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Cosmic History and a Candidate Parent Asteroid for the Quasicrystal-Bearing Meteorite Khatyrka

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Cosmic History and a Candidate Parent Asteroid for the Quasicrystal-Bearing Meteorite Khatyrka COSMIC HISTORY AND A CANDIDATE PARENT ASTEROID FOR THE QUASICRYSTAL-BEARING METEORITE KHATYRKA Matthias M. M. Meier1*, Luca Bindi2,3, Philipp R. Heck4, April I. Neander5, Nicole H. Spring6,7, My E. I. Riebe1,8, Colin Maden1, Heinrich Baur1, Paul J. Steinhardt9, Rainer Wieler1 and Henner Busemann1 1Institute of Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland. 2Dipartimento di Scienze della Terra, Università di Firenze, Florence, Italy. 3CNR-Istituto di Geoscienze e Georisorse, Sezione di Firenze, Florence, Italy. 4Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, USA. 5De- partment of Organismal Biology and Anatomy, University of Chicago, Chicago, USA. 6School of Earth and Environ- mental Sciences, University of Manchester, Manchester, UK. 7Department of Earth and Atmospheric Sciences, Uni- versity of Alberta, Edmonton, Canada. 8Current address: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, USA. 9Department of Physics, and Princeton Center for Theoretical Science, Princeton University, Princeton, USA. *corresponding author: [email protected]; (office phone: +41 44 632 64 53) Submitted to Earth and Planetary Science Letters. Abstract The unique CV-type meteorite Khatyrka is the only natural sample in which “quasicrystals” and associated crystalline Cu,Al-alloys, including khatyrkite and cupalite, have been found. They are suspected to have formed in the early Solar System. To better understand the origin of these ex- otic phases, and the relationship of Khatyrka to other CV chondrites, we have measured He and Ne in six individual, ~40-μm-sized olivine grains from Khatyrka. We find a cosmic-ray exposure age of about 2-4 Ma (if the meteoroid was <3 m in diameter, more if it was larger). The U,Th-He ages of the olivine grains suggest that Khatyrka experienced a relatively recent (<600 Ma) shock event, which created pressure and temperature conditions sufficient to form both the quasicrystals and the high-pressure phases found in the meteorite. We propose that the parent body of Khatyrka is the large K-type asteroid 89 Julia, based on its peculiar, but matching reflectance spectrum, evidence for an impact/shock event within the last few 100 Ma (which formed the Julia family), and its location close to strong orbital resonances, so that the Khatyrka meteoroid could plausibly have reached Earth within its rather short cosmic-ray exposure age. 1. INTRODUCTION crystals (e.g., five-fold). First proposed by Levine and Steinhardt (1984), they were first The motivation for this noble gas study is the synthesized and identified in the laboratory by curiosity and fascination with the origin of an Shechtman et al. (1984). The ensuing multi- exotic type of material: quasicrystals. Short for decade search for natural quasicrystals was quasi-periodic crystals, they are materials show- eventually successful when a powder from a ing a quasi-periodic arrangement of atoms, and millimeter-sized rock sample (later called the rotational symmetries forbidden to ordinary “Florence Sample”) from the collection of the Page 1 of 27 Museum of Natural History in Florence, Italy, al. (2014) found high-pressure mineral phases displayed a diffraction pattern with a five-fold (including stishovite and ahrensite) indicating symmetry (Bindi et al., 2009). This first natural Khatyrka was at one point exposed to pressures quasicrystal, with a composition Al63Cu24Fe13, >5 Gigapascals (GPa) and temperatures >1200 was named icosahedrite. In the host rock of the °C, followed by rapid cooling. Synthetic icosa- Florence sample, silicates and oxides are par- hedrite remains stable under these conditions tially inter-grown with exotic copper-alu- (Stagno et al., 2015). These conditions are also minium-alloys (khatyrkite, CuAl2, and cupalite, typical for asteroid collisions (Stoeffler et al., CuAl) which contain the quasicrystals. The oxy- 1991). In dynamic shock experiments, Asimow gen isotopic compositions of silicates and ox- et al. (2016) successfully synthesized icosahe- ides in the Florence Sample suggest an extrater- dral phases with a composition close to that of restrial origin (Bindi et al., 2012), as they match icosahedrite, and Hamann et al. (2016) reported the ones found in some carbonaceous chon- the shock-induced formation of khatyrkite drites. The provenance of the Florence Sample (CuAl2), mixing of target and projectile mate- was eventually traced back to a Soviet prospect- rial, and localized melting along grain bound- ing expedition to the Far East of Russia in 1979 aries, resembling the assemblages found in (Bindi and Steinhardt, 2014). To find more of Khatyrka. Lin et al. (2017) argued that a two- the exotic material, an expedition was launched stage formation model is needed to explain the to the Koryak mountain range in the Chukotka mineralogical and petrographic features ob- Autonomous Region in 2011. Eight millimeter- served in Khatyrka: first, an event as early as sized fragments of extraterrestrial origin were 4.564 Ga in which quasicrystals with icosa- found by panning Holocene (>7 ka old) river hedrite composition formed; and, second, a deposits (MacPherson et al., 2013). Again, more recent impact-induced shock that led to Cu,Al-alloys were found attached to, or inter- the formation of a second generation of qua- grown with, extraterrestrial silicates and oxides. sicrystals, with a different composition. Ivanova The oxygen isotopes, chemistry and petrology et al. (2017) suggested the possibility of an an- of the eight new fragments all suggest a CVox thropogenic origin of the Cu,Al-alloys from (carbonaceous chondrite, Vigarano-type, oxi- mining operations, but this explanation is in- dized subtype) classification for the meteorite, compatible with the chemistry and the thermo- now named Khatyrka (Ruzicka et al., 2014). dynamic conditions needed to explain all obser- The achondritic, diopside-hedenbergite-rich and vations listed above (see also MacPherson et al., Cu,Al-alloy-encrusted Florence Sample sug- 2016). gests that Khatyrka is an unusual chondritic breccia which accreted both achondritic and ex- In this work, we aim to better understand the otic materials (MacPherson et al., 2013). origin and history of the quasicrystal-bearing Khatyrka meteorite and its relationship to other So far, no other meteorite is known to contain meteorites, in particular other CV chondrites. Cu,Al-alloys or quasicrystals. The lack of We do this by measuring the helium and neon 26Mg/24Mg anomalies suggests that the Cu,Al-al- (He, Ne) content of individual Khatyrka olivine 26 loys formed after the primordial Al (t1/2 = 0.7 grains, in order to determine their cosmic-ray million years, Ma) had decayed away, i.e., at exposure and radiogenic gas retention (based on least ~3 Ma after formation of the oldest con- uranium-thorium-helium = U,Th-He) ages. Due densates in the Solar System 4.567 billion years to the extremely small mass available from (Ga) ago (MacPherson et al., 2013). Hollister et Khatyrka (total mass <0.1 g), destructive analy- Page 2 of 27 ses have to be reduced to the minimum. We in in Fiji/ImageJ (Schindelin et al., 2012), which worked with single grains of ca. 40 μm diame- finds 3D-connected objects with a voxel bright- ter, which were all part of a chondrule from ness above a user-defined 8-bit (256) gray-scale Khatyrka fragment #126 (recovered in the 2011 value (called the “threshold”). We first searched expedition). Because of the very small gas a range of thresholds where the grains are well- amounts expected, we used the high-sensitivity resolved from the background, then searched for “compressor-source” noble gas mass spectrome- the sequence of four thresholds for which the ter at ETH Zurich (Baur, 1999). This unique in- volume change between adjacent gray-scale strument has previously been used in a similar steps was minimal. We then corrected for sur- way to measure He and Ne in individual mineral face resolution effects (i.e., for voxels close to grains returned from asteroid Itokawa (Meier et the actual grain which are only partially in-filled al., 2014). by grain material, and thus have a reduced voxel brightness: they fall below the threshold al- 2. METHODS though they contain a sub-voxel-sized part of the grain) by interpolating between the average 2.1. Volumes from X-ray tomography brightness of unambiguous (internal) grain vox- To determine cosmic-ray exposure and radio- els and background voxels (e.g., if the grain ma- genic gas retention ages, we need noble gas con- terial brightness is 120 and the background centrations (cm3STP/g), which requires the de- brightness is 40, a surface-near voxel with termination of the masses of the individual brightness 80 is assumed to be half in-filled grains. This is not possible to do both safely and with grain material). Grain #126-06 was too reliably on a micro-balance for such small (<1 small, and too fragmented to be of use for this µg) grains. To determine their masses, we first study, and was thus not further analyzed. The measure their volumes using high-resolution X- grain volume uncertainties in Table 1 corre- ray tomography (nano-CT), and then multiply spond to the range in volume within the se- these volumes with the densities calculated from quence of four thresholds with minimal volume their mineralogical composition (determined changes (after correction for surface effects). with SEM-EDS) and textbook mineral densities. Grain #126-05 fragmented after nano-CT scan- At the University of Florence, seven olivine ning (as visible from the SEM images, see sec- grains from Khatyrka fragment #126 (named tion 2.2.). Only two of its fragments could be #126-01 through -07, or 1 through 7 for short in transferred to the sample holder for noble gas the figures), all fragments from a chondrule analysis. Their volume was estimated from their (judging from their size and chemistry), were cross-sectional area in the SEM images and an transferred with a micro-manipulator needle empirical relationship between cross-sectional from a TEM grid to a carbon tape.
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