Variations of divalent cation concentrations in pore water and the precipitation of Mg-rich authigenic mineral during early diagenesis of Toyoma Formation

The variations of Fe, Mn and Mg concentrations in pore water were deduced from the chemistry of calcite. With depth, the Fe/Ca, Mn/Ca, Mg/Ca ratios of pore water decrease. The decrease in the Fe/Ca ratio is due to the precipitation of pyrite, whereas the decrease in the Mn/Ca ratio is due to the precipitation of Mn-rich calcite. The depletion of Mg in pore water is ascribed to the precipitation of either Mg-rich smectite or Mg-rich chlorite. A-type rock is further divided into A1 and A2 subtypes, on the basis of δ13C. Excesses of MgO and deficiencies of K舀 O above the calculated whole-rock compositions are observed mainly in the phosphatic nodules and A1 subtype. This suggests that the formation of Mg-rich smectite or Mg-rich chlorite took place at very early stage of diagenesis of the Toyoma Formation.Article信州大学理学部紀要 38: 23-34(2004)departmental bulletin pape

The genesis of phosphatic and carbonate rocks in the Toyoma Formation, Northeastern Japan

Various phosphatic and carbonate rocks occur in the Upper Permian Toyoma Formation. They are classified into phosphatic nodules, A-, B-and C-type rocks.The isotopic ratios of whole-rock calcite,pyrite and apatite in the rocks and the results of pyrite microanalysis using a high-resolution secondary ion mass spectrometer are presented. The carbon and sulfur isotopic compositions indicate that the rocks were formed through microbial sulfate reduction. Three distinct zones have been recognized within the Toyoma Formation that define its early diagenesis. The three zones’compositions differ in their mole fractions of authigenic material and the δ13C of their calcite deposits. Zone 1 is the region of oxic surface sediment in which sulfur-oxidizing bacteria grew.Because sulfuric acid is generated by the chemosynthesis of the bacteria, biogenic phosphate debris such as fish bones and scales were dissolved.The phosphate ions thus formed were the source of the phosphatic rocks of the Toyoma Formation.Zone 2 is the anoxic zone that underlay Zone 1. Anaerobic sulfate-reducing bacteria grew in this zone, resulting in increased pore water alkalinity and the precipitation of pyrite and apatite. Phosphatic nodules and A-and B-type rocks began to grow in this zone.Zone 3 is characterized by calcite precipitation. The pore water, saturated with calcite components, segregated from the sediments into rocks allowing the rocks to be calcareous. As sediments became buried deeper and deeper, they experienced the conditions of each successive zone. However, the period of final consolidation of the rocks differed from one rock type to another as evidenced by the varying δ1C values for calcite contained in the different rock types. Although the rocks are mixtures of authigenic minerals originating in different zones, it was possible to estimate the range of diagenetic zones in which each rock type was formed.ArticleJournal of the Faculty of Science Shinshu University 44: 1-57(2012)departmental bulletin pape

中新世青木層から産した，緑泥石に富む頁岩団塊の成因

Chlorite-rich shale nodules have been discovered in the Miocene Aoki Formation in Matsumoto City, Nagano Prefecture, central Japan. The present work assessed the mode of occurrence and the internal structures of these nodules along with the proportion of chlorite in each. Analyses were performed using powder X-ray diffraction (XRD) and other techniques to evaluate the compositions and morphologies of the chlorite grains and to elucidate the origin of these nodules. Nodules both with and without pyrite cores were found. The former specimens each comprised a pyrite core, a gray-colored shaly mantle surrounding the core and a margin, designated as Zones C, B and A, respectively. The chlorite content of Zone B was remarkably high, with a maximum concentration of 56 wt%. The chlorite in this region was also very fine grained and the grains had euhedral or acicular to blade-like shapes with Fe/(Fe+Mg) ratios on the order of 0.7. This chlorite was not detrital but rather comprised authigenic crystals precipitated during early diagenesis of the sediments. Oriented samples from Zone B exhibited odd-order line broadening in XRD patterns, indicating that the mineral was not purely chlorite but rather a randomly interstratified serpentine/chlorite mineral. The mineral precipitated during early diagenesis is thought to have been serpentine-like 7 Å berthierine. The crystallization of this berthierine was evidently associated with the cessation of sulfate reduction, based on the zonal structure of the nodules having pyrite cores. Following the depletion of SO4 2– in interstitial water, reactive iron minerals such as iron oxyhydroxides (FeOOH) contained in the sandy mudstone were reduced by organic matter. Fe2+ ions were thus formed and added to the interstitial water, leading to the precipitation of berthierine. Following crystallization of the berthierine, this material transitioned to a randomly interstratified serpentine/chlorite mineral due to the depth of the Aoki Formation.ArticleJournal of the Faculty of Science Shinshu University 51 : 2-24(2024)departmental bulletin pape