Geochemistry, trace elements and minerals at DSDP Holes 80-550B and 80-551

The Leg 80 basalts drilled on the Porcupine Abyssal Plain 10 km southwest of Goban Spur (Hole 550B) and on the western edge of Goban Spur (Hole 551), respectively, are typical light-rare-earth-element- (LREE-) depleted oceanic tholeiites. The basalts from the two holes are almost identical; most of their primary geochemical and mineralogical characteristics have been preserved, but they have undergone some low-temperature alteration by seawater, such as enrichment in K, Rb, and Cs and development of secondary potassic minerals of the "brownstone facies." K/Ar dating fail to give realistic emplacement ages; the apparent ages obtained become younger with alteration (causing an increase in K2O). Hole 551 basalts are clearly different from the continental tholeiites emplaced on the margins of oceanizing domains during the prerift and synrift stages.大洋钻探第80航次所钻取的玄武岩，分别采自戈班海脊（Goban Spur）西南10公里处的豪猪深渊平原（Porcupine Abyssal Plain）550B钻孔，以及戈班海脊西缘的551钻孔，二者均为典型的轻稀土元素（LREE）亏损型大洋拉斑玄武岩。两处钻孔的玄武岩特征几乎完全一致，多数原始地球化学与矿物学特征得以保留，但经历了一定程度的海水低温蚀变作用，表现为钾（K）、铷（Rb）、铯（Cs）元素富集，并发育“褐相”次生钾质矿物。钾氩（K/Ar）定年无法获得可靠的就位年龄，所得视年龄随蚀变程度加剧（伴随K₂O含量升高）而逐渐年轻化。551钻孔玄武岩与裂谷前及同裂谷期形成于洋化域边缘的大陆拉斑玄武岩存在显著差异。