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

• Applicant: Professor Dr. Christian Gerhard Ballhaus
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314568258

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.

• DFG Programme: Research Grants