Physical properties and geochemistry at DSDP Leg 70 Holes

Ashes occurring both as distinct layers and mixed with pelagic sediments of the hydrothermal mounds lying south of the Galapagos Rift are mainly rhyolitic and basaltic. The ashes, of rhyolitic to intermediate composition, appear to belong to a calc-alkalic series and were probably derived from Plinian eruptions in Ecuador or Colombia. Basaltic ashes are made of nonvesicular sideromelane spalling shards and are of tholeiitic composition. They probably were derived locally from fault scarps. Most rhyolitic and basaltic glass shards studied are fresh except for hydration of the rhyolitic shards. Some shards are severely altered, however. Basaltic ash may be more common in pelagic sediments deposited near accretion zones and may be a source of silica and other elements released during diagenesis产于加拉帕戈斯裂谷（Galapagos Rift）以南热液丘（hydrothermal mound）的火山灰，既以独立层状产出，亦与该热液丘的远洋沉积物（pelagic sediment）混合产出，其成分以流纹质（rhyolitic）与玄武质（basaltic）为主。其中流纹质至中性成分的火山灰属于钙碱性系列，推测其源自厄瓜多尔或哥伦比亚的普林尼式喷发（Plinian eruption）。玄武质火山灰由无气孔铁镁玻璃（sideromelane）剥落碎屑构成，具有拉斑玄武质成分，其物源可能为当地的断层崖（fault scarp）。本次研究的多数流纹质与玄武质玻璃碎屑保存完好，仅流纹质碎屑存在水化作用；不过部分碎屑发生了强烈蚀变。在增生带（accretion zone）附近沉积的远洋沉积物中，玄武质火山灰可能更为常见，且可能是成岩作用（diagenesis）过程中释放的硅质及其他元素的来源之一。