H2O-rich island arc low-K tholeiite magma inferred from Ca-rich plagioclase-melt inclusion equilibria

Abstract

In order to quantify H 2 O content and chemical composition of island arc low-K tholeiite magma that crystallized Carich plagioclase, melt inclusions of a typical island arc tholeiite from Izu-Oshima volcano (34°N 44′, 139°E 24′) were analyzed. Composition of studied plagioclase ranges widely from An 83 to An 95 . Composition of studied melt inclusions also shows wide variation, which suggests that the melt inclusions represent various stages of crystallization differentiation at Izu-Oshima volcano. Ca/Na ratios of plagioclase-hosted melt inclusions are comparable with compositions of aphyric lava, which preclude an exotic origin for the Ca-rich plagioclase. Analyzed H 2 O content of the melt inclusions ranges from 0.2 to 2.4 wt.% (0.2 to 1.4 wt.% for plagioclase-hosted melt inclusions and 0.8 to 2.4 wt.% for olivine-hosted melt inclusions).Ca/Na partition coefficient between plagioclase and hydrous basaltic melt, K D(Ca-Na) plag-melt , is empirically calibrated based on experimental data as Based on the equilibria between host plagioclase and melt inclusion and taking effect of overgrowth into consideration, 3 to 6 wt.% H 2 O in melt is required. The lower H 2 O content of the analyzed melt inclusions is probably due to the leakage of volatiles through the host crystal during decompression, eruption and quench. Variation in estimated H 2 O content in the melt at the time of crystallization of plagioclase (3 to 6 wt.%) can be due to polybaric crystallization from H 2 O-saturated melt.Keywords: melt inclusion, Ca-rich plagioclase, island arc low-K tholeiite magma, H 2 O content in melt, hydrous melting experiment tic melt inclusions from the subaqueous Izu arc front ranges from 1 to 5 wt.% (Straub and Layne, 2003). Takagi et al. (2005) claimed that optimal H 2 O content for crystallization of Ca-rich plagioclase from island arc tholeiite magma is ~5 wt.%. However, quantitative discussions on H 2 O content of island arc basalt at the volcanic front have not reached consensus to date. Sakuyama (1979) proposed lateral variation of H 2 O content in northeast Japan arc based on the crystallization sequence and temperature, which lead to the conclusion that the H 2 O content of island arc basalt at the volcanic front was small. Aoki et al. (1981) also proposed similar lateral variations of H 2 O content based on fluorine geochemistry assuming constant proportions of K 2 O:H 2 O:F during fractional crystallization. The 1986 eruption of Izu-Oshima volcano (34°N 44′, 139°E 24′), which is a typical frontal arc volcano and issues island arc low-K tholeiite magma, stimulated the discussion on H 2 O content in island arc basalt. Aramaki and Fujii (1988) calculated the H 2 O content of basaltic melt to explain the INTRODUCTIONQuantifying the H 2 O content of island arc basalt at the volcanic front, especially island arc low-K tholeiite magma, is fundamental in understanding the generation and differentiation of magma at island arc settings. Carich plagioclase (mostly An ≥90 ), commonly found in island arc basalt at the volcan...

Polybaric crystallization differentiation of H2O-saturated island arc low-K tholeiite magmas: a case study of the Izu-Oshima volcano in the Izu arc

“…The bulk H 2 O content ranged from 1 to 6 wt.%. Details of the procedures and techniques have been described by Hamada and Fujii (2007). In the melting experiments on MA43, plagioclase crystallized as the liquidus phase at all H 2 O contents (≤6 wt.%) and the crystallization temperature of plagioclase was linearly suppressed with increasing bulk H 2 O content (Figure 7a).…”

“…Other compositional attributes such as Ca/Na and Al/Si ratios are also critical in crystallizing Ca-rich plagioclase from basaltic melts (Hamada and Fujii 2007). While some researchers have debated whether island arc low-K tholeiite magma is dry or wet, other researchers have stated that both dry and wet primary magmas can be generated beneath frontal arc volcanoes (Tamura et al 2005;England and Katz 2010).…”

“…In their second series of experiments, Hamada and Fujii (2007) conducted hydrous melting experiments on two drilling core samples of volcanic rocks from the Izu-Oshima volcano, labeled as MA43 and MA44 (MgO approximately 5 wt.%; Additional file 2: Table S2), to constrain the origin of Ca-rich plagioclase. Synthesis of the hydrated glasses of MA43 and MA44 revealed that these two samples represent less differentiated liquid compositions on the higher-and lower-Al/Si trends, respectively (Figure 3).…”

Cooperation of upper and lower boundary-layer fractionations in a sheet-like intrusion: Composition and microstructure of the Aosawa dolerite sill in Yamagata Prefecture, northeastern Japan

“…The water content in the intruded magma is estimated from the equilibrium between the initial melt, the composition of which was esti- mated from the chilled margin compositions with the phenocrysts subtracted, and the average An content of the plagioclase -phenocryst core (An = 87.3). The result, according to Hamada and Fujii (2007), yields 2.3 wt% at 1150 °C (1.5 wt% at 1100 °C and 3.3 wt% at 1200 °C). We also performed MELTS calculations (Ghiorso and Sack, 1995) to obtain 1.2 wt% H 2 O to stabilize plagioclase in the initial melt at 1150 °C and 0.15 GPa.…”

The geochemical and petrological characteristics of prenatal caldera volcano: a case of the newly formed small dacitic caldera, Hijiori, Northeast Japan

“…No special techniques were applied to obtain X-ray signals except for Na, S, Cl, and F analysis, given as follows: Na loss associated with migration during electron bombardment was prevented using the defocused electron probe as well as by analyzing Na in first sequence with a short (4 second) counting time. Counting time was 20-s for Si, Ti, Fe, Ca, and K. The use of a defocused electron probe is one of the efficient ways to minimize Na loss and enhance the Na count (Hamada and Fujii 2007). For the S, Cl, and F analysis, a 50-s counting time was applied to reduce statistical error, yielding a reproducibility that was better than ±0.02 wt%.…”