^1,2Enrico Bonatti, ^2,3Dee Breger, ⁴Tommaso Di Rocco, ¹Fulvio Franchi, ¹Luca Gasperini, ¹Alina Polonia, ⁵John Anfinogenov, ⁶Yana Anfinogenova
¹Istituto di Scienze Marine, CNR, U.O.S. Bologna, via Gobetti 101, 40129, Bologna, Italy
²Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA
³Micrographic Arts, P.O. Box 3088, Saratoga Springs 12866, NY, USA
⁴Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universität, Goldschmidtstraβe 1, D-37077 Göttingen, Germany
⁵Faculty of Geology and Geography, National Research Tomsk State University, 36 Lenin Avenue, Tomsk, 634050, Russia
⁶Institute of Physics and Technology, National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk, 634050, Russia

An exotic meter-size quartzitic boulder known as John’s Stone was found by John Anfinogenov in 1972 buried in permafrost close to the epicenter of the 1908 Tunguska blast in a region of Siberia dominated by Permian-Triassic Siberian Trap basalts. The boulder is made almost entirely of well-cemented quartz grains, mostly around 100μm in size; it contains zones with coarser or finer grain sizes. Rare zircon and rutile crystals are scattered within the quartz matrix. Quartz is often dissected by strain lamellae. The rock contains abundant scattered internal vugs rimmed by euhedral quartz crystals. We cannot exclude that John’s Stone is a fragment of a Permian granite-derived sandstone unit. However, based on structure, mineralogy and chemistry the quartzitic boulder may have originated due to silica deposition from hydrothermal solutions that had reacted with basaltic rocks. Anfinogenov et al. (2014) interpreted features observed in the permafrost at the base of the boulder as indicating it impacted from above, suggesting the boulder may be a meteorite, possibly of Martian origin, given the reported presence on Mars of silica-rich deposits. Triple oxygen isotope ratios determined on two samples of the quartzite reveal a terrestrial rather than a Martian meteorites composition. Oxygen isotope data suggest also that the precipitation of SiO2 could have occurred in equilibrium with hydrothermal water (δ18Ow ≈ -19.5 ‰) at the temperature of about 50°C. The thermal event that generated the quartzite may be related either to the century-old Tunguska event, or, more probably, to Permian-Triassic Siberian Traps magmatism, although an extraterrestrial origin cannot be completely ruled out.

Reference
Bonatti E, Breger D, Di Roccod T, Franchi F, Gasperini L, Polonia A, Anfinogenov J, Anfinogenova Y (2015)
Origin of John’s Stone: A quartzitic boulder from the site of the 1908 Tunguska (Siberia) Explosion. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.06.018]

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