Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu–Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab

Abstract

The arc-front volcanoes of Sumisu (31 58N, 1408E) andTorishima (30 58N, 140 38E) in the central Izu^Bonin arc are similar in size and rise as relatively isolated edifices from the seafloor.Together they provide valuable along-arc information about magma generation processes. The volcanoes have erupted low-K basalts originating from both wet and dry parental basaltic magmas (low-Zr basalts and high-Zr basalts, respectively). Based on models involving fluid-immobile incompatible element ratios (La/Sm), the parental basalts appear to result from different degrees of partial melting of the same source mantle ( 20% and 10% for wet and dry basalt magmas, respectively). Assuming that the wet basalts contain greater abundances of slab-derived components than their dry counterparts, geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab. Using an extensive set of new geochemical data fromTorishima, where the top of the downgoing slab is about 100 km deep, we find that Cs, Pb, and Sr are variably enriched in the low-Zr basalts, which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting. These elements are interpreted to be selectively concentrated in slab-derived metasomatic fluids. Variations in K, high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high-Zr basalts; these elements are not contained in the slab-derived fluids. Rb and Ba exhibit variable behaviour in the low-Zr basalts, ranging from immobile, similar to K, to mildly enriched in some low-Zr basalts.We suggest that the K-rich mica, phengite, plays an important role in determining the composition of fluids released from the downgoing slab. In arcfront settings, where slab depth is 100 km, phengite is stable, and the fluids released from the slab contain little K. In back-arc settings, however, where the slab is at 100^140 km depth, phengite is unstable, and K-rich fluids are released.We conclude that cross-arc variations in the K content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting.