The acidity of submarine hydrothermal solutions - an experimental study employing in-situ sampling techniques

海底喷口的热液可能是非常酸性的。在大洋中脊的热液系统中，25°C(PH25)的pH值通常在3-5左右。在后弧处的热液系统中，PH25的值似乎更加多变，通常也更酸性。已报道的PH25最低值均来自西太平洋地区的弧后盆地，范围在1至3之间。在热液系统中，确定了几种有效的反应机制，包括从地下岩浆来源的SO2脱气，热海水与大洋岩石圈岩石圈的岩性之间的阳离子交换，以及可能从海水中提取羟基(OH)的氢氧化物和羟基硫酸盐的沉淀。一个潜在的重要反应是氯化钠的水解(生成盐酸和氢氧化钠)。当高温沸腾时，由于HCl比NaOH更易挥发，两种pH活性物种(HCl和NaOH)可以通过气-液分馏在物理上相互分离。由于海水加热几个pH活性相(波兰石、水镁石、卡明石)都具有逆溶能力，因此只能在简单的水-氯化钠体系中定量测定氯化钠的水解程度。本文介绍了一种新的实验技术来研究水-氯化钠溶液在250℃到390℃之间的水解度。目的是评估平衡的氯化钠(Liq)H2O=HCl(Vap)NaOH(Liq)在对水热溶液施加酸性条件时的潜在作用。

Hydrothermal solutions at submarine vents can be quite acidic. In hydrothermal systems at mid-ocean ridges, pH values at 25°C (pH25) typically range around 3-5. In hydrothermal systems at back arcs the pH25 values appear to be more variable and are typically more acidic. The lowest reported pH25 values, all from back arc basins of the Western Pacific region, range between 1 and 3. Several reaction mechanisms are identified in being pH effective in hydrothermal systems, including the degassing of SO2 from sub-surface magma sources, the exchange of cations between hot seawater and lithologies of the oceanic lithosphere, and the precipitation of hydroxides and hydroxy-sulfates that may extract hydroxyl (OH) from seawater. A potentially important reaction is the hydrolysis of NaCl (to HCl and NaOH). When boiling occurs at high temperature, the two pH-active species (HCl and NaOH) may be separated physically from each other via liquid-vapor fractionation, since HCl is more volatile than NaOH. Because seawater is capable of precipitating upon heating several pH-active phases (portlandite, brucite, caminite), all with retrograde solubility, the extent of NaCl hydrolysis can only be quantified in the simple H2O-NaCl system. Novel experimental techniques are introduced here to study the extent of NaCl hydrolysis in H2O-NaCl solutions between 250 and 390°C. Aim is to assess the potential role of the equilibrium NaCl (liq) + H2O = HCl (vap) + NaOH (liq) in imposing acidic conditions on hydrothermal solutions.