5:15 PM - 7:15 PM
[BCG06-P13] Marine Os isotopic composition during the middle Eocene reconstructed from the umber deposit in the Setogawa Belt, Shizuoka, central Japan.
Keywords:umber, strata-bound ferromanganese deposit, hydrothermal sediment, Os isotopic ratio, Setogawa Belt, middle Eocene
The osmium (Os) isotopic ratio (187Os/188Os) of seawater is governed by the relative contributions of different Os sources: the continental crust, which has a radiogenic signature (187Os/188Os = ~1.4), and mantle- and extraterrestrial-derived materials, which exhibit an unradiogenic signature (187Os/188Os = ~0.126) [1]. Therefore, the seawater Os isotopic ratio has been extensively utilized as a geochemical tracer for reconstructing solid-Earth processes, including continental weathering, magmatism, hydrothermal activity, and meteorite impact events throughout Earth’s history. Previous studies have reconstructed seawater Os isotope records using hydrothermal sediments enriched in Fe-Mn oxides, as well as hydrogenous ferromanganese crusts, both recovered from seafloor [2]. In addition, reconstructions of ancient seawater Os isotopic compositions have been pursued using older seafloor sediments that were subducted and incorporated into accretionary complexes. These offer a unique geological archive for capturing the long-term variations in seawater Os isotopic ratios throughout the Phanerozoic [3-8].
In this study, we aim to reconstruct seawater Os isotopic ratios in the geologic past, using strata-bound Fe-Mn deposits (hereafter referred to as "umber") preserved within Japanese accretionary complexes. Umber is a mudstone with highly enriched in Fe-Mn oxides and was exploited as a low-grade Fe-Mn resource decades ago. Because umber typically occurs in association with mid-ocean ridge basalt (MORB), most umber deposits are considered to have originated as hydrothermal sediments accumulated near mid-ocean ridges. During the deposition of hydrothermal sediments, Fe-Mn oxide particles absorbed various trace elements, including Os, from ambient seawater. Thus, umber is considered an ideal material for reconstructing the Os isotopic ratio of ancient seawater [6–13].
This study focuses on the Ichiizawa umber deposits within the Setogawa subbelt, a part of the Setogawa Belt (belong to the Shimanto Belt), Shizuoka, in the Japanese accretionary complex. The major lithology accompanied by the umber deposits are basalt, limestone, and bedded chert [14]. Basalts outcropped in proximity to the Ichiizawa deposit exhibits a geochemical composition similar to MORB [15]. Based on age of the MORB-like basalts [15], the depositional age of the Ichiizawa deposit is estimated to be in the middle Eocene (48–42 Ma). In this presentation, we present a detailed geochemical characteristics of the Ichiizawa umber and their surrounding rocks. We then discuss the origin and depositional environment of umber within the Setogawa Belt. Furthermore, we examine the Os isotope ratio of middle Eocene seawater, as reconstructed from these umber deposits.
[1] Peucker-Ehrenbrink and Ravizza, 2000, Terra Nova, 12, 205-219, [2] Peucker-Ehrenbrink and Ravizza, 2020, Geologic Time Scale 2020 (Chapter 8), 239-257, [3] Tejada et al., 2009, Geology, 37, 855–858, [4] Sato et al., 2013, Nature Communications, 4, 2455, [5] Matsumoto et al., 2020, Sci. Rep., 10, 12601, [6] Fujinaga et al., 2022, Ore Geol. Rev., 142, 104683, [7] Kuwahara et al., 2022, Glob. Planet. Change, 216, 103920, [8] Yano et al., 2023, J. Asian Earth Sci., 241, 105480, [9] Fujinaga et al., 2001 Shigen Chishitsu, 51 29-40, [10] Kato et al., 2005b, Resour. Geol., 55, 291-299, [11] Ravizza et al., 2001, Earth Planet. Sci. Lett., 188, 369–381, [12] Kato et al., 2005a, Geochem. Geophys. Geosyst., 6, Q07004, [13] Kato et al., 2011, Gondwana Res., 20, 594-607, [14] Sugiyama and Shimokawa, 1989, J. Struct. Geol. Soc. Japan 34, 173-188, [15] Sakamoto et al., 1993, J. Geol. Soc. Japan, 99, 9-28.
In this study, we aim to reconstruct seawater Os isotopic ratios in the geologic past, using strata-bound Fe-Mn deposits (hereafter referred to as "umber") preserved within Japanese accretionary complexes. Umber is a mudstone with highly enriched in Fe-Mn oxides and was exploited as a low-grade Fe-Mn resource decades ago. Because umber typically occurs in association with mid-ocean ridge basalt (MORB), most umber deposits are considered to have originated as hydrothermal sediments accumulated near mid-ocean ridges. During the deposition of hydrothermal sediments, Fe-Mn oxide particles absorbed various trace elements, including Os, from ambient seawater. Thus, umber is considered an ideal material for reconstructing the Os isotopic ratio of ancient seawater [6–13].
This study focuses on the Ichiizawa umber deposits within the Setogawa subbelt, a part of the Setogawa Belt (belong to the Shimanto Belt), Shizuoka, in the Japanese accretionary complex. The major lithology accompanied by the umber deposits are basalt, limestone, and bedded chert [14]. Basalts outcropped in proximity to the Ichiizawa deposit exhibits a geochemical composition similar to MORB [15]. Based on age of the MORB-like basalts [15], the depositional age of the Ichiizawa deposit is estimated to be in the middle Eocene (48–42 Ma). In this presentation, we present a detailed geochemical characteristics of the Ichiizawa umber and their surrounding rocks. We then discuss the origin and depositional environment of umber within the Setogawa Belt. Furthermore, we examine the Os isotope ratio of middle Eocene seawater, as reconstructed from these umber deposits.
[1] Peucker-Ehrenbrink and Ravizza, 2000, Terra Nova, 12, 205-219, [2] Peucker-Ehrenbrink and Ravizza, 2020, Geologic Time Scale 2020 (Chapter 8), 239-257, [3] Tejada et al., 2009, Geology, 37, 855–858, [4] Sato et al., 2013, Nature Communications, 4, 2455, [5] Matsumoto et al., 2020, Sci. Rep., 10, 12601, [6] Fujinaga et al., 2022, Ore Geol. Rev., 142, 104683, [7] Kuwahara et al., 2022, Glob. Planet. Change, 216, 103920, [8] Yano et al., 2023, J. Asian Earth Sci., 241, 105480, [9] Fujinaga et al., 2001 Shigen Chishitsu, 51 29-40, [10] Kato et al., 2005b, Resour. Geol., 55, 291-299, [11] Ravizza et al., 2001, Earth Planet. Sci. Lett., 188, 369–381, [12] Kato et al., 2005a, Geochem. Geophys. Geosyst., 6, Q07004, [13] Kato et al., 2011, Gondwana Res., 20, 594-607, [14] Sugiyama and Shimokawa, 1989, J. Struct. Geol. Soc. Japan 34, 173-188, [15] Sakamoto et al., 1993, J. Geol. Soc. Japan, 99, 9-28.