238U-230Th radioactive disequilibrium in the northern Izu arc: (230Th/232Th) in the sub-arc mantle

Abstract

Major and trace element abundances, Sr and Nd isotopic ratios, and 238 U-230 Th radioactive disequilibria have been analyzed for samples from five volcanoes, Oshima, Miyakejima, Niijima, Teishi Knoll and Fuji, located in the northern Izu arc to investigate across-arc and regional variations of chemical compositions of mantle and fluids expelled from a subducting slab. The abundances and abundance ratios of both fluid mobile and immobile trace elements show across-arc variations.All samples but one have radioactive disequilibria, with ( 238 U/ 230 Th) greater than unity (herein, a ratio in parentheses denotes the activity ratio). The observed disequilibrium is similar to those reported for other arc systems, such as Mariana, Tonga, Kermadec, Chile. It originates from higher mobility of uranium relative to thorium during dewatering of the subducting slab. Th) contributed from sediment melt, which dominates the budget of Th in the sub-arc mantle. It also suggests that influence of fluid addition before the final one had not caused a variation in Th isotopic compositions of the sub-arc mantle.

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).…”

Sources and physicochemical characteristics of fluids along a subduction‐zone megathrust: A geochemical approach using syn‐tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex

“…After centrifugation, Ca and Ba were separated using 2 M HCl and 2 M HNO 3 , respectively, with a cation exchange resin (AG50W‐X8; Bio‐Rad Laboratories Inc., USA), and the REEs were recovered by 6 M HCl prior to analysis. The precision of measured REE concentrations in rock samples is ∼10% of the quoted abundances (Table 2) [ Fukuda et al , 2008].…”

Molybdenum isotopic compositions in volcanic rocks from Northern Izu arc (Izu-Oshima, Niijima, and Kozushima)

“…(2) The δ 98/95 Mo values of N-Izu VF basalts (Izu-Oshima) are higher than those of MORB and DM, which is due to the heavy Mo isotopes preferentially escaping into aqueous fluids on the VF side due to the stability of light Mo isotopes in rutile in the subducting slab during dehydration. Fukuda et al, 2008;Kimura et al, 2010), yellow triangles: Toshima (Isshiki., 1978;Kimura et al, 2010), purple circles: Udonejima (Isshiki, 1987;Kimura et al, 2010), green diamonds:…”

“…Niijima (filled : this study / open : Arakawa et al, 2017Arakawa et al, , 2019Fukuda et al, 2008;Kimura et al, 2010;Koyaguchi, 1986) red triangles: Kozushima (filled: this study / open: Isshiki, 1982Taniguchi et al, 1990;Yamashita et al, 2005), light green crosses: Teishi knoll (Fukuda et al, 2008), gray circles: Miyakejima (Freymuth et al, 2016b;Fukuda et al, 2008;Yokoyama et al, 2003), indigo blue circles: Hachijojima and nearby seamount chains (Ishizuka et al, 2008;Nakano et al, 1997;Oiwa et al, 2023), gray squares: Izu-Torishima (Tamura et al, 2007).…”