^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]

Copyright Elsevier

¹A. Kereszturi, ²S. Ormandi, ²S. Jozsa
¹Research Center for Astronomy and Earth Sciences, Konkoly Astronomical Institute, Budapest, Hungary
²Department of Petrology and Geochemistry, Faculty of Science, Hungarian Academy of Sciences, Eötvös Lorand University of Sciences, Budapest, Hungary

In analyzing a thin section of the NWA 6604 CK4 meteorite, only altered chondrules and various components that are probably left behind the destruction of former chondrules can be observed. We suggest that melting, grain size decrease, resorption of the original chondrules, and crystallization of opaque minerals were the main processes that destroyed the chondrules. Four different events could be identified as having occurred during this alteration. First, opaques crystallized along former fractures producing chains of separated grains. Later, opaques and Ca-rich minerals crystallized together in veins and large melt pockets; this was the strongest recrystallization phase involving the largest volume of melt. This occurred along different fractures than the first phase above. During the third phase, only Ca-rich plagioclase crystallized along thin veins, and in a fourth phase, fractures formed again, partly along those formed during the second phases but without substantial mineral infill. Two simple possible case models should be considered for this meteorite: alteration by purely impact-driven processes or mainly by melt-driven processes. Although for CK4 chondrites, the shock-produced alteration driven by impact is the more accepted and widespread approach, melting is also compatible with the observed textural characteristics of chondrule destruction. During melting, recrystallization took place producing iron-rich minerals earlier and Ca-Si-rich ones later. The penetration of melts into veins contributed in the chondrule destruction. The stress directions also changed during these alterations, and minerals that formed later filled differently oriented fractures than the earlier ones. From our observations, we favor a view where heat-driven melting and recrystallization produced the destruction and uniform mineralogy in the sample.

Reference
Kereszturi A, Ormandi S, Jozsa S (2015) Possible melting produced chondrule destruction in NWA 6604 CK4 chondrite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12458]

Published by arrangement with John Wiley&Sons