Application of time–temperature–transformation and time–pressure–transformation diagrams for cooling- and decompression-induced crystallization to estimate the critical cooling rate required to form glassy obsidian

Abstract

The formation of glass from silicic magmas during eruption is an enigmatic process. To understand the formation of obsidian, we used time–temperature–transformation (TTT) and time–pressure–transformation (TPT) diagrams to estimate the critical cooling and decompression rates needed to form glassy obsidian. The TTT diagram is a contour map of the crystallized volume fraction as a function of the crystallization temperature and time. A contour line for a given crystal fraction in a time versus temperature diagram has a cone shape, with the “nose” of the curve corresponding to the minimum time and corresponding temperature required to generate a specific crystal fraction. We also present a TPT diagram, based on the TTT diagram, and liquidus curve for decompression-induced crystallization. We applied the TTT diagram to the Tokachi–Ishizawa obsidian from Shirataki, Hokkaido, Japan, and used it to estimate the critical cooling rate that would have been required to form its glassy texture by cooling-induced crystallization during eruption. For decompression-induced crystallization, we used the TPT diagram to estimate the critical decompression rate needed to form glassy obsidian. Based on our calculation results, the critical cooling and decompression rates are highly dependent on the interfacial energy between crystals and melt. We compared the estimated critical cooling and decompression rates with those estimated from microlite number density and a one-dimensional solution of the heat conduction equation. Our calculation results reveal that glassy obsidian can be formed during an eruption, particularly under conditions of high interfacial energy.