Geochemistry of basalts fropm DSDP Leg 65 basalts@en

Samples of basalt collected on Leg 65 near 22°N on the East Pacific Rise all display the depleted light rare-earth pattern of \"normal\" oceanic crust. Consequently the La/Ta ratio is close to 18, as opposed to the value of 9 associated with the flat or enriched patterns found along parts of the Mid-Atlantic Ridge and the Emperor Seamount chain. The Leg 65 samples are chemically similar to those from the CYAMEX area at 21 °N and to the Leg 54 samples from 9°N, suggesting homogeneity of the upper mantle under the northern part of the East Pacific Rise over a minimum distance of about 1500 km. The geochemistry of the rocks and their field relationships with respect to depth and distance from the axis of the Rise show no pattern of distribution linked to the degree of fractional crystallization and thus cast doubt on any possible model involving large, long-lived magma chambers at the axis of the Rise.在东太平洋海隆（East Pacific Rise）北纬22°附近采集的第65航次（Leg 65）玄武岩样品，均呈现出“正常”洋壳典型的亏损型轻稀土配分模式。因此其镧/钽（La/Ta）比值接近18，而大西洋中脊（Mid-Atlantic Ridge）部分区段及天皇海山链（Emperor Seamount chain）产出的具有平坦型或富集型轻稀土配分模式的样品，其镧/钽比值仅为9。第65航次样品的地球化学特征与北纬21°处CYAMEX区域的样品，以及北纬9°处第54航次（Leg 54）的样品高度相似，这表明东太平洋海隆北部下方的上地幔在至少约1500公里的范围内具有均一性。这些岩石的地球化学特征，以及它们与海隆轴部的深度和距离的野外产出关系，均未呈现出与分离结晶作用程度相关的分布模式，因此对“海隆轴部存在大型长期存留岩浆房”的各类潜在模型提出了质疑。