Distribution, depositional form and geochemical composition of tephra horizons in ODP Sites 186-1150 and 186-1151

Major element geochemical composition was established for 59 tephra horizons from Ocean Drilling Program Sites 1150 and 1151, located in the Japan forearc. These data, encompassing typically between 15 and 30 individual shard analyses per tephra horizon, were used to investigate the degree to which sediment reworking, postdepositional geochemical alteration, and geochemical uniqueness of individual eruptives facilitate or impede the potential for establishing a tephrostratigraphical framework for the Japan Trench, as well as usage of the tephra record to document arc evolution. Evidence was found that hydration (termed phase 1 alteration) of glass shards increases with age in the Pliocene-Pleistocene, but there is no indication that element leaching (phase 2 alteration) has occurred. Post- or syn-depositional differences in preservational style are shown to have no significant bearing on tephrogeochemical homogeneity and suitability for tephrostratigraphical analysis. Overall, therefore, the volcaniclastic record is suitable for investigating medium- to long-term changes in arc geochemistry and, provided consideration is given to the potential for nonunique geochemical signatures, is suitable for erecting tephrochronological frameworks. A limited number of Pleistocene tephra correlations are suggested in furtherance of this framework goal.

针对位于日本弧前区的大洋钻探计划（Ocean Drilling Program, ODP）1150、1151井位的59层火山灰层（tephra horizon），本研究测定了其主量元素地球化学组成。该数据集单火山灰层通常包含15~30次独立玻璃屑（shard）分析结果，以此探讨沉积物再沉积、沉积后地球化学蚀变以及单次喷发产物的地球化学独特性，在多大程度上影响了日本海沟火山灰地层框架（tephrostratigraphical framework）的建立可行性，同时评估利用火山灰记录反演弧演化过程的适用性。研究发现，上新世-更新世地层中玻璃屑的水化作用（第一阶段蚀变，phase 1 alteration）随沉积年代变老而增强，但未发现存在元素淋滤（第二阶段蚀变，phase 2 alteration）的迹象。沉积期后或同沉积期的保存样式差异，对火山灰地球化学均一性以及火山灰地层分析（tephrostratigraphical analysis）的适用性均无显著影响。综上，本次研究获取的火山碎屑记录可用于探究弧地球化学的中-长期变化；若考虑到地球化学特征存在非唯一性的可能，该记录同样适用于构建火山灰年代学框架（tephrochronological frameworks）。为推进该火山灰年代学框架的构建目标，本研究提出了少量更新世火山灰对比方案。