Polybaric degassing of island arc low-K tholeiitic basalt magma recorded by OH concentrations in Ca-rich plagioclase

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”.

Plagioclase is one of the nominally anhydrous minerals (NAMs) and accommodates hydrogen of up to hundreds of wt. ppm H2O. Hydrogen in NAMs can act as a proxy for dissolved H2O in melt prior to eruption. In this study, we analyzed the OH concentration in plagioclase from the Izu-Oshima summit eruptions during 1986–1987 using mid-infrared spectroscopy. The OH concentration in plagioclase (20–300 wt. ppm H2O) differs from eruptive styles; a Strombolian eruption, an effusive eruption emitting lava flow, and a less energetic eruption emitting flaky bombs. The observed variations in OH concentration can be explained by combination of crystallization from H2O-saturated melt and diffusive loss of hydrogen from plagioclase crystal during eruption.Figure optionsDownload high-quality image (123 K)Download as PowerPoint slideHighlights
► The island arc low-K tholeiitic basalt magma is proved to be H2O-saturated.
► The island arc low-K tholeiitic basalt magma undergoes polybaric degassing.
► The OH concentration in plagioclase is a useful proxy for H2O-rich arc magma.
► The H2O concentration of arc tholeiitic basalt magma is higher than ever thought.