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摘要
Calcic amphibole phenocrysts in the Tsurumidake summit (TS) lava, which was produced during the most recent eruption at Tsurumi Volcano (SW Japan) at around 7.5-10.5 ka, have been analyzed to determine the pre-eruptive conditions, such as temperature (T), pressure (P), oxygen fugacity (fO(2)), SiO2 content (SiO2melt), and FeOMgO ratio (FeOMgOmelt), of coexisting silicate melts in the magma reservoir beneath the volcano. Although most of the amphibole phenocrysts have been completely decomposed to a fine-grained opaque symplectite, similar to 6% of the grains remain intact. The degree of amphibole breakdown (DAB), defined as the ratio of the area of symplectite to the area of symplectite plus relict amphibole in each phenocryst, varies from similar to 20% to 100%. Compositional zoning was not observed in the amphibole grains, however, two distinct groups of amphibole phenocrysts have been identified based on their chemical compositions: group-I amphiboles, which are relatively Si-poor, and Al-[6]-rich, and have a relatively high Mg# [100Mg/(Mg + Fe2+)]; and group-II amphiboles, which are Si-rich, and Al-[6]-poor, and have a relatively low Mg#. Empirical equations for geothermobarometry, oxygen barometry and chemometry that exclusively rely on the amphibole composition were applied to estimate the T-P-fO(2)-SiO2melt-FeOMgOmelt conditions of the silicate melts with which the amphibole crystals equilibrated. The results show that group-I and group-II amphiboles equilibrated with andesitic melts (group-I melts) and dacitic-rhyolitic melts (group-II melts), respectively. The T-P-fO(2) conditions of group-I melts were estimated as similar to 374-483 MPa (similar to 13.9-17.9 km depth), similar to 950 degrees C, and NNO + 1.3, respectively, and those of group-II melts were similar to 93-242 MPa (similar to 3.4-9.0 km depth), 824-913 degrees C, and NNO + 0.6-1.7, respectively. The estimated T-P-fO(2)-SiO2melt-FeOMgOmelt conditions were almost constant for group-I melts, whereas the T, Delta NNO, and FeOMgOmelt values of group-II melts decreased with increasing SiO2melt. The DAB decreased as the temperature increased; low-DAB grains are absent in group-II amphiboles. Our results suggest that (1) a deeper reservoir (at a depth of similar to 16 km) stored an andesitic melt coexisting with group-I amphiboles at similar to 950 degrees C; (2) a chemically zoned reservoir at shallower depth (similar to 6.4 km) was filled with dacitic-rhyolitic melts that coexisted with group-II amphiboles at similar to 824-913 degrees C; (3) the TS lava was the product of magma mixing of at least three components, including the andesitic melt derived from the deeper reservoir, the dacitic and rhyolitic melts from the shallower reservoir and a mafic magma derived from the lower crust and/or upper mantle; and (4) the TS lava eruption occurred within a month or less of the magma mixing.
- 出版日期2017-5
