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.

Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc

“…In addition, the Ra excesses in the Izu arc lavas are within the range of Ra excesses observed for 276 MORB (see compilation in Lundstrom, 2003 Oshima and Niijima are similar to previously published data for these islands, eruptions, and in 284 some cases same samples by Yokoyama et al (2003) and Fukuda et al (2008) (Fig. 8).…”

Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

“…The ratios of the 1940 and 1962 rocks are limited in the ranges of 0.70346-0.70352 and 0.70341-0.70347, respectively. 87 Sr/ 86 Sr of the 1983 rocks (0.70338-0.70361) was reported in several papers (Notsu and Aramaki 1984;Kuritani et al 2003;Arakawa et al 1992;Yokoyama et al 2003), which is wider than that of 1940 or 1962. 87 Sr/ 86 Sr of the 18 August 2000 summit bombs (0.70343-0.70348) is similar to that of the 1940-1983 rocks of this study (0.70341-0.70347), whereas that of the 2000 submarine spatter (0.70345-0.70356) is more scattered.…”

Depositional setting, provenance, and tectonic-volcanic setting of Eocene–Recent deep-sea sediments of the oceanic Izu–Bonin forearc, northwest Pacific (IODP Expedition 352)

“…The La versus Th versus Sc discrimination plot (Figure 12 (d)) highlights the similarities of many of the Oligocene-early Miocene, generally up-slope sediments, with typical basalt-andesite compositions of the Izu-Bonin arc (e.g. Fryer et al 1990;Hochstaedter et al 1990Hochstaedter et al , 2001Gill et al 1992;Taylor and Nesbitt 1998;Kuritani et al 2003;Yokoyama et al 2003Yokoyama et al , 2006Tamura et al 2005Tamura et al , 2007Tollstrup et al 2010). In contrast, the mid-Pliocene to Holocene downslope sediments are more similar to volcanic ash from Honshu (e.g.…”