10:15 AM - 10:30 AM
[SVC44-06] Syn-eruptive breakdown of pyrrhotite: a marker of pyroclasts oxidation and air entrainment into eruption columns
Keywords:pyrrhotite, oxidation, air entrainment, Sakurajima, eruption column
Most of the Po grains in pumices from the Plinian eruption partly or entirely breakdown into Fe oxides. The Fe oxides are composite of magnetite (Mt) and hematite (Hm), forming a columnar structure with pores along certain Po crystallographic planes. The Mt in the Fe oxides is Ti-free, indicating that the Fe oxides were quenched immediately after their formation. The Hm stable fO2 is at least ~4 log units higher than the magma chamber fO2 estimated from the (Fe+Cu)/S ratio of Po. This large increase of fO2 within a rapid quenching is attributed to interaction with air during the Plinian eruption. In a clastogenic lava from the same eruption, columnar Fe oxides are almost completely oxidized to Hm. The dominance of Hm is consistent with the formation processes of a clastogenic lava in which high fO2 and high temperature are maintained during welding. Compared with oxidation product of Po reproduced by previous experiments in oxidized gases, the columnar Fe oxides are estimated to have formed as follows: the oxidation reaction produced Mt with outgassing of SO2 (3Fe1-xS + (5-2x)O2 = (1-x)Fe3O4 + 3SO2). Porous reaction zone formed during this outgassing stage allowed the SO2 gas to escape and the oxygen to access to the reaction front easily, leading to the rapid formation of Mt column. Then formation of Hm without outgassing followed (2Fe3O4 + 1/2O2 = 3Fe2O3).
To constrain the timing of columnar Fe oxides formation, we compared the Hm width with reaction distance calculated from the experimentally determined growth rate of Hm assuming a conductive cooling of pumice. The average Hm width of grains located near the surface of pumices from the Sakurajima ranges 0.86–1.22 μm. On the other hand, even in the center of the pumice with 1 cm radius, the calculated Hm width is 0.35 μm. The thicker Hm width of the natural sample indicates that the pumice surfaces were maintained at a high temperature for a long time in the eruption column. From these results, we propose that Po can be a quantitative marker of magma–air interaction applicable to the dynamic processes of volcanic eruptions.