Geochemistry of Neogene volcanic ashs in ODP Leg 124 holes@en

Mineralogical and geochemical analyses were performed on 40 ash layers of Pleistocene to late Miocene age, recovered during Leg 124 in the Celebes and Sulu Seas (Sites 767, 768, and 769). They provide information on alteration processes related to burial diagenesis. The zonal distribution of secondary volcanic products emphasizes a major diagenetic change, characterized by the complete replacement of volcanic glass by an authigenic smectite-phillipsite assemblage, in tephra layers dated at 3.5-4 Ma. This diagenetic \"event\" occurs simultaneously in the two basins, and, on the basis of isotopic data, under low-temperature conditions. It is independent of distinct sedimentation rates and related to a relative quiescence of on-land volcanic activity. This period suggests a more uniform paleooceanographic situation having tectonic significance, and probably reflects a kinetic and environmental control of diagenetic reactions.本次研究对西里伯斯海与苏禄海大洋钻探第124航次（767、768及769站位）回收的40套更新世至晚中新世火山灰层开展了矿物学与地球化学分析。该套分析数据可为与埋藏成岩作用相关的蚀变过程提供研究依据。次生火山产物的带状分布特征凸显了一次显著的成岩转变：在年代为3.5~4 Ma的火山碎屑岩层中，火山玻璃被自生蒙脱石-钙十字沸石组合完全交代。该成岩"事件"在两个盆地中同步发生，结合同位素数据可知其形成于低温环境下，不受显著沉积速率的影响，且与陆上火山活动的相对平静期密切相关。这一时期对应着更均一的古海洋环境，具有构造学意义，其或反映了成岩反应受到动力学与环境因素的共同调控。