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

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处843站位的一处相对未蚀变玄武岩开展的⁴⁰Ar-³⁹Ar阶段加热实验，得到其结晶年龄为110 ± 2 Ma。此前通过间接方法以及南夏威夷海山的放射性测年结果推断的基底年龄，比真实值偏小20~30百万年。843站位玄武岩脉体中的页硅酸盐蚀变矿物形成了一条铷锶（Rb/Sr）等时线，其年龄为94.5 ± 0.5 Ma。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，从而阻碍了海水自由循环进入地壳。蚀变矿物的化学特征表明，热液流体中的稀土元素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。锶同位素比值（⁸⁷Sr/⁸⁶Sr）与海水近似，但锶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，其成因是海水与早期由玄武岩主导的热液流体中沉淀的方解石发生了反应。