Geochemical investigations of volcanic ash layers from ODP Leg 119

Geochemical investigations were carried out on 19 discrete ash layers and on 42 dispersed ash accumulations in Oligocene to Pleistocene sediments from Sites 736, 737, 745, and 746 of ODP Leg 119 (Kerguelen Plateau in the southern Indian Ocean). The chemical data obtained from more than 500 single-grain glass analyses allow the characterization of two dominant petrographic rock series. The first consists of transitional- to alkali-basalts, the second mainly of trachytes with subordinated alkali-rhyolites and rhyolites. Chemical correlation with possible source areas indicates that the tephra layers from the northern Kerguelen Plateau Sites 736 and 737 were probably erupted from the nearby Kerguelen Islands. The investigated ash layers clearly reflect the Oligocene to recent changes in the composition of the volcanic material recorded from the Kerguelen Islands. The dispersed ashes from Sites 745 and 746 in the Australian-Antarctic Basin display almost the same range in chemical compositions as those from the north. Heard Island and other sources may have contributed to their formation, in addition to the Kerguelen Islands. Dispersed ash of calc-alkaline composition is most probably derived from the South Sandwich island arc, indicating sea-ice rafting as an important mechanism of transport.本次研究针对南印度洋凯尔盖朗高原（Kerguelen Plateau）大洋钻探计划（Ocean Drilling Program, ODP）第119航次736、737、745、746四个站位的渐新世至更新世沉积物，开展了地球化学调查，共涉及19层离散火山灰层与42处分散火山灰堆积体。通过对500余颗单颗粒玻璃的化学分析（single-grain glass analyses），可明确两类占主导的岩相岩石系列（petrographic rock series）的特征：其一为过渡型至碱性玄武岩（transitional- to alkali-basalts）系列，其二则以粗面岩（trachytes）为主，伴生少量碱性流纹岩（alkali-rhyolites）与流纹岩（rhyolites）。结合潜在物源区（source areas）的地球化学对比结果显示，凯尔盖朗高原北部736与737站位的火山灰层（tephra layers）大概率源自邻近的凯尔盖朗群岛。本次研究的火山灰层清晰反映了凯尔盖朗群岛所记录的火山物质成分自渐新世至现代的演化变化。位于澳大利亚-南极海盆（Australian-Antarctic Basin）的745与746站位的分散火山灰，其化学成分的分布范围与北部站位几乎一致；除凯尔盖朗群岛外，赫德岛（Heard Island）及其他物源区也可能为其形成贡献了物质。其中钙碱性成分（calc-alkaline composition）的分散火山灰，极有可能源自南桑威奇岛弧（South Sandwich island arc），这表明海冰搬运（sea-ice rafting）是一类重要的火山灰搬运机制。