Rare earth element and strontium isotopic study of seamount-type limestones in Mesozoic accretionary complex of Southern Chichibu Terrane, central Japan: Implication for incorporation process of seawater REE into limestones.
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- Tanaka Kazuya
- Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
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- Miura Noriko
- Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
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- Asahara Yoshihiro
- Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
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- Kawabe Iwao
- Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
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Ishimaki and Tahara limestones occur as exotic blocks juxtaposed in the Mesozoic (Jurassic) accretionary complex of Southern Chichibu Terrane in eastern Aichi Prefecture, central Japan. They are supposed to be of the seamount-type limestone, since they have no terrigenous materials and are intimately associated with greenstones. REE (rare earth elements) and Sr isotopic studies for the limestones have been made in order to know their geochemical characteristics, ages and origins. Their 87Sr/86Sr ratios, when referred to the seawater 87Sr/86Sr curve and relevant geological data, suggest that Ishimaki and Tahara limestones are the late Permian and the Carboniferous to the Early Permian, respectively. Two greenstone fragments found inside the Ishimaki limestone block and one greenstone sample associated with Tahara limestone block, resemble the Hawaiian alkali basalt in their REE and Y patterns. This is supporting the idea that the limestone blocks may be parts of reef limestones on ancient volcanic seamounts. All the limestone samples, except three unusual Tahara ones, show seawater REE and Y signatures in their chondrite-normalized patterns. Their REE/Ca ratios, however, are 102-104 times as high as those ratios of modern biogenic carbonates like corals and the seawater. Accordingly, seawater REE and Y were incorporated into the limestones, when originally biogenic carbonates transformed into inorganic calcite and its secondary growths occurred in diagenesis in contact with sufficient seawater. This view is favored by the reported REE partition experiment between calcite overgrowths and seawater solution. The seawater Ce anomaly as a function of water depth in the modern ocean is a key to infer the water depth of the REE and Y incorporation. The Ce anomalies given by log(Ce/Ce∗) for about a half of Ishimaki samples and most of Tahara ones are between -0.5 and -0.2, which are compatible with the shallow water origin. Another half of Ishimaki samples, however, have log(Ce/Ce∗) values between -0.7 and -1.0, suggesting moderately deep waters (ca. 500-1000 m deep or more). This may reflect such a situation that water depths of REE incorporation into the seamount-type limestones are generally greater than the depositional water depths of original biogenic carbonates because of the fate of limestone-capped volcanic seamounts decided by the oceanic plate motion.
収録刊行物
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- GEOCHEMICAL JOURNAL
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GEOCHEMICAL JOURNAL 37 (2), 163-180, 2003
一般社団法人日本地球化学会
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詳細情報 詳細情報について
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- CRID
- 1390282679527947392
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- NII論文ID
- 130003904718
- 80016197188
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- NII書誌ID
- AA00654975
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- COI
- 1:CAS:528:DC%2BD3sXivFOjsb0%3D
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- ISSN
- 18805973
- 00167002
- http://id.crossref.org/issn/00167002
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- NDL書誌ID
- 6499086
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- 本文言語コード
- en
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- データソース種別
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- JaLC
- NDL
- Crossref
- CiNii Articles
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