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

海底喷口的热液可能非常酸。在大洋中脊的热液系统中，25°C时的pH值通常在3-5左右。在后弧的热液系统中，pH25值似乎变化更大，通常酸性更强。最低的pH25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，pH值有效的反应机制包括：地下岩浆源的SO2脱气、热海水与海洋岩石圈岩性之间的阳离子交换、氢氧化物和羟基硫酸盐的沉淀，这些反应可能从海水中提取羟基（OH）。一个潜在的重要反应是NaCl的水解（生成HCl和NaOH）。高温沸腾时，由于HCl比NaOH挥发性更强，两种ph活性物质（HCl和NaOH）可以通过液汽分馏物理分离。由于海水加热后会析出几种ph活性相（波特兰岩、水镁石、茶石），这些相都具有逆溶性，所以NaCl水解的程度只能在简单的H2O-NaCl体系中进行量化。本文介绍了一种新的实验技术，研究了250 ~ 390°C的NaCl水溶液中NaCl的水解程度。目的是评估平衡NaCl (liq) + H2O = HCl (vap) + NaOH （liq）对水热溶液施加酸性条件的潜在作用。

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.