[S-GC46] Solid Earth Geochemistry, Cosmochemistry

2:30 PM - 2:45 PM JST(5:30 AM - 5:45 AM UTC)

[SGC46-04] Constraints on slab-derived components beneath the Northeast Japan and Izu arcs from Molybdenum isotopic compositions

*Tatsuya Tamura^1,2, Tetsuya Yokoyama¹, James B Gill³, Takeshi Kuritani⁴, Hikaru Iwamori⁵, Kenta Ueki⁶, Takashi Kudo⁷ (1.Institute of Science Tokyo, 2.Tono Geoscience Center Japan Atomic Energy Agency , 3.University of California, Santa Cruz, 4.Hokkaido University, 5.The University of Tokyo, 6.Japan Agency for Marine-Earth Science and Technology, 7.Research Institute of Geology and Geoinformation, Geological Survey of Japan, AIST)

Keywords:

Subduction zone,Mo isotopic composition,Northeast Japan arc,Izu arc

Physicochemical processes associated with subduction zone activities play an important role in the Earth’s material cycling from the surface to the deep mantle, and slab-derived fluids and melts are also crucial for understanding seismic and volcanic activity in subduction zone settings [e.g., 1,2]. Nevertheless, the specifics of this model remain a topic of considerable debate, and the provenance of the slab-derived fluid has yet to be fully elucidated. Over the past decade, Molybdenum (Mo) isotope ratio (δ^98/95Mo = {(⁹⁸Mo/⁹⁵Mo)_sample/(⁹⁸Mo/⁹⁵Mo)_{NIST SRM 3134} – 1}×10³) of volcanic rocks in subduction zone has been used as a one of the valuable tracers to elucidate material transport by slab dehydration and Earth’s material cycling (e.g., [3-6]). However, studies on across-arc variations in δ^98/95Mo values from the volcanic front (VF) to the rear arc (RA), as well as comparative studies of multiple subduction zone volcanoes using consistent analytical approaches, remain limited. To address this issue, we analyzed major and trace elements, as well as Mo, Sr, Nd, and Pb isotopic compositions, of volcanic rocks from the Northeast Japan (NE Japan) arc and the Izu arc, where volcanoes are distributed from the VF to the RA, in order to investigate across arc variations in δ^98/95Mo values of volcanic rocks and arc specific differences in their behavior.
In the Izu arc, volcanic rocks from VF exhibit higher δ^98/95Mo values (δ^98/95Mo = +0.05±0.19‰, 2sd, N = 29 [this study, 5, 6]) than those from the RA (δ^98/95Mo = −0.19±0.08‰, 2sd, N = 9), and these variations correlate with trace element ratios (e.g., Ba/Th and La/Sm) as well as Sr and Nd isotopic compositions. In contrast, NE Japan arc exhibit a wide range of δ^98/95Mo values and do not show a clear across-arc variation comparable to that observed in the Izu arc, even when compared with Ba/Th ratios and Sr isotopic compositions (VF: δ^98/95Mo = +0.03±0.38‰, 2sd, N = 20; RA: δ^98/95Mo = −0.02±0.13‰, 2sd, N = 7). Considering the characteristics of slab-derived components of NE Japan and Izu arcs [8], the contrasting behavior of δ^98/95Mo values in each subduction zone may reflect not only the effects of crustal assimilation and fractional crystallization, but also differences in the relative contribution of sediment-derived components. In the Izu arc, where slab-derived aqueous fluids are dominant, δ^98/95Mo values show strong correlations with Ba/Th ratios and Sr isotopic compositions, suggesting that a progressive decrease in aqueous fluids derived from serpentinite or altered oceanic crust with the depth of the Wadati-Benioff zone is closely associated with variations in the δ^98/95Mo values of volcanic rocks. In contrast, the NE Japan arc is characterized by a relatively low geothermal gradient along the subducting slab, which may allow sediment-derived aqueous fluids that are likely lost at shallower depths beneath the Izu arc to be transported to greater depths beneath the slab [9]. The wide range of δ^98/95Mo values observed in VF samples from the Northeast Japan arc therefore suggests an influence from heterogeneous sediment-derived components.

References
[1] Tatsumi and Eggins 1995, Blackwell Science; Frontiers in Earth Sciences, 211. [2] Stern, 2002, Rev. Geophys., 40.4, 3-1, [3] Li et al., 2021, Nat. Commun., 12.1, 6015, [4] Willbold and Messling, 2023, Geochem. Geophysics. Geosyst., 24.9, e2023GC011007, [5] Tamura et al., 2024, Geochem. J., 58.2, 51-70, [6] Villalobos-Orchard et al., 2020, Geochim. Cosmochim. Acta, 288. 68-82. [7] Li et al., 2024, J. Geophys. Res. Solid Earth. 129.2 e2023JB028169, [8] Nakamura and Iwamori, 2009, Gondwana. Res., 16.3-4, 431-445, [9] Nakamura et al., 2019, Gondwana. Res. 70, 36-49.