Origin of John's Stone: A quartzitic boulder from the site of the 1908 Tunguska (Siberia) explosion

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 100 μm 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. (Anfinogenov, J. et␣al. [2014]. Icarus 243, 139-147) 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.