Origin of manganese carbonates in Jurassic red shale, central Japan

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.

A geochemical constraint on the formation process of a manganese carbonate nodule in the siliceous mudstone of the Jurassic accretionary complex in the Mino Belt, Japan

“…3B). These characteristics are consistent with those of rhodochrosite and the previous reports on the characteristics of Mn carbonate nodules collected from this area (Fujihara, 2009;Minoura et al, 1991). These grains contained almost unbroken radiolarian shells.…”

“…The Furou-fall Park section is located between CH-2 in the north and CH-1 in the south, where red siliceous mudstone with Mn carbonate nodules is exposed. This outcrop is the immediately opposite bank in the stratigraphic strike direction of the Inuyama section, where sedimentological and geochemical studies of Mn nodules were performed in previous studies (Fujihara, 2009;Minoura et al, 1991). In the Furou-fall section, siliceous microfossil radiolarians were extracted from Mn carbonate nodules by diluted HCl (Nagai, 1986(Nagai, , 1989Nagai and Mizutani, 1990), dating the depositional age as Bajocian (170.3-168.3 Ma;Ogg et al, 2012) of the Jurassic.…”

“…The sedimentary rocks of the Jurassic accretionary complexes are composed of the accreted plate-oceanic stratigraphy from the greenstones as a basement through the pelagic chert and hemipelagic siliceous mudstone to the clastic rocks of mudstone and sandstone (Matsuda and Isozaki, 1991). The occurrence of Mn in the Jurassic accretionary complexes are different for the different source locations considered: the stratiform Mn deposits between the chert beds , the stratiform ferromanganese deposit associated with greenstone , and the distinctive nodules associated with laminae composed of spherules of rhodochrosite aggregations in siliceous mudstone and mudstone (Fujihara, 2009;Suzuki and Ogane, 2004;Minoura et al, 1991). This variable occurrence condition of the Mn deposits is presumably caused by the different origins and different formation processes (Nakagawa et al, 2011).…”

“…This fact implies that the radiolarian-bearing Mn nodule in siliceous mudstone of the Jurassic accretionary complexes is a unique characteristic in the sequence in the East Asia. Minoura et al (1991) studied the Mn carbonate nodules in the Mino Belt and suggested that agglutinated foraminiferal tests were replaced by Mn carbonate. Recently, Nakagawa et al (2009) proposed that the Mn deposits in the Sambagawa, Mikabu, Chichibu, and Shimanto belts in Shikoku were formed by the metamorphism of Mn nodules in the siliceous sediments on the ocean floor.…”

“…However, the occurrence condition of Mn deposits is so different that the examination of their origin and formation process using geochemical analysis should be carefully performed while considering the geologic age and the sedimentary structures. Minoura et al (1991) reported a geochemical analysis on Mn carbonate nodules hosted by siliceous mudstone. They reported the carbon and oxygen isotopic ratios of Mn carbonate and suggested that the depositional environment of Mn carbonate in the Mino Belt is a transitional zone between aerobic oxidation and sulfate reduction.…”