[S-GC33] 固体地球化学・惑星化学

[SGC33-P02] Rare earth elements composition of carbonate concretion in the Miocene Morozaki Group, southwestern Japan

*北中 良佑¹、壷井 基裕¹ (1.関西学院大学)

キーワード：炭酸塩コンクリーション、希土類元素

Carbonate concretions are observed in marine sedimentary rocks from all over the world, and are generally spherical in shape. They are mainly composed of calcium carbonate, and resistant to weathering. The formation of carbonate concretions may be related to the organic matters of marine life and formed rapidly in the pores of sediments (1). It sometimes contains well-preserved ammonite or other fossils. In this study, we measured rare earth elements (REE) composition of carbonate concretions and investigate the formation processes of concretion. The studied concretion samples are approximately 1.5 m in scale and the matrix is tuffaceous fine sandstones and mudstones of the Miocene Morozaki Group (15−18 Ma), Chita Peninsula, Aichi Prefecture, Japan. REE are measured by inductively coupled plasma mass spectrometry (ICP-MS). The core, mantle and front part of concretion and, matrix sedimentary rocks were analyzed. REE concentrations were normalized by CI chondrite, deep seawater and matrix sedimentary rocks. The REE patterns normalized by CI chondrite shows negative Eu anomaly and abundant light rare earth elements (LREE) in all samples. It shows negative Ce anomaly in the matrix. REE patterns of concretion, except for core, normalized by the matrix show flat curve. At the core part, LREE show higher value, while heavy rare earth elements (HREE) show lower value in all REE patterns. This may be due to the elemental mobility process or the original organism matters.



(1) Yoshida, H., Ujihara, A., Minami, M., Asahara, Y., Katsuta, N., Yamamoto, K., Sirono, S., Maruyama, I., Nishimoto, S. and Metcalfe, R., 2015, Early post-mortem formation of car-bonate concretions around tusk-shells over week-month timescales. Scientific Reports 5, 14123.