Origin of manganese carbonates in Jurassic red shale, central Japan |
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| Authors: | KOJI MINOURA SHU NAKAYA ATSUSHI TAKEMURA |
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| Institution: | Institute of Geology and Palaeontology, Faculty of Science, Tohoku University, Sendai 980, Japan;Department of Earth Sciences, Hirosaki University, Faculty of Science, Hirosaki 036, Japan;Department of Geology, Hyogo University of Education, Faculty of Education, Yashiromachi 673-14, Japan |
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| Abstract: | Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well-preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn86.7?92.2Ca2.2?2.9Mg2.6?6.7Fe2.6?5.6)CO3 Carbon and oxygen isotope values, which range from ?7.99 to ?4.78‰ and ?4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction. |
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