¹Paola Vannucchi, ¹Jason P. Morgan, ¹Damiano Della Lunga,
²Christopher L. Andronicos, ³W. Jason Morgan
¹Earth Sciences Department, Royal Holloway, University of London, UK
²Earth, Atmospheric and Planetary Sciences Department, Purdue University, IN, USA
³Department of Earth and Planetary Sciences, Harvard, Cambridge, MA, USA

Shock metamorphism is rarely found at the surface of the Earth. The most used structures to identify shock metamorphism are “true Planar Deformation Features” (PDFs) in quartz, now accepted as diagnostic indicators of a meteorite impact. Here we present several lines of evidence for shock metamorphism and PDFs developed in quartz occurring on samples centered on a circular geological structure on Mount Stojkovic (60°54′06″N; 101°55′40″E), which lies within southern surface exposures of the Siberian Traps. The shock event appears to have occurred during the eruption of the surface Siberian Traps basalts that cover this region. Curiously, Mount Stojkovic lies within ∼3 km of the tree fall epicenter of the 1908 Tunguska event. Based on current estimates of the Phanerozoic impact distribution, there is at most a 1 in ∼17 000 chance that the 1908 bolide would randomly fall on the site of a previous impact structure capable of creating shocked quartz. Just as improbable would be an airbust event, incapable of creating a small crater, that could have produced shock metamorphism. Our preferred least implausible hypothesis is that the shock-metamorphism here was associated with a terrestrial event, a hyperexplosive volcanic gas eruption called ‘Verneshot’.

Reference
Vannucchi P, Morgan JP, Lunga DD, Andronicos CL, Morgan WJ (2014) Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska Event. Earth and Planetary Science Letters 409, 168–174
Link to Article [doi:10.1016/j.epsl.2014.11.001]

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^1,2Tsuyoshi Iizuka, ^3,4Akira Yamaguchi, ³Makiko K. Haba, ²Yuri Amelin, ²Peter Holden, ²Sonja Zink, ²Magdalena H. Huyskens, ²Trevor R. Ireland
¹Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
²Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
³National Institute of Polar Research, Tokyo, Japan
⁴Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tokyo, Japan

Non-cumulate eucrites represent basaltic crust that experienced a complex thermal history involving multistage metamorphism and metasomatism, probably on asteroid Vesta. To better constrain the thermal history of these rocks and their parent body, we have integrated high-precision U–Pb age and trace element data for zircon grains with sizes up to 80 μm in the eucrite Agoult. All analyzed zircon grains yielded concordant U–Pb dates that correspond to the precise 207Pb/206Pb age of 4554.5±2.0 Ma4554.5±2.0 Ma. The Ti contents in these zircon grains indicate their crystallization at subsolidus temperatures of ca. 900 °C, which are similar to the inferred conditions of pyroxene exsolution in most basaltic eucrites that occurred during protracted thermal metamorphism. The zircon crystallization temperatures, together with the presence of baddeleyite needles and variable Zr concentration in Agoult ilmenite grains, indicate metamorphic origin of the Agoult zircon through Zr release from ilmenite followed by reaction with silica. We therefore consider the zircon 207Pb/206Pb age as the timing of the widespread thermal metamorphism in Vesta’s crust. The metamorphic age is coincident with the oldest Mn–Cr date for cumulate eucrites, supporting the view that the thermal metamorphism is a result of burial of basaltic crust and subsequent heating from the hot interior rather than collision of asteroids. The zircon rare earth element patterns with restricted Ce positive anomalies suggest that the metamorphism occurred at an oxygen fugacity below the iron–wüstite buffer, implying the absence of oxidizing agents such as aqueous fluid within the crust at that time.

Reference
Iizuka T, Yamaguchi A, Haba MK, Amelin Y, Holden P, Zink S, Huyskens MH, Ireland TR (2014)Timing of global crustal metamorphism on Vesta as revealed by high-precision U–Pb dating and trace element chemistry of eucrite zircon. Earth and Planetary Science Letters 409,182–192
Link to Article [doi:10.1016/j.epsl.2014.10.055]

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¹Takaaki Noguchi, ²Noriaki Ohashi, ²Shinichi Tsujimoto, ²Takuya Mitsunari, ³John P. Bradley, ⁴Tomoki Nakamura,
⁵Shoichi Toh, ⁶Thomas Stephan, ⁷Naoyoshi Iwata, ⁸Naoya Imae
¹Faculty of Arts and Science, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan
²College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito 310-8512, Japan
³University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology, Honolulu, HI 96822, USA
⁴Department of Earth Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan
⁵Department of Applied Physics, Fukuoka University, 8-19-1 Nanakuma, Fukuoka 814-0180, Japan
⁶Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA
⁷Department of Earth and Environmental Sciences, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560, Japan
⁸National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan

Chondritic porous interplanetary dust particles (CP IDPs) collected in the stratosphere are regarded as possibly being cometary dust, and are therefore the most primitive solar system material that is currently available for analysis in laboratories. In this paper we report the discovery of more than 40 chondritic porous micrometeorites (CP MMs) in the surface snow and blue ice of Antarctica, which are indistinguishable from CP IDPs. The CP MMs are botryoidal aggregates, composed mainly of sub-micrometer-sized constituents. They contain two components that characterize them as CP IDPs: enstatite whiskers and GEMS (glass with embedded metal and sulfides). Enstatite whiskers appear as <2-μm-long acicular objects that are attached on, or protrude from the surface, and when included in the interior of the CP MMs are composed of a unit-cell scale mixture of clino- and ortho-enstatite, and elongated along the [100] direction. GEMS appear as 100–500 nm spheroidal objects containing <50 nm Fe–Ni metal and Fe sulfide. The CP MMs also contain low-iron–manganese-enriched (LIME) and low-iron–chromium-enriched (LICE) ferromagnesian silicates, kosmochlor (NaCrSi2O6)-rich high-Ca pyroxene, roedderite (K, Na)2Mg5Si12O30, and carbonaceous nanoglobules. These components have previously been discovered in primitive solar system materials such as the CP IDPs, matrices of primitive chondrites, phyllosilicate-rich MMs, ultracarbonaceous MMs, and cometary particles recovered from the 81P/Wild 2 comet. The most outstanding feature of these CP MMs is the presence of kosmochlor-rich high-Ca pyroxene and roedderite, which suggest that they have building blocks in common with CP IDPs and cometary dust particles and therefore suggest a possible cometary origin of both CP MMs and CP IDPs. It is therefore considered that CP MMs are CP IDPs that have fallen to Earth and have survived the terrestrial environment.

Reference
Noguchia T, Ohashi N, Tsujimoto S, Mitsunari T, Bradley JP, Nakamura T, Toh S, Stephan T, Iwata N, Imae N (2014) Cometary dust in Antarctic ice and snow: Past and present chondritic porous micrometeorites preserved on the Earth’s surface. Earth and Planetary Science Letters 410,l 1-11
Link to Article [doi:10.1016/j.epsl.2014.11.012]

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