Mantle metasomatism and rapid ascent of slab components beneath island arcs:: Evidence from 238U-230Th-226Ra disequilibria of Miyakejima volcano, Izu arc, Japan -: art. no. 2329

U-238-Th-230-Ra-226 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. U-238-Th-230-Ra-226 disequilibria observed in lavas with large U-238 and Ra-226 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 (Th-230/Th-232) 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 Ra-226 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.