北上山地,ペルム系登米統の頁岩中に産する含燐酸塩岩・炭酸塩岩の成因 : 炭素・硫黄同位体による研究(<特集>地球における炭素の循環(続))

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Carbon and oxygen isotope ratios of carbonates, and sulfur isotope ratios of pyrite in the phosphate and carbonate rocks in the Upper Permian Toyoman Series were measured. The phosphate and carbonate rocks have been divided into the following four types; 1) nodules (phosphate-bearing calcareous shale), 2) A type (same as nodules), 3) B type (calcareous sandstones) and 4) C type (carbonate rocks), on the basis of their occurrence and lithology. These rocks are intercalated within black shale. A, B and C type rocks are sometimes interbedded with each other. Nodules and A type are rich in phosphate minerals (apatite), while B and C type are rich in carbonate minerals (calcite). The δ^<13>C_<PDB> values of each rock type are; 1) nodules: -10. 5〜-6. 8‰, 2) A type: -11. 5〜-8.9‰, 3) Btype: -13.2〜-9.5‰, 4) C type: -15.8〜-11.5‰. The δ^<13>C values decrease in the following order; nodules, A, B, and C types, and the neighboring values overlap with each other to some extent. The ^<18>O_<SMOW> values of the rocks fall within the range of 14.1 to 20.7‰ and each rock type apparently does not have characteristic oxygen isotope values. Both the carbon and oxygen isotope ratios of the rocks are significantly lower than those of the marine limestones from the Southern Kitakami Belt. Black shale contains framboidal pyrite abundantly and its δ^<34>S_<CDT> values are in the range between -32.7 and -24.3‰. The δ^<34>S values of pyrite in the phosphate rocks are higher than those in the black shale by 8〜10‰. The modes of occurrence of C type carbonate rocks were studied in detail. It became clear that the rocks are not marine limestone but was originally formed by calcareous segregation during the diagenesis of the sediments. Several lines of evidence suggest that the phosphate and carbonate rocks were formed through microbial sulfate reduction. It is seen that the oxygen isotope ratios of the carbonates do not maintain the primary signatures of diagenesis. The following diagenetic stages are postulated for the formation of the phosphate and carbonate rocks in the Toyoman Series. Stage 1: Sediments rich in organic matter deposited on the continental shelf or slope. With the increase of sediment burial, bacterial sulfate reduction occurred. Pyrite in the black shale with lower δ^<34>S values formed in this stage. Stage 2: The δ^<34>S values of sulfate ions in porewaters became higher as the result of pyrite precipitation in the preceding stage. Pyrite rich in ^<34>S precipitated in the core parts of nodules at this stage. Apatite also started to crystallize in this stage forming P_2O_5-rich nodule. Porewaters in this stage have not yet been saturated with carbonate ions; thus the nodules of this stage mostly lack carbonate. Stage 3 A: In this stage, the porewaters became saturated with carbonate ions due to the progress of sulfate reduction. This caused the precipitation of carbonate at the rim of the nodules. The carbonates in the nodules show the highest δ^<13>C values among the four rock types; this supports its shallowest origin. Stage 3 B: The δ^<13>C values of carbonate ions became lower with further progress of sulfate reduction. A type rocks were formed during this stage and subsequently, B type rocks wereformed. Stage 4: The last stage of sulfate reduction. Porewaters saturated with carbonate ions, which has the lowest δ^<13>C values, segregated from black shale to form C type carbonate veins. Sulfate reduction oxidized organic matter and liberated PO_4^<3-> into porewaters. It is concluded that sulfate reduction within organic-rich sediments was responsible for the formation of the phosphatic rocks in the Toyoman Series.
1994-09-25

北上山地,ペルム系登米統の頁岩中に産する含燐酸塩岩・炭酸塩岩の成因 : 炭素・硫黄同位体による研究(<特集>地球における炭素の循環(続))

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