Molybdenum isotopic compositions in Oki-Dogo Island basalts

Abstract

Subduction zones play an important role not only in surface volcanism and seismic activity but also in material cycling from the Earth's surface to mantle [e.g., 1]. In recent years, Molybdenum (Mo) isotopic compositions have been used to trace the origin of slab-derived fluids in subduction zones and the recycled oceanic crust [e.g., 2]. Mo isotopic compositions have also been studied in volcanic rocks from several oceanic arcs and oceanic island basalts (OIB), but the mechanism of recycle processes of subducted oceanic crust are still unclear [e.g., 2-4]. Oki-Dogo Island is an island in the Japan Sea, situated on the rear arc side of southwestern Japan, where a variety of alkali volcanic rocks from the Miocene to Quaternary period have been reported [e.g., 5]. In this study, we attempted to estimate the recycled slab material into the source magma in alkali volcanic rocks after Oki-Dogo Island by determining the Mo isotopic compositions of alkali basalts using the double-spike technique with TIMS.Mo isotopic compositions of Oki-Dogo Island basalts show relatively lower values (-0.30 to -0.11, N=4) than the estimated depleted mantle [6]. This is also similar to the range of Mo isotopic compositions in the OIB and Mariana Trough in previous studies [4,7]. Previous studies have suggested that such Mo isotopic compositions, which are lower than the estimated DM value, together with less radiogenic Pb isotopic composition and Ce/Mo ratios in volcanic rocks, can be explained by the effect of recycled oceanic crust [3,4]. Oki-Dogo Island basalts also show less radiogenic Pb isotopic compositions and low Ce/Mo ratios, similar to or lower than those in previous studies, indicating that they may have received contributions from the recycled component. In addition, the Mo isotopic compositions of the Oki-Dogo Island basalts shows no clear correlation with Mo concentration but a negative correlation with the La/Yb ratio, suggesting that the Mo isotopic composition may have changed with magmatic differentiation within the basalts. Therefore, it is necessary to investigate this issue in more detail in the future.References [1] Spandler and Perard, 2013, Lithos., [2] Freymuth et al., 2015, EPSL, [3] Cai et al., 2024, Chem. Geol., [4] Willbold and Elliott, 2023, EPSL, [5] Uchimizu, 1966, J. Fac. Sci. Univ. Tokyo, [6] Uto et al., 1994, GJ, [7] Kobayashi et al., 2002, GKK, [8] McCoy-West et al., 2019, Nat. Geosci., [9] Li et al., 2021a, Geol.