Chemical profiles in sediment and basalt at DSDP Hole 74-525A

Forty sediment and four basement basalt samples from DSDP Hole 525A, Leg 74, as well as nine basalt samples from southern and offshore Brazil, were subjected to instrumental neutron activation analysis. Thirty-two major, minor, and trace elements were determined.
The downcore element concentration profiles and regression analyses show that the rare earth elements (REE) are present in significant amounts in both the carbonate and noncarbonate phases in sediments; Sr is concentrated in the carbonate phase, and most of the other elements determined exist mainly in the noncarbonate phase.
The calculated partition coefficients of the REE between the carbonate phase and the free ion concentrations in seawater are high and increase with decreasing REE ionic radii from 3.9 x 10**6 for La to 15 x 10**6 for Lu. Calculations show that the lanthanide concentrations in South Atlantic seawater have not been changed significantly over the past 70 Ma.
The Ce anomaly observed in the carbonate phase is a redox indicator of ancient seawater. Study of the Ce anomaly reveals that seawater was anoxic over the Walvis Ridge during the late Campanian. As the gap between South America and West Africa widened and the Walvis Ridge subsided from late Campanian to late Paleocene times, the water circulation of the South Atlantic improved and achieved oxidation conditions about 54 Ma that are similar to present seawater redox conditions in the world oceans.
The chemical compositions of the basement rocks correspond to alkalic basalts, not mid-ocean ridge basalts (MORBs). The results add more evidence to support the hypothesis that the Walvis Ridge was formed by a series of volcanos moving over a "hot spot" near the Mid-Atlantic Ridge.
From the chemical composition and REE pattern, one 112 Ma old basalt on the Brazilian continental shelf has been identified as an early stage MORB. To date, this is the oldest oceanic tholeiite recovered from the South Atlantic. This direct evidence indicates that the continental split between South America and Africa commenced > 112 Ma.

本研究对深海钻探计划（DSDP）第74航次525A钻孔的40件沉积物样品、4件基底玄武岩样品，以及巴西南部及近海海域的9件玄武岩样品开展了仪器中子活化分析，共测定了32种常量元素、微量元素与痕量元素的含量。岩心元素浓度垂向剖面与回归分析结果显示，稀土元素（REE）在沉积物的碳酸盐相和非碳酸盐相中均有显著赋存；锶（Sr）主要富集于碳酸盐相，其余多数测定元素则主要赋存于非碳酸盐相中。计算得到的稀土元素在碳酸盐相与海水自由离子浓度间的分配系数较高，且随稀土元素离子半径减小而升高，从镧（La）的3.9×10^6升至镥（Lu）的15×10^6。计算结果表明，过去70 Ma以来南大西洋海水中的稀土元素浓度未发生显著变化。碳酸盐相中观测到的铈（Ce）异常可作为古海水氧化还原环境的指示指标。对铈异常的研究显示，坎潘晚期沃尔维斯海岭上方的海水处于缺氧状态。自坎潘晚期至古新世晚期，随着南美与非洲大陆间距不断扩大、沃尔维斯海岭持续沉降，南大西洋的水循环得到改善，约在54 Ma时达到与现今全球海洋海水氧化还原条件相似的氧化环境。基底岩石的化学成分属于碱性玄武岩，而非洋中脊玄武岩（MORBs）。该研究结果为"沃尔维斯海岭是由一系列在大西洋中脊附近热点上方移动的火山形成"这一假说提供了更多支撑证据。基于化学成分与稀土元素配分模式，巴西大陆架上一件形成于112 Ma的玄武岩被鉴定为早期洋中脊玄武岩。这是迄今为止南大西洋采获的最古老大洋拉斑玄武岩。该直接证据表明，南美与非洲大陆的裂离作用始于112 Ma之前。