Syn-eruptive desulfidation of pyrrhotite in the pumice of the Sakurajima 1914-15 eruption: Implication for potential magma ascent rate meter

Keiko Matsumoto, Michihiko Nakamura

Research output: Contribution to journalLetterpeer-review

3 Citations (Scopus)

Abstract

Pyrrhotite (Po) occurs as inclusions and as isolated crystals in pumice from the 1914-15 eruption of the Sakurajima volcano, Kyushu, Japan. The Po crystals have partly reacted to form spongy Fe oxides. A similar texture has been reported in some previous studies (Hattori, 1993), but the mineral phases and formation processes of the spongy Fe oxides have not been clarified. Our quantitative and compositional map analyses with electron probe microanalysis (EPMA) reveal that the spongy Fe oxides are mostly magnetite (Mt), with a thin rim (<3 μm) of hematite on rare occasions. The spongy texture includes unreacted regions of Po, mesh-like pores, and S-rich spots, showing that it was formed by desulfidation of Po. No Ti was detected, even in the outermost rim; this indicates that the reaction occurred syn-eruptively. According to diffusion calculations, the spongy Mt was formed during the 4 h preceding quenching. Thermodynamic calculations showed that Po is stable at log fO2 < NNO + 2 at a pressure of 1 bar and magmatic temperature, which is 1-2 log units higher than the usual magmatic fO2. These constraints on the timing and oxidation condition of desulfidation lead to the conclusion that the reaction was caused by oxidation of the magma in a shallow volcanic conduit, not in magma chamber processes. The pumice groundmass consists mostly of glass, indicating that the rate of the desulfidation reaction is faster than the decompression-induced crystallization of microlites in the andesitic magma. Therefore, the desulfidation reaction of Po has the potential to be used as a geospeedometer for very fast magma ascent in vigorous explosive eruptions.

Original languageEnglish
Pages (from-to)206-211
Number of pages6
JournalJournal of Mineralogical and Petrological Sciences
Volume107
Issue number5
DOIs
Publication statusPublished - 2012

Keywords

  • Desulfidation
  • Magma ascent rate
  • Pyrrhotite
  • Sakurajima volcano
  • Spongy magnetite

ASJC Scopus subject areas

  • Geophysics
  • Geology

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