Establishment of the stable isotopic stratigraphy of Banded Iron Formations

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Banded iron formations (BIFs), bedded manganese deposits, and silica sinters are the typical examples of chemical sediments that possess clear banding texture. These sedimentary rocks have precipitated on seafloor or subaerial surface, from seawater or hydrothermal solutions with the decrease of solubilities of silica, iron, and manganese in solutions. Variation of stable isotopic compositions of oxygen, carbon, and sulfur in these banded chemical sedimentary rocks have been measured to clarify their genesis.1.Newly developed CO_2-laser microprobe fluorination technique was applied to the Brockman Iron Formation, Hamersley basin, Western Australia, to investigate submillimeter scale oxygen isotopic compositions of quartz both hosted in chert and iron-oxide mesobands. Our data show that delta^<18>O values of BIFs vary between 16.0 and 23.0*, on the contrary to the previous studies that BIFs are isotopically homogeneous. The wide variation of delta^<18>O probably suggests that hydrothermal activities have been responsible for the formation of Hamersley BIFs.2.Isotopic values of organic and carbonate carbon in shale macrobands of the Marra Mamba Iron Formation, Western Australia were measured. The very low delta^<13>C values of organic C,-41.0--37.1*, suggest the activity of the metanogens and metanotrophs in the ocean. The delta^<13>C of dissolved bicarbonate in seawater estimated from carbon isotopic compositions of carbonate was delta^<13>C=--12*. This low value, compared to that of the modern marine HCO_3^- (--1.0*), was also explained as the results of a carbon-cycle system affected by the activities of metanogens and methanotrophs.3.The delta^<18>O values of quartz in silica sinter from the Kitano-oh gold deposits, and those of manganese silicate minerals from the metamorphosed bedded manganese deposits of the Noda-Tamagawa mine were measured. The origin of hydrothermal solutions is discussed.