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Thermal History and Origin of the IVB Iron Meteorites and Their Parent Body

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Thermal History and Origin of the IVB Iron Meteorites and Their Parent Body Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 4493–4506 www.elsevier.com/locate/gca Thermal history and origin of the IVB iron meteorites and their parent body Jijin Yang a,*, Joseph I. Goldstein a, Joseph R. Michael b, Paul G. Kotula b, Edward R.D. Scott c a Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, MA 01003, USA b Materials Characterization Department, Sandia National Laboratories, P.O. Box 5800, MS 0886, Albuquerque, NM 87185, USA c Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA Received 11 November 2009; accepted in revised form 6 April 2010; available online 21 April 2010 Abstract We have determined metallographic cooling rates of 9 IVB irons by measuring Ni gradients 3 lm or less in length at kama- cite–taenite boundaries with the analytical transmission electron microscope and by comparing these Ni gradients with those derived by modeling kamacite growth. Cooling rates at 600–400 °C vary from 475 K/Myr at the low-Ni end of group IVB to 5000 K/Myr at the high-Ni end. Sizes of high-Ni particles in the cloudy zone microstructure in taenite and the widths of the tetrataenite rims, which both increase with decreasing cooling rate, are inversely correlated with the bulk Ni concentrations of the IVB irons confirming the correlation between cooling rate and bulk Ni. Since samples of a core that cooled inside a ther- mally insulating silicate mantle should have uniform cooling rates, the IVB core must have cooled through 500 °C without a silicate mantle. The correlation between cooling rate and bulk Ni suggests that the core crystallized concentrically outwards. Our thermal and fractional crystallization models suggest that in this case the radius of the core was 65 ± 15 km when it cooled without a mantle. The mantle was probably removed when the IVB body was torn apart in a glancing impact with a larger body. Clean separation of the mantle from the solid core during this impact could have been aided by a thin layer of residual metallic melt at the core-mantle boundary. Thus the IVB irons may have crystallized in a well-mantled core that was 70 ± 15 km in radius while it was inside a body of radius 140 ± 30 km. Ó 2010 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Bulletin Database see http://tin.er.usgs.gov/meteor/metbull. php), the group is well studied, as the irons appear to be a The IVB iron meteorites are ataxites with microscopic fairly representative suite of samples from the core of an Widmansta¨tten patterns consisting of kamacite platelets interesting asteroid with a relatively simple history. <20 lm in width, which appear in section as platelets The IVB irons are chemically unique as they contain embedded in matrices of plessite, a fine mixture of kama- exceptionally low concentrations of moderately volatile ele- cite and taenite. The small size of the kamacite platelets ments and high Ni contents: element/Ni ratios are depleted reflects the high bulk Ni concentrations of the IVB irons, relative to CI chondrites by factors of 10–104 and decrease 15–18 wt.%. Low-Ni IVB irons have very few kamacite in order of increasing volatility: Au, P and As, Cu and Ga, platelets, and phosphides whereas high-Ni IVB irons have and Ge (Kelly and Larimer, 1977; Rasmussen et al., 1984; many more kamacite platelets and phosphides (Fig. 1). Wasson, 1985). The low volatile abundances reflect high Although only 13 IVB irons are known (Meteoritical nebular temperatures when their parent body accreted or volatile loss during one or more impacts (Goldstein et al., 2009a). The high Ni concentrations in group IVB favor for- * Corresponding author. mation in an oxidizing environment (Campbell and Huma- E-mail address: [email protected] (J. Yang). yun, 2005). Despite the unusual bulk composition of group 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.04.011 4494 J. Yang et al. / Geochimica et Cosmochimica Acta 74 (2010) 4493–4506 Fig. 1. Reflected light photomicrographs of low-Ni IVB iron meteorite, Cape of Good Hope (left) and high-Ni IVB iron, Weaver Mountains (right) showing kamacite platelets and phosphides in a plessite matrix. The abundance of kamacite platelets and phosphides in IVB irons increases with increasing bulk Ni concentration. Figure modified from Goldstein and Michael (2006). IVB, variations in the concentrations of Ni, P and other ele- zone particles and tetrataenite widths in the IVB irons as ments within the group are well modeled by simple frac- these parameters provide additional constraints on cooling tional crystallization in a vigorously convecting core using rates at 300–400 °C(Goldstein et al., 2009a). The cloudy experimentally determined solid–liquid distribution coeffi- zone formed below 350 °C by spinodal decomposition cients (Rasmussen et al., 1984; Campbell and Humayun, and grew by diffusion controlled coarsening (Oswald Rip- 2005; Walker et al., 2008). ening). Thus the size of the high-Ni particles is inversely re- Although much is known about the formation of the IVB lated to cooling rate (Yang et al., 1997) and has been used irons, the direction of crystallization of the IVB core and its to help understand the origin and formation of the parent subsequent thermal and collisional history is poorly known. asteroid of the IVA irons (Yang et al., 2007; Goldstein Precise metallographic cooling rates are difficult to obtain for et al., 2009b). Similarly, the tetrataenite band between the the IVB irons with the 1 lm spatial resolution of the electron cloudy taenite and the kamacite platelet formed below probe microanalyzer (EPMA) because the Ni gradients in 400 °C and its width is inversely related to the cooling rate taenite at the boundary with the kamacite platelets are (Goldstein et al., 2009a). 3 lm or less in length. In addition, the kamacite platelets Our first goal was to find out whether the cooling rates are too far apart for impingement to occur so that M-shaped in the IVB irons were uniform or if they were correlated profiles are not generated in the intervening taenite. Given with bulk Ni, and to reconcile their thermal histories with these limitations, the conventional taenite central Ni vs half their simple fractional crystallization history. A second goal width Wood method (Wood, 1964), and the profile matching was to understand core formation and evolution of the IVB method for taenite lamellae (Goldstein and Ogilvie, 1965) parent body. cannot be applied. Cooling rates for the IVB irons have been obtained from the measurement of kamacite bandwidths and 2. SAMPLES AND ANALYSES have values as high as 10,000 K/Myr (Rasmussen, 1989). However, the measured band-widths and the corresponding We examined 9 out of the 13 known IVB irons (Table 1). cooling rates have large and unknown uncertainties because Samples were first prepared by standard metallographic the orientation of the kamacite platelets with respect to the procedures, mounting, grinding, polishing, and etching polished surface was not measured and their nucleation tem- with 2% nital, and observed using light optical microscopy. peratures were poorly constrained. Samples for electron probe microanalysis were prepared the The fast cooling rates for IVB irons inferred from their same way but without etching. kamacite bandwidths have been interpreted as evidence In order to determine the nucleation mechanism and the for core formation and cooling in a small parent body with nucleation temperature of the kamacite platelets, it is neces- a radius of 2–4 km (Rasmussen, 1989; Chabot and Haack, sary to have accurate bulk Ni and P contents for each mete- 2006). However, the inferred radius of 2–4 km is much orite. Although bulk Ni and P contents of the IVB irons have smaller than the minimum radius of 10–20 km for a body been measured (Moore et al., 1969; Buchwald, 1975; Camp- to be melted and differentiated by 26Al decay (Hevey and bell and Humayun, 2005; Walker et al., 2008), there are sig- Sanders, 2006). nificant differences among the measured values, especially The purpose of this study was to obtain accurate cooling the P content. Therefore, we re-measured the bulk Ni and P rates during kamacite formation for a diverse suite of IVB content of each meteorite using X-ray area scans obtained irons by measuring Ni gradients at the kamacite–taenite with wavelength dispersive spectrometers on a Cameca SX- boundary with the analytical transmission electron micro- 50 electron probe microanalyzer (EPMA) at the University scope in thin sections cut from oriented samples with a fo- of Massachusetts. An operating voltage of 15 kV and a beam cused ion beam instrument. These Ni profiles are then current of 40 nA were used. Two major elements (Fe and Ni) compared with calculated Ni profiles generated by model- and two minor elements (Co and P) were measured. Pure Fe, ing kamacite growth (Yang and Goldstein, 2006). In addi- Ni, Co and (Fe–Ni)3P in the Grant meteorite (15.5 wt.% P) tion, we wanted to measure the dimensions of the cloudy were used as standards. The bulk Fe, Ni, Co and P composi- Thermal history and origin of IVB iron meteorites 4495 tions were measured by averaging 8–27 side by side The size of the high-Ni particles in the cloudy zone next 100 Â 80 lm2 area scans for each IVB iron. The total area to the cloudy zone/tetrataenite boundary (Yang et al., scanned for each meteorite was dependent on its structure 1997) was measured directly from the STEM image or the and ranged from 64,000 lm2 to 216,000 lm2 (Table 1).
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