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

Abstract

Hydrogen in nominally anhydrous minerals (NAMs) in volcanic rocks can be used as a proxy for dissolved H 2O in melt prior to eruption. Plagioclase is a NAM that accommodates hydrogen in concentrations of up to hundreds of wt. ppm H 2O. 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 H 2O-saturated and undergo polybaric degassing during the magma ascent. The analyzed OH concentrations in plagioclase range from 20 to 300 wt. ppm H 2O, and three levels of OH (20–80 wt. ppm H 2O, 100–180 wt. ppm H 2O, and 220–300 wt. ppm H 2O) are found. These variations in OH indicate that crystallized plagioclase is equilibrated with H 2O-saturated melt at three depths beneath the Izu-Oshima volcano prior to eruption: near the surface level (≈ 1 wt.% H 2O in melt), at a 4-km-deep magma chamber (≈ 3 wt.% H 2O in melt), and at a 8–10-km-deep magma chamber (≈ 5 wt.% H 2O in melt). It is proposed that deep-seated island arc low-K tholeiitic basalt magmas erupting from frontal-arc volcanoes are richer in H 2O than previously thought, containing approximately 1 wt.% H 2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at “low-pressure conditions”.