(Table 1) Water samples collected from basaltic basement at DSDP Holes 68-501 and 69-504B

The first attempts to sample formation waters from basaltic basement in the oceanic crust were made at Sites 501, 504, and 505 of the Deep Sea Drilling Project. Two methods were used. In the first, the water that occupied the hole was sampled some time after pumping had stopped. In the second, water from both the hole and the surrounding rocks was sampled by sealing off a 3-meter section at the bottom of the hole and opening a large-volume sampler, thereby creating negative pressure. Neither method produced a sample that contained an appreciable and unambiguous component of true formation water, although the samples generally showed large compositional differences from seawater. Samples from Holes 501 and 505B were mainly mixtures of the surface seawater used as drilling fluid and pore water from sediment that fell down the hole. Samples from Hole 504B contained a large fraction of seawater that displayed large chemical changes due to reaction with basement basalts. Tritium analyses revealed the samples to be surface seawater that had been pumped into the formation a few days earlier and had reacted rapidly with basalt at the in situ temperature of 80°C. The solutions had gained Ca and lost Mg on a mole-for-mole basis, lost K and possibly SO4, and gained Si. The Si/Ca ratio increased with depth. These results are consistent with those of laboratory experiments in which seawater reacted with basalt at 70 °C and indicate that at 80 °C in situ reaction rates in the crust are sufficiently rapid to produce large changes in solution chemistry almost instantaneously. The absence of an 18O shift in the solutions indicates that the amount of rock that reacted with the solution during its brief residence in the crust was negligible.

首次针对洋壳玄武岩基底开展地层水（formation water）采样的尝试，依托深海钻探计划（Deep Sea Drilling Project）的501、504与505三个站位开展。本次研究采用两种采样方案：其一为在抽水作业停止一段时间后，采集孔内留存的水体；其二则通过封堵孔底3米井段并开启大体积采样器以形成负压，同步采集孔内水体及周围岩石中的含水组分。尽管所有采样样品与海水的成分差异均较为显著，但两种方法均未获得含有可观且明确原生地层水组分的有效样品。501孔与505B孔的样品主要为用作钻井液的表层海水与沿孔道下落沉积物中的孔隙水的混合物。504B孔的样品则以海水占比居多，且因与基底玄武岩发生水-岩反应而产生了显著的化学组分变化。氚同位素分析表明，这些样品实为数天前被泵入地层的表层海水，且在80℃的原位温度下与玄武岩发生了快速反应。该溶液以等摩尔比的方式获得钙（Ca）并消耗镁（Mg），同时失去了钾（K）与可能的硫酸根（SO₄），并获得了硅（Si）。硅钙比随采样深度的增加而升高。上述结果与70℃下海水与玄武岩反应的室内模拟实验结果一致，表明在80℃的原位环境中，洋壳内的反应速率足够快，可在近乎瞬时的尺度上使溶液的化学性质发生显著改变。溶液中未观测到氧-18（¹⁸O）同位素偏移现象，说明该溶液在洋壳内的停留时间极短，与之发生反应的岩石量可忽略不计。