Title
Polybaric degassing of island arc low-K tholeiitic basalt magma recorded by OH concentrations in Ca-rich plagioclase
Author
Morihisa Hamada, Tatsuhiko Kawamoto, Eiichi Takahashi, Toshitsugu Fujii
Year
2011
Journal
Earth and Planetary Science Letters
Abstract
Hydrogen in nominally anhydrous minerals (NAMs) in volcanic rocks can be used as a proxy for dissolved H2O in melt prior to eruption. Plagioclase is a NAM that accommodates hydrogen in concentrations of up to hundreds of wt. ppm H2O. The species of hydrogen in volcanic plagioclase is structural OH. We report the analytical results of OH concentrations in Ca-rich plagioclase from the 1986?1987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in Izu arc. We demonstrate that the island arc low-K tholeiitic basalt magmas erupting from the frontal-arc volcanoes are H2O-saturated and undergo polybaric degassing during the magma ascent. The analyzed OH concentrations in plagioclase range from 20 to 300 wt. ppm H2O, and three levels of OH (20?80 wt. ppm H2O, 100?180 wt. ppm H2O, and 220?300 wt. ppm H2O) are found. These variations in OH indicate that crystallized plagioclase is equilibrated with H2O-saturated melt at three depths beneath the Izu-Oshima volcano prior to eruption: near the surface level (? 1 wt.% H2O in melt), at a 4-km-deep magma chamber (? 3 wt.% H2O in melt), and at a 8?10-km-deep magma chamber (? 5 wt.% H2O in melt). It is proposed that deep-seated island arc low-K tholeiitic basalt magmas erupting from frontal-arc volcanoes are richer in H2O than previously thought, containing approximately 1 wt.% H2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at “low-pressure conditions”
Instrument
IRT-5000
Keywords
FTIR microscopy, Water in nominally anhydrous minerals, island arc basalt, volcanic frontmelt inclusion, excess degassing, Izu arc