Geochemistry of lower sheeted dike complex samples of ODP Holes 137-504B and 140-504B@en

Rocks of the lower sheeted dike complex of Hole 504B sampled during Leg 140 were analyzed for major and trace element compositions to investigate the effects of igneous processes and hydrothermal alteration on the compositions of the rocks. The rocks are relatively uniform in composition and similar to the shallower dikes. They are moderately evolved mid-ocean-ridge basalts (MORB) with relatively high MgO (7.9-10 wt%) and Mg# (0.60-0.70), and have unusually low incompatible element contents (TiO2 = 0.42-1.1 wt%, Zr = 23-62 ppm). Discrete compositional intervals in the hole reflect varying degrees of differentiation, and olivine and plagioclase accumulation in the rocks, and may be related to injection of packets of dikes having similar compositions. Systematic depletions of total REE, Zr, Y, TiO2, and P2O5 in centimeter-size patches are most likely attributed to exclusion of highly differentiated, late-stage interstitial liquids from small portions of the rocks. […]本研究对140航次采集的504B孔下部席状岩墙杂岩中的岩石样品开展主量与微量元素组成分析，以探究火成作用与热液蚀变对岩石化学成分的影响。这些岩石的化学成分相对均一，且与浅层岩墙的组成特征相似。该套岩石属于中等演化程度的洋中脊玄武岩（mid-ocean-ridge basalts, MORB），具有较高的MgO含量（7.9~10 wt%）与Mg#值（0.60~0.70），且不相容元素含量异常偏低（TiO₂：0.42~1.1 wt%，Zr：23~62 ppm）。钻孔中离散的化学成分区间反映了岩石经历了不同程度的岩浆分异以及橄榄石与斜长石的堆积作用，且可能与具有相似组成的岩墙群的注入事件相关。厘米级斑块中总稀土元素（rare earth elements, REE）、Zr、Y、TiO₂与P₂O₅的系统性贫化，最可能源于小体积岩石中排除了高度演化的晚期粒间流体。[…]