Generation of rear-arc magmas induced by influx of slab-derived supercritical liquids: Implications from alkali basalt lavas from Rishiri Volcano, Kurile arc

Magma generation processes were investigated for alkali basalt lavas from Rishiri Volcano, located at the rear of the Kurile arc, using major and trace elements and Sr, Nd, Pb and Th isotopic data. The Numaura and the Araragiyama lava flows, investigated in this study, show a significant variation in TiO2 contents (1.0-1.4wt %) despite a limited variation in SiO2 content (48.5-50.0wt %); TiO2 contents correlate positively with Nd-143/Nd-144 and negatively with Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204. The compositional variations of the lavas cannot be explained by magma chamber processes, such as fractional crystallization, crustal assimilation and magma mixing, and they are suggested to have formed principally during magma generation. The variation of the TiO2 contents essentially reflects a variation of the degree of partial melting (from similar to 2 to similar to 3%) of the source mantle, and it is inferred that the melting degree correlated positively with amounts of slab-derived materials influxed into the melting region. The melting appears to have occurred progressively under isothermal and isobaric conditions, as slab-derived materials were continuously supplied. The geochemical variations in the lavas can be explained by mixing of depleted mid-ocean ridge basalt source mantle with slab-derived materials consisting of an altered oceanic crust component and a sediment component. The slab-derived materials are likely to have contained not only Sr, Ba, Pb and U, but also significant amounts of Nd and Th that are not highly soluble in aqueous fluids. The materials are thus suggested to have been supercritical liquids, and it is suggested that magma generation occurred at depths greater than that at which supercritical liquids were decomposed into aqueous fluid and silicate melt components. The lava samples show U-238-(230) Th disequilibrium with 10-20% of Th-230 excess; this Th-230 enrichment resulted primarily from the high-Th nature of the slab-derived materials.