11:00 AM - 11:15 AM
[SVC34-08] Pyrrotite oxidation as a proxy for air entrainment in eruption columns
Keywords:pyrrhotite, oxidation, explosive eruption, eruption column
Mineral phases of magmatic sulfide and occurrence of pseudomorph (Mt and Hm) are different among the three eruption samples. Po is observed in the groundmass of the pumice from the Asama 1783 eruption and the Sakurajima 2010 eruption, whereas the sulfide in the Shinmoedake 2011 groundmass is intermediate solid solution (iss; Cu1-xFe2+yS2-z). Po is sometimes replaced to form pseudomorph, while iss shows no replacement. The porous texture of the Po pseudomorph, which indicates a reaction of Po with air, is similar to that observed for the oxidation product having columnar microstructure reported in the pumice from the Sakurajima 1914 eruption (Matsumoto and Nakamura, 2017). Approximately half of the Po grains are partly or entirely replaced by the pseudomorph in the Asama 1783 pumice, as observed in the Sakurajima 1914 samples. On the other hand, proportion of the pseudomorphic Po grains is less than 20% in the Sakurajima 2010 pumice. This difference indicates that the Asama 1783 pumice, together with the Sakurajima 1914 pumice, has been more efficiently oxidized than the Sakurajima 2010 pumice. The preservation of iss in the Shinmoedake 2011 pumice is probably due to the difference in oxidation mechanism between iss and Po. The efficient oxidation of Po during the two Plinian eruptions (i.e., the Asama 1783 and the Sakurajima 1914 eruptions) can be attributed to the longer duration in which temperature of pumice were kept high in the eruption columns than that in the Vulcanian column. These results underscore the potential use of Po oxidation for the purpose of constraining the temperature and fO2 of eruption columns that reflect eruption intensity of explosive eruptions.