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The Fukang Pallasite: Characterization and Implications for the History of the Main-Group Parent Body

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The Fukang Pallasite: Characterization and Implications for the History of the Main-Group Parent Body Meteoritics & Planetary Science 54, Nr 8, 1781–1807 (2019) doi: 10.1111/maps.13313 The Fukang pallasite: Characterization and implications for the history of the Main-group parent body Daniella N. DELLAGIUSTINA 1, Namrah HABIB1, Kenneth J. DOMANIK1, Dolores H. HILL1, Dante S. LAURETTA 1, Yulia S. GOREVA2, Marvin KILLGORE3, Yang HEXIONG4, and Robert T. DOWNS 4 1Lunar and Planetary Laboratory, 1629 E University Blvd Tucson, Arizona 85721, USA 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA 3Southwest Meteorite Laboratory, PO Box 95, Payson, Arizona 85547, USA 4Department of Geosciences, University of Arizona, 1040 4th St, Tucson, Arizona 85721, USA *Corresponding author. E-mail: [email protected] (Received 21 December 2017; revision accepted 23 April 2019) Abstract–We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main-group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe-Ni matrix with 9–10 wt% Ni and low-Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe-Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low-Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main-group pallasites, including clinopyroxene (augite), tridymite, K-rich felsic glass, and an unknown Ca-Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two-pyroxene barometry (0.39 Æ 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main-group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius. INTRODUCTION Main-group pallasites comprise the majority of grouped pallasites, and share mineralogical, elemental, Pallasites are stony-iron meteorites that consist of two and isotopic similarities that imply a common origin major phases: olivine and Fe-Ni metal typically in near (Scott 1977a; Clayton and Mayeda 1996). Despite these equal amounts. It is widely accepted that pallasites formed similarities, Main-group specimens have a range of olivine at the core–mantle boundary (CMB) of a differentiated grain morphologies (Scott 1977a; Boesenberg et al. 2012), planetesimal of chondritic bulk composition that compositional variations (Buseck 1977; Scott 1977a; underwent core formation, thereby providing a potential Wasson and Choi 2003; Boesenberg et al. 2012), and link between iron and chondritic meteorites (Buseck 1977). metallic cooling rates (Yang et al. 2010). Recent work has Pallasites have been classified into two established groups: explained differences among Main-group pallasites by the Main-group and the Eagle Station group (PES). Along proposing that they originated from one parent body that with ungrouped members, such as Milton and the cooled at different rates with depth (Scott and Taylor pyroxene-bearing pallasites, this meteorite class may 1990; Scott et al. 2010; Yang et al. 2010). represent at least five different parent bodies (Boesenberg Similarities in oxygen isotope, trace element, and Ni et al. 2012; Ruzicka et al. 2017). Each group differs in compositions have been used to suggest that the IIIAB siderophile trace element abundances, olivine composition, iron meteorites represent the core of the Main-group and oxygen isotope ratios (Scott 1977a; Boesenberg et al. pallasite parent body (Scott 1977b; Clayton and 2000; Jones et al. 2003; Bunch et al. 2005). Mayeda 1996). Like the IIIAB irons, pallasites often 1781 © The Meteoritical Society, 2019. 1782 D. N. DellaGiustina et al. display a Widmanstatten€ crystallization pattern that represents very slow cooling of the Fe-Ni phase. Studies of Ni diffusion profiles in pallasites have yielded metallic cooling rates of 2.5–20 K MyrÀ1 (Yang et al. 2010). These cooling rates indicate that pallasites formed deep within a planetary interior. Because thermal conduction establishes that the core responsible for the IIIAB iron meteorites should have cooled more slowly than the surrounding CMB, it is surprising that metallographic cooling rates yield the opposite: the IIIAB irons appear to have cooled more rapidly at 50– 350 K MyrÀ1 (Yang et al. 2010). Studies based on metallographic cooling rates and siderophile trace Fig. 1. Cross section of the Fukang meteorite (91 cm wide). elements have questioned the genetic relationship Note the large olivine nodules. Photo by S. Platts (UA-LPL). between Main-group pallasites and IIIABs (Wasson and (Color figure can be viewed at wileyonlinelibrary.com.) Choi 2003; Yang and Goldstein 2006; Yang et al. 2010), though recent oxygen and sulfur isotope measurements continue to support formation in the same parent body To account for the abundance of pallasites that (Franchi et al. 2013; Dottin et al. 2018). It also remains exist in the world’s meteorite collections (112 finds and plausible that the crustal remnants of the Main-group 4 falls at the time of this publication), one must be able parent body were transported to the inner solar system to explain the efficient mixing of dense molten metal through the same mechanisms that delivered the and low-density olivine despite their immiscibility. Even pallasites and IIIABs, and are thus represented at the core of a planetesimal, the duration for somewhere in the meteorite collection. separating olivine from molten metal is relatively short Assuming that pallasites originated at the CMB of on planetary time scales when compared to the metal a planetesimal, several theories to explain their solidification rate (Anders 1964). Despite the surplus of formation exist. Among them is an equilibrium process, formation theories, the genesis of Main-group pallasites inferred from the smooth texture of rounded olivine remains unknown. To successfully explain their origin, grains found in some pallasites (Wood 1978). During it is necessary to account for the wide range of metallic this process, the intercumulate silicate melt between cooling rates (2.5–20 K MyrÀ1), variation in olivine cumulate mantle olivine is slowly replaced by molten textures, lack of prominent shock features, and a low metal (Buseck 1977). An alternate theory involves abundance of troilite. nonequilibrium, impact-induced shockwave that In this work, we report the results of an extensive violently mixed mantle olivine crystals, and molten study of the Fukang pallasite (Fig. 1). The 1003 kg metal from the core (Scott 2017). Combinations of these Fukang main mass was recovered in 2001 in the Gobi two processes have also been suggested (Scott and Desert of China. The enormous main mass of Fukang Taylor 1990). More exotic proposals to explain the makes it the third largest pallasite find, and possibly the origin of pallasites include crystallization near the largest single-mass pallasite. Measurements of Fukang’s surface of an externally heated asteroid (Mittlefehldt metal and olivine phases are consistent with a Main- 1980), crystallization of impact melts (Malvin et al. group pallasite classification; this is in agreement with a 1985), and nebular condensation (Kurat 1988). More prior oxygen isotope study that demonstrated Fukang’s recently, glancing collisions of already differentiated Main-group affinity (Greenwood et al. 2015). However, parent asteroid bodies or mostly molten differentiated Fukang exhibits small but important inclusions not yet planetesimals have been invoked to explain pallasite observed in other Main-group pallasites. Here, we formation (Asphaug et al. 2006; Yang et al. 2010). discuss the implications of these findings for the origin These impacts are predicted to result in smaller objects of pallasites and the history of their parent body. that cooled at different rates (Scott 2012). Additionally, Mn/Mg ratios and low-Sc concentrations may ANALYTICAL METHODS AND TECHNIQUES demonstrate that the olivines in pallasites are not cumulates, but residues from high degrees of partial Optical Microscopy melting (Mittlefehldt and Rumble 2006). This finding potentially invalidates all previous theories of pallasite We examined four Fukang olivine thin sections formation that involve the presence of cumulate olivine (20 9 30 mm) using transmitted-light and polarized- from fractional crystallization. light optical microscopy on a Leica DMLP petrographic The Fukang pallasite 1783 polarizing microscope. These include sections labeled MS (LA-ICP-MS). Selected locations for LA-ICP-MS University of Arizona (UA) 2103 TS1, TS2, TS3, and analysis were first measured for major element TS4, made from Southwest Meteorite Laboratory compositions using EPMA. The laser ablation system is Fukang specimen SWL86,7d2. composed of a CETAC LSX-213 laser ablation peripheral with the ThermoFinnigan Element2 ICP-MS Electron Microprobe Analysis at the Paradiso ICP-MS Analytical Laboratory at LPL. Polished slabs ~1 9 3 cm and 10 9 5 cm (UA We determined elemental abundances for metals, 2103,5 and UA 2103,7) of Fukang and thin sections phosphides, and sulfides by measuring isotopes 31P, 34S, (20 9 30 mm) of olivine-rich areas were prepared to 48Ti, 51V, 53Cr, 57Fe, 59Co, 60Ni, 63Cu, 66Zn, 69Ga, 74Ge, analyze the metal, silicate, and minor phases of the 75As, 77Se, 95Mo, 101Ru,
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