Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu–Bonin Arc, Japan.

Basalt–basaltic andesite (<55 wt % SiO2) and dacite–rhyolite (66–74 wt % SiO2) are the predominant eruptive products in the Sumisu caldera volcano, Izu–Bonin arc, Japan. The most magnesian basalt (8·5% MgO), as well as some of the other basalts, has a low Zr content (20–25 ppm), and cannot yield basalts with higher Zr contents (29–40 ppm) through fractionation and/or assimilation. The high- and low-Zr basalts have different phenocryst assemblages, olivine, plagioclase and pyroxene phenocryst chemistries, REE (rare earth element) patterns, and fluid-mobile element/immobile element ratios. Estimated primary olivine compositions are more magnesian (>Fo91) in the low-Zr basalts compared with those in high-Zr basalts (<Fo89). The low-Zr basalts contain up to 11 vol. % augite, but many high-Zr basalts are free of augite, which appears only in their more differentiated products. The low-Zr basalts are considered to be hydrous magmas in which olivine crystallizes first followed by augite and plagioclase, whereas the high-Zr basalts are dry. The low-Zr basalts have higher U/Th ratios than the high-Zr basalts. We suggest that both dry and wet primary basalts existed in the Sumisu magmatic system, each having different trace element concentrations, mineral assemblages and mineral chemistry. The lower contents of Zr and light REE and magnesian primary olivines in the wet basalts could have resulted from a higher degree of partial melting (∼20%) of a hydrous source mantle compared with ∼10% melting of a dry source mantle. The Sr, Nd and Pb isotope compositions of the wet and dry basalts are similar and are limited in range. These lines of evidence indicate that a mantle diapir model might be applicable to satisfy the configuration of such a mantle source region beneath a single volcanic system such as Sumisu. [ABSTRACT FROM AUTHOR]