Geochemistry of altered phases, basalts and sediments of ODP Site 136-843@en

40Ar-39Ar incremental heating experiments on a relatively unaltered basalt from Site 843 yield a crystallization age of 110 ± 2 Ma for the central Pacific Ocean igneous basement near Hawaii. Previous estimates of the age of the basement inferred by indirect methods and from radiometric dates of the South Hawaiian Seamounts are too young by 20-30 m.y. Phyllosilicate alteration minerals from veins in the Site 843 basalts define a Rb/Sr isochron with an age of 94.5 ± 0.5 Ma. The isochron records the last equilibration of the phyllosilicate minerals with a hydrothermal fluid at about 16 m.y. after the formation of the igneous basement. The last event recorded by calcite veins is the sealing of the crust by a sufficient thickness of sediment to impede the free circulation of seawater into the crust. The chemistry of the alteration minerals indicates the rare earth elements in the hydrothermal solutions were derived from alteration of the basalts and, furthermore, were transported in solution as metal species and carbonate complexes. Calcite with approximately seawater 87Sr/86Sr, but Sr contents too low to precipitate directly from seawater, is suggested to have formed at a late stage in the alteration history of the crust by the reaction of seawater with calcite precipitated earlier from basalt-dominated hydrothermal fluids.对夏威夷附近中太平洋火成基底843站位的一处相对未蚀变玄武岩开展的40Ar-39Ar阶段升温实验，获得其结晶年龄为110 ± 2 Ma。此前通过间接方法推断的基底年龄，以及基于南夏威夷海山放射性测年得到的年龄，均比该真实年龄偏年轻20~30百万年。843站位玄武岩脉体中的页硅酸盐蚀变矿物，定义出年龄为94.5 ± 0.5 Ma的铷-锶（Rb/Sr）等时线，该等时线记录了火成基底形成约16百万年后，页硅酸盐矿物与热液流体达到最终平衡的事件。方解石脉所记录的最后一次地质事件，是地壳被足够厚度的沉积物覆盖，从而阻断了海水自由循环进入地壳的过程。蚀变矿物的地球化学特征显示，热液流体中的稀土元素（rare earth elements）源自玄武岩的蚀变作用，且以金属物种和碳酸盐络合物的形式在流体中运移。87Sr/86Sr比值近似于海水，但锶（Sr）含量过低而无法直接从海水中沉淀的方解石，被认为形成于地壳蚀变历史的晚期阶段，由海水与早期从以玄武岩为主的热液流体中沉淀的方解石反应生成。