Chemical and oxygen isotope behavior of deep sea sediments during diageneti clay mineral formation

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The project was planned to estimate the diagenetic temperature of sediments, because we have not had good geothermometers at low temperatures to interpret the thermal history in the shallow crust.First, authigenic mineral formation mechanism and its related chemical behavior during early stage of diagenesis was precisely studied. The natural authigenic minerals (clay minerals and zeolites), obtained from the 1300 m sediment column in the Nankai trough, were mineralogically and chemically studied mainly by a transmission electron microscope and analytical electron microscope. The mechamism of clay mineral formation was mainly dissolution-precipitation even at the diagenetic condition. The authigenic mineral assemblages were different from the volcanic ash layrs (zeolites were formed) and shales (illite and illite/smectite mixed layred clay minerals were formed) in the same sediment column. Chemical composition of the authigenic smectite would be useful for the geothermometer, however, the mineral assemblage is not useful for estimation of diagenetic temperature.The fractionation factors of oxygen isotope ratios were tried to determine by the two different approaches ; one was the measurement of oxygen isotope ratios of authigenic clay mineral, same as used for the study of formation mechanism. The oxygen isotope ratios of separated authigenic clay minerals vary with increasing depths (equal to the temperature), and it is useful for low temperature geothermometer. The other was laboratory experiment of oxygen isotope exchange of water-clay minerals at hydrothermal condition. Two sets of hydrothermal reaction bombs were designed, and used to determine the oxygen isotope fractionation factors between illite-water at the temperature lower than 200ﾟC.The bombs were worked well at the temperature lower tham 300ﾟC under the hydrostatic conditions. Artificial and natural illite samples were selected for the exchange experiments of oxygen isotopes between illite and water.