Silicate-metallic spherules and the problem of the ignimbrite eruption mechanism: The Yakutinskaya volcanic depression

In the course of detailed mineralogical, geochemical, and petrological studies of volcanic rocks in the Yakutinskaya volcanic depression of the Eastern Sikhote-Alin volcanic belt (Primorye) silicate-metallic formations (small balls) were found to occur widely. This suggests a new approach to the evaluation of the mechanism that is responsible for the formation of specific large-volume “hot” explosions. The metallic nuclei (whose compositions vary from low-carbon cast iron to cohenite) are surrounded by symplectite, which consists of quartz, magnetite, and siliceous-potassic glass (without Na); thus, the entire formation can be considered as a siliceous-metallic spherule. Based on analyses of the gas phase, the spherules consist of hydrogen and methane. The calculation of oxidation reactions that involve these gases shows that the process entails a significant decrease of volume and heat release. This suggests that the mechanism of “hot” ignimbrite explosions consists of the collapse of the roof of the magma chamber into the explosion zone and the formation of a collapse caldera with the ejection of the melt and its subsequent dispersion on the surface. This is facilitated by significant overheating of magma and its saturation with hydrogen, which is accompanied by a sharp decrease in viscosity by several orders of magnitude, even for such highly siliceous melts.