[S-GC33] Solid Earth Geochemistry, Cosmochemistry

[SGC33-03] Mo-Sr-Nd-Pb isotopic compositions in Quaternary volcanic rocks: Northeast Japan (Hokkaido, Tohoku) arc and Izu islands

*Tatsuya Tamura¹, Tetsuya Yokoyama¹, Takeshi Kuritani², Mitsuhiro Nakagawa², James B Gill³, Hikaru Iwamori⁴, Kenta Ueki⁵ (1.Department of Earth and Planetary Science, School of Science, Tokyo Institute of Technology, 2.Department of Earth and Planetary Science, Faculty of Science, Hokkaido University, 3.University of California, Santa Cruz, 4.Earthquake Research Institute, Division of Earth and Planetary materials science, University of Tokyo, 5.Japan Agency for Marine-Earth Science and Technology, Research Institute for Marine Geodynamics)

Keywords:Northeast Japan arc, Izu islands, Molybdenum isotopic composition, Subduction zone

Subducting oceanic crusts plays an important role in the return of surface rocks, sediments, and seawater to the mantle. Furthermore, subduction zone processes such as dehydration of subducting slab induces volcanic and seismic activities [e.g., 1-3]. Several models have been proposed to elucidate the link between fluid source and volcanoes [e.g., 2,3], but uncertainties remain, especially for the formation of fluid sources in the rear arc (RA) region. In this study, we focus on the Mo isotope ratio (δ^98/95Mo = (^98/95Mo_sample / ^98/95Mo_NIST3134 - 1) × 1000) of arc volcanic rocks, which has been recognized as a useful tracer for elucidating the transport of material by aqueous fluid from subducting slabs. Volcanic rock samples with different subduction depths (DWB: Depth of Wadati-Benioff Zone) in Hokkaido, Tohoku and Izu Islands were analyzed using a double-spike method combined with thermal ionization mass spectrometry (TIMS). Sr-Nd-Pb isotope measurements were also performed using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) to compare variations in slab-derived components in subduction zone volcanoes with changes in DWB.
The δ^98/95Mo values in the Izu Islands decreased with increasing DWB, showing positive correlations with ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios, as well as with ¹⁴³Nd/¹⁴⁴Nd ratio [e.g., 4]. Samples from Rishiri in the RA showed lower values (δ^98/95Mo = -0.16 to -0.14‰) compared with those from Mt. Usu, Mt. Tokachi, and Mt. Meakan in the Hokkaido volcanic front (VF) (δ^98/95Mo = 0.20 to 0.35‰). These samples showed positive correlations with ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios, and a negative correlation with the ¹⁴³Nd/¹⁴⁴Nd ratio. On the other hand, samples from Tohoku VF showed higher δ^98/95Mo values (δ^98/95Mo = 0.26 to 0.67‰) compared to those from Izu Islands and Hokkaido. The Tohoku RA volcanoes also exhibited comparably high δ^98/95Mo values (δ^98/95Mo = 0.06 to 0.52‰), and a clear decreasing trend between VF and RA, as suggested by previous studies, was not observed (e.g., [5]).
The decrease in ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios in volcanic rocks from the VF to the RA in the Northeast Japan region indicates a decrease in the contribution of aqueous fluids with increasing DWB, while ¹⁴³Nd/¹⁴⁴Nd ratio suggests an increase in sedimentary and mantle components in the magma [e.g., 6]. Consequently, variations in Mo isotope ratios in Hokkaido have been suggested to fluctuate in response to the decrease in aqueous fluid components and the increase in slab melt components toward RA. Similarly, in the Izu Islands, the trends in δ^98/95Mo values and ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios suggest fluctuations associated with a decrease in aqueous fluid components and an increase in slab melt components toward the RA, consistent with previous studies [4]. Furthermore, the positive correlation in the ¹⁴³Nd/¹⁴⁴Nd ratio may reflect mantle heterogeneity of the wedge mantle caused by ancient mantle metasomatism driven by slab-derived fluid [e.g., 4].
A clear trend in δ^98/95Mo values related to DWB was not observed in the Tohoku region, which may be associated with the observation that a considerable number of samples in the Northeast RA exhibit higher ²⁰⁸Pb/²⁰⁴Pb ratios compared to other RA volcanoes. Given the analogous behavior of Pb and Mo in slab-derived fluids [7], it is plausible that RA samples with δ^98/95Mo values similar to those of the Tohoku VF underwent an event in which the δ^98/95Mo value and ²⁰⁸Pb/²⁰⁴Pb ratio increased concurrently. Alternatively, their δ^98/95Mo values and ²⁰⁸Pb/²⁰⁴Pb ratios could differ from the inferred mantle values, suggesting formation from sources with relatively high δ^98/95Mo values and ²⁰⁸Pb/²⁰⁴Pb ratios.
Reference [1] Bott and Dean, 1973, Nature, [2] Elliott et al., 1997, JGR, [3] Hawkesworth et al., 1993, Rev. EPS, [4] Villalobos-Orchard et al., 2020, GCA, [5] Li et al., 2021, Nature, [6] Kimura and Yoshida, 2006, J. Petrol., [7] Freymuth et al., 2015, EPSL