 2–33 tetrataenite bands was measured in the SEM images of each of the 21 pallasites and four IIIAB 4.3. Accuracy and precision of cooling rates irons (Tables 1 and 2). For the remainder, either we could not determine the crystallographic orientation of the taenite Errors in the measurement of the Ni profile and in the region or the SEM image quality was insufficient. Since metallographic cooling rate model contribute to the total er- most of the taenite regions were not oriented perpendicular ror of the metallographic cooling rates (Yang and Goldstein,

Fig. 5. Metallographic cooling rates for eight main group pallasites (MG Pall.) and 14 IIIAB irons (Yang and Goldstein, 2006) plotted as a function of bulk Ni concentration in the metal. Error bars are 2 the standard error of the mean. Since nearly all MG pallasites cooled 5–100 times slower than the IIIAB irons, they could not have formed around a IIIAB core. In addition, the spread in cooling rates of at least a factor of four shows that the MG pallasites did not cool at a core–mantle boundary. The abrupt increase in cooling rate at 8.5% Ni in group IIIAB reflects a change in the nucleation mechanism and a corresponding increase in nucleation temperature for kamacite formation. The parent asteroidal body most likely cooled more rapidly at higher temperatures (Yang and Goldstein, 2006). Thermal history and origin of main-group pallasites 4481

Fig. 6. (a) Tetrataenite bandwidth and (b) cloudy zone particle size vs. bulk Ni concentration in the metallic Fe–Ni of main group pallasites (MG Pall.), Eagle Station group pallasites (ES Pall.), and IIIAB irons. Error bars are 2 the standard error of the mean.

Fig. 7. Tetrataenite bandwidth vs. cloudy zone particle size in 20 main group pallasites (MG Pall.) and four IIIAB irons. Error bars are 2 the standard error of the mean. 4482 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

2006; Yang et al., 2008). The uncertainty factors caused by The Mn–Cr age of the olivine in the Omolon pallasite measurements of Ni concentration, taenite distance and implies that it cooled at 20–40 K/Myr at 1000 °C, which kamacite–taenite orientation are 1.1, 1.2, and 1.2, respec- is probably compatible with a metallographic cooling rate tively (Yang et al., 2008) and the combined uncertainty is of 10–17 K/Myr (Lugmair and Shukolyukov, 1998; Ito at most a factor of 1.3. This uncertainty value is less than and Ganguly, 2006). We did not measure the cooling rate the mean 2r uncertainty factors for the eight pallasites (Ta- of Omolon but since it is a MG pallasite, there does not ap- ble 3). pear to be a major conflict between the Mn–Cr age and the The cooling rate uncertainty caused by model parame- metallographic cooling rate. ters such as kamacite nucleation mechanism and tempera- Cooling rates for a few pallasites of 2–200 K/yr were ture, phase diagram and diffusion coefficients has been derived from element zoning profiles (e.g., Cr, Ca, Mn, discussed in detail for iron meteorites (Yang et al., 2008). and Fe) in olivine by Miyamoto (1997) and Miyamoto In this study, we used the same set of diffusion coefficients et al. (2004). These rates, which are 105–7 times faster than and phase diagrams that were used for the iron meteorite the results of this study, were derived using an implausible studies. For pallasites, we consider two possible effects on model. The olivine crystals were assumed to be initially the measured cooling rate due to the uncertainty in the homogeneous and equilibrated at 1100 °C with the ob- kamacite nucleation temperature. One effect is the uncer- served core compositions. Theoretical zoning profiles were tainty of the kamacite nucleation temperature for a fixed derived by assuming that the olivine crystals were trans- Ni and P. For the local bulk Ni in the metal of 8.5– ferred to a totally different environment in which the sur- 15 wt%, the nucleation temperature for the Widmansta¨tten face compositions of the crystals were somehow fixed at pattern is about 975–875 K (700–600 °C). For a ±10 K their measured rim composition while the crystals cooled. uncertainty in the nucleation temperature for a fixed local However, chemical zoning profiles in pallasitic olivines, like bulk Ni, the cooling rate is essentially the same. The second those in kamacite and taenite, reflect kinetic and thermody- effect is the difference in nucleation temperature of Wid- namic factors that change continually on cooling. To deter- mansta¨tten kamacite due to differences in local bulk Ni. mine reliable cooling rates from a zoned olivine crystal, one Our measured local bulk Ni contents are 0–3 wt% higher needs to know what phases were reacting with olivine, their than the overall bulk Ni for a given pallasite. The kamacite equilibrium phase compositions as a function of tempera- nucleation temperature for the Widmansta¨tten pattern ture, and the various diffusion rates in olivine, the reacting may therefore vary across each pallasite. For example, phases, and the medium that separated them. Although kamacite will nucleate at 913 K (640 °C) for a local some improvements have been made to the olivine-based bulk Ni of 12 wt% while kamacite will nucleate at 873 K cooling rate model, the olivine cooling rates are still 1–3 or- (600 °C) for another taenite region with a local bulk Ni ders of magnitude faster than the metallographic ones of 15 wt%. After applying the Ni profile matching method (Tomiyama and Huss, 2006). to both taenite bands, we observe that the difference in the measured cooling rates for the two bands is negligible, 5. DISCUSSION despite the difference in local bulk Ni and nucleation temperature. 5.1. Cooling rates of pallasites The values for 2 the standard error of the mean for each pallasite (2SEM in Table 3), expressed as a percentage Although we rely solely on the taenite Ni compositional of the mean cooling rate, vary between 12% and 50%. These data for quantitative cooling rates of pallasites (Fig. 5, Ta- values represent the precision of our cooling rates. The 2r ble 3), the sizes of cloudy zone particles and tetrataenite uncertainty factors in the cooling rates listed in Table 3 bandwidths provide excellent quantitative constraints on are 1.4–2.4 with a mean value of 1.6. The uncertainty fac- relative cooling rates as both parameters are inversely cor- tors, which are much larger than the precision, except for related with the cooling rates derived from oriented taenite the Finmarken data, can be used to estimate the total error lamellae. Fig. 8a and b shows that cloudy zone particle sizes in the cooling rates. and the tetrataenite bands in eight MG pallasites increase systematically with decreasing metallographic cooling rate 4.4. Comparison with published pallasite cooling rates as observed in other meteorite groups (Goldstein et al., 2009a). Since the pallasites with the lowest and highest Our cooling rates for pallasites of 2.5–18 K/Myr are cooling rates in Fig. 5 (Glorieta Mountain and Finmarken) much higher by a factor of 5–6 than the previously mea- lie at opposite ends of the MG range in Fig. 7, we infer that sured cooling rates (0.5–2 K/Myr) based on taenite edge all 28 analyzed MG pallasites cooled through 975–775 K compositions (Buseck and Goldstein, 1968, 1969). The (700–500 °C) at between 2.5 and 18 K/Myr. metallographic cooling rates are now more consistent with One might argue from Fig. 5 that the MG pallasites the lower limit of 5 K/Myr that Pellas et al. (1983) in- actually lie on an extrapolation of the IIIAB trend to ferred from Pu fission tracks in Marjalahti phosphate. higher Ni concentrations. However, the data for the The change in metallographic cooling rate is not surpris- two IIIAB irons with 10 wt% Ni show that the IIIAB ing given the significant improvements, especially in trend levels off with increasing bulk Ni. In addition, understanding the effect of P on Widmansta¨tten growth our measurements of the sizes of cloudy taenite particles and the improvement in the relevant diffusion coefficients and tetrataenite widths show that the cooling rates of and phase diagrams. MG pallasites are not correlated with bulk Ni (Fig. 6a Thermal history and origin of main-group pallasites 4483

Fig. 8. (a) Cloudy zone particle size and (b) tetrataenite bandwidth vs. metallographic cooling rate for eight main group pallasites. The solid line is the best fit to the main group pallasite data; the dashed line is the best fit line for data from mesosiderites, pallasites, IIIAB, IVA, and IVB irons; see Appendix A. The excellent correlation and the close agreement between the lines suggests that cooling rates of pallasites and other meteorites can be reliably inferred from cloudy taenite particle sizes and tetrataenite bandwidths. and b). There is therefore no justification for extrapolat- shown by analytical transmission electron microscopy of ing the trend in IIIAB irons with <8.5% Ni into the MG several kamacite/taenite interfaces or any evidence for pallasites. shock reheating shows that it cooled faster than The inverse correlation between cloudy taenite particle 5000 K/Myr, the cooling rate of the irons with the finest size and metallographic cooling rate, Fig. 8a, validates observed cloudy taenite intergrowths (Goldstein et al., our conclusion that the range of cooling rates we deter- 2009a). Theoretical constraints on the formation mecha- mined for the MG pallasites of 2.5–18 K/Myr is not an nism of cloudy taenite and tetrataenite bands and the artifact resulting from errors in the phase diagram or dif- relation between cloudy taenite particle size, tetrataenite fusion coefficients. Fig. 8a also shows that the correlation bandwidth and metallographic cooling rates are given in trend defined by the MG pallasites is within error of that the Appendix A. defined by all the meteorites including the mesosiderites, From our metallographic cooling rates and the con- which cooled slower than the MG pallasites, and the straints from cloudy taenite particle size and tetrataenite IIIAB, IVA, and IVB iron meteorites, which cooled faster bandwidth we draw the following conclusions. (Goldstein et al., 2009a). Thus we can infer that the two Eagle Station group pallasites cooled through 975–775 K (1) MG pallasites cooled at 2.5–18 K/Myr during kama- (700–500 °C) at a cooling rate of 13–16 K/Myr. The ab- cite growth below 975 K, more slowly than the IIIAB sence of any cloudy taenite in the Milton pallasite as irons, which cooled at 50–350 K/Myr. 4484 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

(2) The range of cooling rates among MG pallasites is they called MG-am pallasites. Each one has anomalous not an artifact of errors in the phase diagram or dif- concentrations of between one and three of the elements, fusion data or differences in kamacite nucleation tem- Au, Co, Ga, Ge, and Ir. However, the concentrations of perature as the cooling rates are correlated with two the other siderophiles appear quite typical for main-group other parameters: cloudy taenite particle size and tet- pallasites. Of the five MG-am pallasites that we analyzed, rataenite bandwidth. Argonia, Brenham, Glorieta Mountain, Huckitta, and Kra- (3) Cooling rates among MG pallasites are not corre- snojarsk, only Glorieta Mountain has an extreme cooling lated with bulk Ni, as they are in IIIAB irons (also rate (Fig. 9a). However, its cooling rate was very similar in group IVA and IVB). to that of the MG pallasite, Esquel, so that excluding Glo- (4) The range of cooling rates observed in MG pallasites rieta Mountain would not affect our conclusions about the shows that they were not located at the core–mantle origin of the MG pallasites in any way. boundary of a single parent body when kamacite, tet- Wasson and Choi (2003) also identified on their inter- rataenite, and cloudy taenite formed as they cooled element diagrams three “unusual” MG pallasites, which through the temperature range 975–575 K (700– have moderately high Ir concentrations. Our tetrataenite 300 °C). and cloudy taenite data in Fig. 9a show that the two we (5) The Eagle Station grouplet cooled at a rate compara- analyzed, Finmarken and Marjalahti, are both fast cooled ble to that of the fast cooled MG pallasites, viz. MG pallasites: Finmarken clearly cooled much faster than 15 K/Myr. any other MG pallasite in our set. However, tetrataenite (6) The ungrouped pallasite, Milton, cooled at >5000 K/ and cloudy taenite dimensions are not correlated with bulk Myr. Ir concentrations, and excluding Finmarken from the MG would not affect our conclusions. Like Wasson and Choi For the remainder of this paper we focus solely on the (2003), we infer that the anomalous compositional features origin of the MG pallasites. in these nine pallasites reflect local processes within the MG parent body such as solid–liquid metal mixing. 5.2. Source(s) of main group pallasites A single parent body for all MG pallasites is supported by the high precision oxygen isotopic analyses of olivine by Could the diversity of cooling rates of the MG pallasites Greenwood et al. (2006, 2008) who found uniform oxygen reflect their formation at the core–mantle boundaries of isotopic compositions in a set of 12 MG pallasites that in- two or more bodies having differing sizes and compositions? cluded two MG-am, one MG-as, and two high Ir MG Since MG pallasites are much more diverse than magmatic members. Ziegler and Young (2007) questioned whether groups of iron meteorites in their concentrations of minor MG pallasites could come from one source as they found and trace elements in metal and other minerals and abun- a somewhat bimodal distribution of D17O values in seven dances of minerals like phosphate and troilite, the possibil- MG pallasites. However, their total range of D17O values ity that they come from multiple sources needs to be for MG pallasites is much wider than that of Greenwood readdressed using our cooling rate data. Wasson and Choi et al. (2006) despite a smaller number of analyzed meteor- (2003) listed several MG pallasites with unusual properties, ites. Ziegler and Young (2007) also found that individual which we have identified on a plot of tetrataenite band- pallasites showed the same wide range of D17O values, con- widths vs. cloudy taenite particle sizes (Fig. 9a). They con- trary to Greenwood et al. (2006, 2008). We conclude that cluded that these pallasites formed in the same location as the oxygen isotopic compositions of MG pallasites are the other MG pallasites and that the mineralogical and probably consistent with a single body. chemical heterogeneity of this group reflects the complexity Since the shape of olivine grains in pallasites probably of processes operating within a single body, rather than der- reflects the degree of annealing of olivine–metal mixtures ivation from multiple bodies (Wasson and Choi, 2003). when they contained residual S-rich Fe–Ni–S liquid (Scott, Most MG pallasites have 11–13.5 mol.% fayalite in their 1977b; Saiki et al., 2003), one might expect that the main- olivine but four are more Fe-rich with 16–19 mol.% faya- group pallasites with rounded olivines cooled in separate lite. These pallasites with anomalous silicate compositions locations from those with angular olivines. However, the were called MG-as pallasites by Wasson and Choi (2003), five MG pallasite samples that contain rounded olivine, who suggested that oxygen-rich magmatic gas in voids oxi- Rawlinna, Krasnojarsk, Springwater, Brenham, and Seym- dized Fe and P causing a subsequent increase in Fe concen- chan, have cooling rates that are spread throughout the tration in their olivine. This explanation might explain the MG range (Fig. 9b). In addition, Seymchan contains re- presence in Springwater, the only multi-kg MG-as pallasite, gions with angular olivine fragments and regions with of multi-cm wide regions where phosphates, mostly farring- rounded olivines (Fig. 2a). It therefore seems likely that tonite Mg3(PO4)2, replaced metal between the olivine crys- the rounded olivine texture was formed by annealing before tals (Buseck, 1977; Davis and Olsen, 1991). Two of the four olivine fragmentation and slow cooling. MG-as pallasites, Springwater and Rawlinna, are plotted in Although MG pallasites have diverse properties, there is Fig. 8a, and do not appear to have extreme cooling rates. In no evidence that this requires that they formed in multiple addition we found that fayalite concentration in olivine and parent bodies. Their diverse cooling rates are best inter- cloudy taenite particle size are uncorrelated. preted as evidence that they cooled at different depths in Wasson and Choi (2003) identified six main-group pall- one body rather than at the core–mantle boundaries of sev- asites with somewhat deviant metal compositions, which eral bodies. Thermal history and origin of main-group pallasites 4485

Fig. 9. Tetrataenite bandwidth vs cloudy zone particle size for 20 main group pallasites. (a) Symbols show those classified by Wasson and Choi (2003) as normal main group pallasites (MG Pall.), those with anomalous metal compositions (MG Pall.-AM), those with anomalous silicate compositions (MG Pall.-AS) and those with “moderately high Ir” (MG Pall.-Hi Ir). (b) Symbols show pallasites with rounded and angular olivine crystals. The metallographic cooling rate, which is correlated with cloudy taenite particle size and tetrataenite bandwidth, is not related to olivine shape or the composition of the metal or silicate phases. Error bars are 2 the standard error of the mean.

5.3. Possible relationships between main group pallasites and the impact that mixed olivine and residual molten metal to other meteorites make the MG pallasites also removed the IIIAB solid core so that it cooled with little mantle? 5.3.1. IIIAB irons If the IIIAB core had solidified outwards, then a com- Three different techniques show that the IIIAB irons all mon ancestral body for the IIIAB irons and MG pallasites cooled significantly faster than the MG pallasites (Figs. 5, would be feasible. But the cooling rate data of Yang and 7, 8, A1, and A2), and that the latter therefore did not cool Goldstein (2006) (see Fig. 5) preclude outwards solidifica- at the core–mantle boundary of the parent body of the tion as the low-Ni irons, which formed first, cooled faster IIIAB irons. Given the oxygen isotope and siderophile than the high-Ni irons. Our measurements of the sizes of abundance data supporting a common source, we also need the high-Ni cloudy taenite particles in IIIAB irons and tet- to ask whether the MG pallasites and the IIIAB irons could rataenite bandwidths confirm this inverse correlation be- be derived from a common precursor body. Since the IIIAB tween bulk Ni and cooling rate (Fig. 6a and b). Thus irons cooled in a core surrounded by only a very thin layer three techniques suggest that IIIAB solidified inwards so of silicate under a few kilometers in thickness (Yang and that the residual Ir-poor liquid was near the center of the Goldstein, 2006; Goldstein et al., 2009b), is it possible that core. Removal of the residual liquid would have required 4486 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492 that the solid core was broken open, destroying any rela- 5.4. Formation of main group pallasites tionship among IIIAB irons between bulk Ni concentration and burial depth. We conclude that MG pallasites did not Although our cooling rate data effectively negate the form from residual liquid from the IIIAB core. compositional arguments in favor of a link with IIIAB irons, it remains probable that the metal in MG pallasites was lar- 5.3.2. Other iron meteorites and olivine achondrites gely derived from residual molten metal during the final If IIIAB irons are not derived from the core of the MG stages of crystallization from a IIIAB-like source (Scott, pallasite parent body, is it possible that another group of ir- 1977b; Wasson and Choi, 2003). Only by invoking frac- ons sampled the core? Since Seymchan was once classified as tional crystallization of 80% of a metallic Fe–Ni in a core a IIE-anomalous iron before olivine-rich samples were rec- can one plausibly explain the highly fractionated, low-Ir ognized (Wasson and Wang, 1986), a link between MG pall- metallic compositions in most MG pallasites, which show asites and IIE irons might be suspected on the basis of metal Ir/Ni ratios that are 0.05–0.005 those found in chondrites. compositions. However, silicates in the two groups have Since the associated iron meteorites appear to be absent in very different oxygen isotopic compositions (Franchi, meteorite collections, we infer that the core of the body that 2007). In addition, IIE irons have lower Co concentrations supplied the metal in the main-group pallasites crystallized and almost uniformly higher Ir concentrations (Wasson outwards, like the IVB core (Yang et al., 2010), so that the and Wang, 1986). Group IIIE irons show a very minor com- solid inner core could have been separated from the sur- positional overlap with the MG pallasites. However, IIIE ir- rounding residual molten metal by an impact. Fig. 10 shows ons nearly all have lower Ni, Co, Au, and As concentrations schematically how the MG pallasites may have formed from as well as carbides which are absent in the pallasites (Wasson a mixture of mantle fragments and the 20 vol% of residual et al., 1998). Group IIICD irons are also close in composi- molten Fe–Ni. We suggest that removal of the solid core tion to metal in a few MG pallasites but their rare chondritic may have occurred in a glancing hit-and-run collision when silicates have different oxygen isotopic compositions the differentiated MG pallasite body struck a larger body (Clayton and Mayeda, 1996) arguing against formation in (Asphaug et al., 2006). If all of the 20 vol% of residual mol- a common body. Although the metal in MG pallasites has ten metal had been mixed with twice the volume of mantle the low-Ir concentrations that favor formation from the core fragments, the pallasitic body would have had 60 vol% of a body that fractionally crystallized, no group of iron of the original core in the differentiated body in Fig. 10a, meteorites appears to be derived from the same parent body. or 84% of the core’s radius. Three MG pallasites have olivine-free regions of metal Since the pallasites with rounded olivines did not cool that are several tens of centimeters in size—Brenham, Glo- below 925 K any slower than the pallasites with angular oli- rieta Mountain, and Seymchan—and Seymchan was classi- vines, we suggest that the former owe their shape to prior fied as an ungrouped iron before olivine-bearing samples annealing at the core–mantle boundary before the impact were discovered. Is it possible, therefore, that other un- (Fig. 11). The rounded olivines may have formed either grouped irons may have been derived from olivine-free me- from olivine fragments created at the boundary by earlier tal zones in the MG pallasite body? Although two milder collisions, or from olivine crystals that were sub- ungrouped irons, Ban Rang Du and Tres Costillos, have merged into the molten core by the weight of the olivine compositions that approach that of Pavlodar, which is the mantle (Davis, 1977; Wood, 1978, 1981). Pallasites like MG pallasite with the lowest Ni, Au, and As and the sec- Pavlodar with relatively high Ir concentrations, near chon- ond highest Ir concentration (Wasson et al., 1998; Wasson dritic Ir/Ni ratios, and rounded olivines could have formed and Choi, 2003), there are currently no ungrouped irons in the pallasitic layer prior to the impact that broke up the with compositions close to the majority of MG pallasites, differentiated body. For pallasites like Brenham with low-Ir which have low-Ir concentrations. Lonaconing, which was metal and rounded olivines, the metal may have solidified once classified as an ungrouped iron, closely matches the around the rounded olivine intergrowth in the pallasite compositions of Fe–Ni in many MG pallasites but was body after the impact. The presence of olivine-free metal transferred to a subgroup of the IAB complex by Wasson veins in Brenham is compatible with such an origin. Palla- and Kallemeyn (2002). sites like Seymchan that have regions with rounded olivines Olivine achondrites that might be linked to the MG pall- and regions with angular olivines (Fig. 2a) appear to have asites are rare. Mineralogically, the dunite, NWA 2968, formed from a mixture of fragments of olivine mantle, might qualify but its oxygen isotopic composition shows rounded olivine intergrowth (Fig. 11), and molten metal. it is not from the same body as the MG pallasites (Green- How pallasites with angular olivines but high Ir concentra- wood et al., 2006; Scott et al., 2009b). We conclude that tions, like Finmarken, formed is not known. few if any ungrouped irons or olivine achondrites are re- Unlike other models that envisage formation and cool- lated to the MG pallasites. Since the olivine-free metal re- ing of pallasites at the core–mantle boundary, our model gions should survive impacts better than pallasites, we can account for the incorporation of some material origi- infer that olivine-free metal zones were small and rare in nally located well above the core–mantle boundary. For the body that supplied the MG pallasites. Assuming the example, some phosphates in MG pallasites contain high MG pallasites are representative of the body in which they concentrations of rare-earth elements and negative Eu cooled, its mean composition should be similar to that of anomalies and appear to have crystallized from residual sil- large pallasite slices: viz., 65% olivine and 35% metal and icate melt (Davis and Olsen, 1991; Hsu, 2003). Traces of associated phases (Buseck, 1977; Ulff-Møller et al., 1998). residual silicate melt from cooler regions could have been Thermal history and origin of main-group pallasites 4487

Fig. 10. Cartoon showing how typical main group pallasites, which have angular olivines and metal with sub-chondritic Ir/Ni ratios, may have formed. (a) Differentiated asteroid or protoplanet with an olivine-rich mantle and a metallic Fe–Ni core that is 80% solidified impacts a larger body at a glancing angle. (b) The differentiated body is torn apart forming the pallasite body from residual molten Fe–Ni, which has a low-Ir/Ni ratio, and fragments of olivine mantle. The remainder of the mantle and the solid core fail to accrete to the pallasite body. If all the residual Fe–Ni melt accretes with twice the volume of mantle fragments to form a single pallasitic body, its radius would be 90% of the radius of the solid Fe–Ni core. (Not drawn to scale.)

Fig. 11. Cartoon showing how the main group pallasites with rounded olivines may have formed from a pallasitic layer of intergrown rounded and annealed olivine crystals or fragments at the core–mantle boundary. Pallasites like Pavlodar that have a chondritic Ir/Ni ratio could have formed from metallic Fe–Ni that solidified in the pallasitic layer prior to the impact that broke up the differentiated body. Pallasites like Brenham with sub-chondritic Ir/Ni ratios may have formed from rounded olivine intergrowth and molten metal that solidified in the pallasite body after the impact. mixed with mantle olivine and residual molten Fe–Ni when 5.5. Thermal model for main group pallasite body the pallasitic body accreted. In addition, cooler olivine mantle fragments that were originally located far above A thermal model of the pallasite parent was developed, the core–mantle boundary may have been added to the pall- assuming that the MG pallasite parent body is composed of asitic mixture causing incipient crystallization of molten mantle olivine and metal with similar proportions to the metal around the olivine, hindering the tendency for molten large slices. The pallasite material was treated as a particu- metal and olivine to separate gravitationally. late composite with olivine as particle and metal as matrix. 4488 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

We assume that the pallasite body has an olivine/metal vol- a very weak inverse correlation between log Ir in the metal ume ratio of 2/1, and that the mixture rule can be used to and cloudy taenite particle size, but this is very dependent obtain the density and specific heat of pallasites. The ther- on the two end-members, Finmarken and Glorieta Moun- mal conductivity of pallasites can be calculated by an tain.) Since all the MG pallasites, including those with ex- empirical equation given by Kaviany (2002) for particulate treme metal or olivine compositions, appear to have come composite, giving a thermal conductivity of 7.5 W/m-K. from a single body, the MG pallasites probably cooled at We also assume that a hot isothermal and spherical pall- various depths in a single body. asite body was exposed to space after the collision took Six group IIIAB irons with 7.7–10.0% Ni and cooling place and that regolith was absent. An initial temperature rates of 50–350 K/Myr (Yang and Goldstein, 2006) have of 1750 K and a surface temperature of 200 K were as- cloudy taenite particle sizes of 40–60 nm and tetrataenite sumed, and any internal heat generation in the body during bandwidths of 300–450 nm. These data confirm that cool- cooling (for example by 60Fe decay) was neglected. The ing rates of IIIAB irons are faster than those of MG palla- thermal conductive equation for these assumed conditions sites and decrease with increasing bulk Ni. The widespread was solved using the code developed by Yang et al. (2007). belief that MG pallasites formed and cooled at the core– The calculation shows that if the center of the body mantle interface of the IIIAB core (Mittlefehldt et al., cooled at 2.0 K/Myr, equivalent to the slowest cooled 1998; Wasson and Choi, 2003) is incompatible with the fol- MG pallasites (Table 3), the pallasite body was 400 km in lowing observations: (a) IIIAB irons have cooling rates, tet- radius. If we assume that pallasite body contained rataenite widths and cloudy zone particle sizes showing that 20 vol% of original core that was liquid and twice the vol- their cooling rates decrease with increasing bulk Ni; (b) MG ume of olivine mantle fragments, the protoplanet in pallasites have cooling rates, tetrataenite widths and cloudy Fig. 10 would have been about 950 km in radius. However, zone particle sizes which show that their cooling rates differ the pallasite body and protoplanet would have been much by a factor of five or more; and (c) three techniques show smaller if the pallasite body cooled under a silicate regolith, that all the MG pallasites cooled more slowly than the as seems plausible (see Haack et al., 1990). IIIAB irons. Formation of IIIAB irons and MG pallasites in a single body by mixing residual core melt with mantle 6. SUMMARY material prior to impact destruction and core–mantle sepa- ration is incompatible with the inverse correlation between We have determined metallographic cooling rates below bulk Ni and cooling rate for IIIAB irons, which shows that 975 K (700 °C) for eight MG pallasites by measuring Ni the IIIAB core crystallized inwards. We infer that MG pall- profiles across taenite lamellae of known crystallographic asites and IIIAB irons are not derived from a common par- orientation in metallic Fe–Ni regions with Widmansta¨tten ent body even though they have indistinguishable oxygen patterns and comparing these profiles with those calculated isotopic compositions (Clayton and Mayeda, 1996; Fran- for diverse cooling rates. Local bulk concentrations of Ni chi, 2007). and P were determined for each taenite lamella to correct We suggest that MG pallasites formed from an impact for variations due to prior growth of swathing kamacite that mixed residual Ir-poor molten metal from the outer- around olivine grains. Mean cooling rates for each pallasite, most part of a IIIAB-like core that was 80% fractionally which were determined by averaging rates inferred for 7–10 crystallized from the center (like the IVB core; see Yang taenite lamellae, ranged from 2.5 ± 0.3 K/Myr for Glorieta et al., 2010) with roughly twice the volume of olivine mantle Mountain to 18 ± 9 K/Myr for Finmarken. The quoted fragments. The absence of associated iron meteorites or uncertainties, which are twice the standard error of the olivine achondrites suggests that olivine-free metal regions mean, are 0.3–1.3 K/Myr, except for Finmarken. Total er- and metal-free olivine zones were relatively rare in the rors due to analytical errors and errors in measuring dis- MG pallasite body after it had solidified. Separation of tances normal to the plane of the kamacite–taenite the solid core and most of the associated mantle may have interface are probably less that the mean 2r uncertainty fac- resulted from a grazing hit-and-run impact with a larger tor of 1.6 in the cooling rates determined for each pallasite. protoplanet or asteroid (Fig. 10). Such an impact would Sizes of high-Ni particles in cloudy taenite at the tetrata- have been more effective than a head-on collision in sepa- enite interface in 28 MG pallasites (86–170 nm) and the rating core and mantle material without intense shock widths of the tetrataenite bands in 20 MG pallasites (Asphaug et al., 2006). The possibility that a hit-and-run (1050–2170 nm) are positively correlated with each other collision created more than one MG pallasite parent body and negatively correlated with the metallographic cooling cannot be excluded. rates. Results from three different techniques show that Since MG pallasites with rounded olivines did not cool the MG pallasites cooled below 925 K (700 °C) at rates any slower than those with angular fragments of olivine, that differed by a factor of 5 or more. Since samples from we infer that the annealing of olivine and metal that con- a core–mantle interface should have indistinguishable cool- verted angular to rounded olivine occurred prior to the im- ing rates, we infer that the MG pallasites did not cool at the pact that created the pallasite body. Thermal calculations core–mantle interface of a differentiated asteroid. show that if the pallasite body lacked a regolith and cooled Cooling rates for 28 MG pallasites inferred from cloudy at 2 K/Myr at its center, like the slowest cooled MG palla- taenite particle sizes are not correlated with bulk Ni concen- site, the pallasite body would have been 400 km in radius. tration in metallic Fe–Ni or with the FeO concentration However, it would have been considerably smaller if it and oxygen isotopic composition of the olivine. (There is had a silicate regolith, as seems likely. Thermal history and origin of main-group pallasites 4489

ACKNOWLEDGMENTS We should not expect all meteorites in Fig. A1 (and A2) to define a single line as the taenite compositions that deter- The financial support from NASA through grants mine the metallographic cooling rate reflect cooling rates at NNX08AG53G (J.I. Goldstein, P.I.), NNX08AE08G (K. Keil, temperatures that are about 200 K higher than the forma- P.I.) and NNX08AI43G (E. Scott, P.I.) is acknowledged. We thank tion temperatures of cloudy zone and tetrataenite. The Haiqi Yu and Miaoyong Zhu for help during thermal model devel- cooling rates during formation of cloudy zone particles opment, Eric Asphaug for helpful discussions, and Allan Lang for kindly supplying the Seymchan photographs. We thank Tim and tetrataenite should be slower than the cooling rates McCoy (Smithsonian Institution), Diane Johnson (Open Univer- during Widmansta¨tten growth by an amount that depends sity), Clarita Nunez (Field Museum), Marvin Killgore (Southwest on the nature of the body. For the IVA irons, the thermal Meteorite Lab), Horton Newsom (Univ. of New Mexico), Caroline model of the asteroid (Yang et al., 2007, 2008) indicates Smith (Natural History Museum), Rebekah Hines and Laurence that the cooling rate below 675 K (400 °C) is a factor of Garvie (Arizona State University), Joseph Bosenberg (Americal 1–5 slower than the cooling rate below 875 K (600 °C) Museum of Natural History), and Alan Rubin (UCLA) for loaning for a metallic body. But for a slow cooled parent body like meteorite samples, and Richard Greenwood, Henning Haack, and the pallasite asteroid, the cooling rate below 675 K is most Christian Koeberl (associate editor) for their helpful and detailed likely 1–3x slower than the cooling rate below 875 K. reviews. The cloudy zone microstructure formed by spinodal decomposition followed by Oswald ripening. If we neglect the initial particle size before coarsening, the cloudy zone APPENDIX A. THEORY OF CLOUDY TAENITE AND particle size should follow the power law for the coarsening TETRATAENITE FORMATION growth process where

A1. Cloudy zone particle size Dn ¼ mt ðA2Þ

The widths of the cloudy zone particles at the tetratae- and D is the size of the particle, m is the coarsening rate nite interface in several chemical groups of iron meteorites constant, and t is time. The parameter n is a constant which and stony irons are related to the cooling rate determined depends on the mechanism which controls the coarsening from the growth of Widmansta¨tten kamacite as shown in process. A classical view of the coarsening process under Fig. A1. The equation which best fits the data is isothermal conditions suggests that n = 4 for an interface- m controlled growth process (Zhu et al., 1999), n = 3 for a d2:9 ¼ ðA1Þ CZ CR bulk diffusion-controlled growth process (Lifshitz and Sly- ozov, 1961; Wagner, 1961), and n = 2–3 for a trans-inter- where dCZ is the cloudy particle size in nm, m is constant face-controlled growth process (Ardell and Ozolins, 2005). which equals to 7,620,000, and CR is the metallographic The cloudy zone particle size follows a power law, given cooling rate in K/Myr. Fig. 8a shows that this line is within by Eq. (A1), where n = 2.9. For a continuous cooling pro- error of the best fit line for the MG pallasite data. cess t=DT/CR, where DT is the temperature interval for

Fig. A1. Cloudy zone particle size vs. metallographic cooling rate for iron and stony-iron meteorites: IVA irons (Goldstein et al., 2009a), IIIAB irons (Yang and Goldstein (2006) and this study), main group pallasites (this study), IVB irons (Yang et al., 2010) and mesosiderites (Hopfe and Goldstein, 2001). 4490 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4471–4492

Fig. A2. Tetrataenite bandwidth vs. cooling rate for iron and stony-iron meteorites: IVA irons (Goldstein et al., 2009a), IIIAB irons (Yang and Goldstein, 2006 and this study), main group pallasites (this study), and IVB irons (Yang et al., 2010). spinodal decomposition and the coarsening process. For Asphaug E., Agnor C. B. and Williams Q. (2006) Hit-and-run cloudy zone formation DT is about 150 K. Since n = 2.9 planetary collisions. Nature 439, 155–160. is very close to either a bulk diffusion-controlled (n =3) Boesenberg J. S., Davis A. M., Prinz M., Weisberg M. K., Clayton or a trans-interface controlled (n = 2–3) growth process, R. N. and Mayeda T. K. (2000) The pyroxene pallasites, the empirical expression of power law for growth of cloudy Vermillion and Yamato 8451: not quite a couple. Meteorit. Planet. Sci. 35, 757–769. zone particle given by Eq. (A1) is consistent with coarsen- Buchwald V. F. (1975) Handbook of Iron Meteorites. Their History, ing theory. Distribution, Composition and Structure. University of Califor- nia Press, Berkeley and Los Angeles, CA. A2. Tetrataenite bandwidth Buseck P. R. (1977) Pallasite meteorites—mineralogy, petrology and geochemistry. Geochim. Cosmochim. Acta 41, 711–740. The widths of the tetrataenite bands in three groups of Buseck P. R. and Goldstein J. I. (1968) Pallasitic meteorites: iron meteorites and the MG pallasites are inversely corre- implications regarding the deep structure of asteroids. Science lated with the cooling rates determined from the growth 159, 300–302. of Widmansta¨tten kamacite as shown in Fig. A1. Buseck P. R. and Goldstein J. I. (1969) Olivine compositions and The equation of the best fit line to the data plotted in cooling rates of pallasitic meteorites. Geol. Soc. Am. Bull. 80, 2141–2158. Fig. A1 is Clayton R. N. and Mayeda T. K. (1996) Oxygen isotope studies of k achondrites. Geochim. Cosmochim. Acta 60, 1999–2017. d2:3 ¼ ðA3Þ Davis A. M. (1977) The Geochemical History of the Pallasites. Tt CR Ph.D. Thesis, Yale University. Davis A. M. (1982) Phosphorus in main group pallasites. Mete- where dTr is the tetrataenite bandwidth in nm, CR is cooling rate in K/Myr, and k is constant which equals to oritics 17, 203. Davis A. M. and Olsen E. J. (1991) Phosphates in pallasite 14,540,000. meteorites as probes of mantle processes in small planetary The isothermal diffusion-controlled growth of one phase bodies. Nature 353, 637–640. in a two-phase alloy obeys the parabolic law Desrousseaux A., Doukhan J. C., Leroux H. and van Duysen J. C. (1997) An analytical electron microscope investigation of some x2 Kt ¼ ðA4Þ pallasites. Phys. Earth Planet. Int. 103, 101–115. where x is the thickness of new phase, K is a constant and t Eugster O., Herzog G. F., Marti K. and Caffee M. W. (2006) Irradiation records, cosmic-ray exposure ages, and transfer is time. The exponent 2.3 in Eq. (A3) is very close to the times of meteorites. In Meteorites and the Early Solar System II exponent 2 in Eq. (A4) above and is consistent with the par- (eds. D. S. Lauretta and H. Y. McSween Jr). University of abolic law for growth of the tetrataenite band. Arizona Press, Tucson, pp. 829–851. Franchi I. A. (2007) Oxygen isotopes in asteroidal materials. Rev. REFERENCES Miner. Geochem. 68, 345–397. Goldstein J. I. and Ogilvie R. E. (1965) The growth of the Ardell A. J. and Ozolins V. (2005) Trans-interface diffusion- Widmansta¨tten pattern in metallic meteorites. Geochim. Cos- controlled coarsening. Nature Mater. 4, 309–316. mochim. Acta 29, 893–920. Thermal history and origin of main-group pallasites 4491

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