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

由于我们还没有良好的低温地温计来解释浅层地壳的热历史，因此该项目计划估算沉积物的成岩温度。首先，对成岩作用早期的自生矿物形成机制及其相关化学行为进行了详细的研究。对南开海槽1300m沉积物中的天然自生矿物(粘土矿物和沸石)进行了矿物学和化学研究，主要采用了透射电子显微镜和分析电子显微镜。即使在成岩条件下，粘土矿物的形成机制仍以溶解-沉淀为主。自生矿物组合不同于同一沉积柱中的火山灰层(形成沸石)和页岩(形成伊利石和伊利石/蒙皂石混合层状粘土矿物)。自生蒙皂石的化学成分可用于地温计，但矿物组合不能用于成岩温度的估算，尝试通过两种不同的方法测定氧同位素分馏系数：一种是测定自生粘土矿物的氧同位素比值，用于研究形成机制。分离出的自生粘土矿物的氧同位素比值随深度的增加而变化(与温度相等)，可用于低温地温计。二是水-粘土矿物在热液条件下氧同位素交换的室内实验。设计了两套水热反应弹，用于在低于200ﾟC的温度下测定伊利石-水之间的氧同位素分馏系数。在静水条件下，炸弹在低于300ﾟC的温度下工作良好。选取人工伊利石样和天然伊利石样进行了伊利石与水的氧同位素交换实验。

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.