[S-CG48] 海洋底地球科学

[SCG48-P37] 海洋-堆積物間のNd質量収支ボックスモデルを用いたレアアース泥生成を支配する環境因子の検討

*松波 亮佑¹、安川 和孝¹、中村 謙太郎^1,2、加藤 泰浩^1,2 (1.東京大学、2.千葉工業大学)

キーワード：レアアース泥、ネオジム、質量収支、モデリング、海底堆積物、海底鉱物資源

Deep-sea sediments highly enriched in rare-earth elements (REE), termed “REE-rich mud,” have recently attracted attention as a potential mineral resource for the industrially critical metals [1]. Previous studies on geochemistry and mineralogy of REE-rich mud have suggested that a sedimentation rate is a key for the formation of REE-rich mud [2]. Therefore, Earth system dynamics affecting a sedimentation rate in the pelagic realm should have controlled the formation, ore grades, and distributions of REE-rich mud on a geological time scale. However, the environmental factors controlling the formation of REE-rich mud and their secular variations have never been studied systematically and quantitatively. In this study, we constructed a multi-box model of Nd mass balance considering the interaction between ocean and seafloor sediments, and examined the dominant factors in the formation of REE-rich mud in the Pacific Ocean. The results of a sensitivity analysis show that the dilution effect by eolian dust which is not enriched in REE results in decrease of REE content in the REE-rich mud, whereas the discharge of REE from the continental margin to the ocean can have significantly positive impact on the REE content in the REE-rich mud. Long-term simulations through the Cenozoic era demonstrates that the background trend of the REE content of REE-rich mud is primarily controlled by dust fluxes. The range of the calculated REE contents is consistent with that of observed data in the North and South Pacific REE-rich mud. On the other hand, the model could not reproduce temporary peaks in REE concentrations observed in actual REE-rich muds. This implies that such REE peaks reflect local/regional process(es), as proposed by a previous study [3].

[1] Kato et al. (2011) Nat. Geosci. 4, 535-539. [2] Yasukawa et al. (2016) Sci. Rep. 6, 29603. [3] Ohta et al. (2020) Sci. Rep. 10, 9896.