Pyrrhotite oxidation as a proxy for thermal structure of eruption clouds: A comparative study of Plinian and Vulcanian eruptions of Asama and Sakurajima volcanoes, Japan

Abstract

Thermal expansion of a volcanic eruption cloud by entrainment of air is a primary control of its behavior. Oxidation of the pyroclasts in the eruption cloud can be an indicator of the degree of air entrainment and thermal disequilibrium during its development. Here, the oxidation texture of sulfide minerals (pyrrhotite) in four andesitic pyroclastic eruptions of Asama and Sakurajima volcanoes was investigated, in order to highlight the contrast between the air entrainment and thermal disequilibrium processes of the Plinian and Vulcanian eruption columns. Though oxidized pyrrhotite crystals were commonly found in the pumice-fall deposit of these four Plinian and Vulcanian eruptions, significant differences in the component proportion and degree of oxidation of the pyrrhotite crystals were found between Plinian and Vulcanian products. Greater proportions of oxidized pyrrhotite crystals in the Asama 1783 and Sakurajima 1914 Plinian pumice deposits than those in the Asama 2004 and Sakurajima 2010 Vulcanian eruptions indicate that heat retainment of pumice clasts and the continuous ejection of eruption clouds in the Plinian eruptions kept pumice in the eruption cloud at high temperature during the entrainment of air and expansion of the cloud. In contrast, the rapid thermal equilibrium of pumice clasts and the short-lived episodic ejection of Vulcanian eruptions reduced the temperature of pumice in the eruption cloud quickly. This suggests that pyrrhotite crystals have the potential to be indicators of the development of temperature inside an eruption cloud, reflecting the process of thermal disequilibrium of pyroclasts and of air entrainment.