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

Metamorphic rocks of the Cape Hinode district, East Antarctica

Microprobe analyses of amphiboles, biotites, clinopyroxenes, feldspars, garnets and Fe-Ti oxides in various metamorphic rocks of the Cape Hinode district, East Antarctica are presented. The analytical results of these minerals show that they crystallized under the condition of amphibolite facies. The composition of hornblendes belongs to a low pressure type, and indicates that they might have crystallized at about 5kb or lower. Garnet-biotite geothermometry reveals that the estimated temperature is about 650℃. Biotite included in garnet crystal in sillimanite-garnet-biotite gneiss has the composition rich in Mg and poor in Ti and might have been formed at lower temperature than matrix ones

Chemical Characteristics of Biotites and Hornblendes from Metamorphic Rocks around Lutzow-Holmbukta, East Antarctica

Chemical compositions of biotites and hornblendes in metamorphic rocks around Lutzow-Holmbukta show that these minerals have the characteristics of granulite facies ones. Some low-Ti biotites and hornblendes may be due to low TiO_2 content of host rocks. Chemical natures of biotites and hornblendes reflect those of their host rocks, that is, biotites from metabasites are phlogopite and Na_2O-rich ones, while those from paragneisses are Fe-rich and Na_2O-poor ones. The replacement of R^ by R^ ions in octahedral sites in biotites increases from metabasites through charnockites to paragneisses. Hornblendes from metabasites are richer in Na_2O than K_2O, but the proportion is reversed in hornblendes from charnockites and hornblende gneisses. The values of log (ƒ_/ƒ_) of fluid phase coexisting with biotites during crystallization are 4.3 in metabasites and 3.8 in paragneisses, respectively. From the available data of the F content of hydrous minerals in various metamorphic terrains, it can be presumed that the ratio of ƒ_/ƒ_ decreases during progressive metamorphism

Major Element Chemistry of Metamorphic Rocks of the Cape Hinode District, East Antarctica

Metamorphic rocks of the Cape Hinode district have the mineral assemblages of the amphibolite facies and have the chemical composition poor in K_2O even in SiO_2-rich rocks. They show lower K_2O/Na_2O and higher Fe_2O_3/FeO ratios than those of the Lutzow-Holmbukta region. The fluorine content in metabasites of the Cape Hinode and Lutzow-Holmbukta districts shows positive correlation with K_2O, and the F/K_2O ratio is slightly higher than that of the continental basalt series. The amount of fluorine decreases with increasing SiO_2 content in most of rock types