Mantle metasomatism and rapid ascent of slab components beneath island arcs: Evidence from 238U‐230Th‐226Ra disequilibria of Miyakejima volcano, Izu arc, Japan

Abstract

238U‐230Th‐226Ra systematics in lavas from Miyakejima volcano, Japan, are presented to estimate the timescale of magmatic processes beneath an island arc. Miyakejima volcano has four recent eruptive stages (Stages 1–4) starting >7000 BP. 238U‐230Th‐226Ra disequilibria observed in lavas with large 238U and 226Ra excesses imply metasomatism of depleted mantle by fluid‐related processes. This metasomatism is also suggested by trace element and Sr‐Nd‐Pb isotopic systematics in the same lavas. In the equiline diagram, the trends for two magmatic stages (Stages 1 and 2) are regarded as two different isochrons with a common initial (230Th/232Th) ratio, although the trend for Stages 3 and 4 is a magma mixing line. Our model calculations show that slab‐derived fluids can deliver some Th and a very rapid ascent time of the slab components in the mantle wedge (< 7 kyr) is inferred. This rapid ascent can be explained by nearly instantaneous material transport in the mantle wedge by a hydrofracture model for fluid and a channel flow model for melt. Such a timescale estimate is not increased even if melting processes that enhance 226Ra are taken into account. The age difference in the equiline diagram corresponds to the interval of individual fluid‐release events (13 kyr between Stages 1 and 2, and 5 kyr between Stages 2 and 3). Thus fluid release from the slab and subsequent magma generation occur as episodic events on a several‐kiloyear timescale.