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

海底喷口的热液溶液可能是相当酸性的。在大洋中脊的热液系统中，25°C时的pH值通常在3-5之间。在弧后的热液系统中，pH 25值似乎更易变，通常更酸性。报告的最低pH 25值均来自西太平洋区域的弧后盆地，范围在1至3之间。几种反应机制被确定为在热液系统中有效的pH值，包括从地下岩浆源的SO2脱气，热海水和海洋岩石圈的岩石之间的阳离子交换，以及氢氧化物和羟基硫酸盐的沉淀，可以从海水中提取羟基（OH）。一个潜在的重要反应是NaCl的水解（生成HCl和NaOH）。当在高温下发生沸腾时，两种pH活性物质（HCl和NaOH）可以通过液-汽分馏彼此物理分离，因为HCl比NaOH更易挥发。由于海水能够在加热几种pH活性相（氢氧钙石、水镁石、钾镁石）时沉淀，所有这些都具有逆行溶解度，因此NaCl水解的程度只能在简单的H2O-NaCl系统中量化。本文介绍了一种新的实验技术，用于研究250 ° C和390°C之间H2O-NaCl溶液中NaCl水解的程度。目的是评估平衡NaCl（liq）+ H2O = HCl（vap）+ NaOH（liq）在对热液溶液施加酸性条件中的潜在作用。

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.