Experimental superheating of water and aqueous solutions

The metastable superheated solutions are liquids in transitory thermodynamic equilibrium inside the stability domain of their vapor (whatever the temperature is). Some natural contexts should allow the superheating of natural aqueous solutions, like the soil capillarity (low T superheating), certain continental and submarine geysers (high T superheating), or even the water state in very arid environments like the Mars subsurface (low T) or the deep crustal rocks (high T). The present paper reports experimental measurements on the superheating range of aqueous solutions contained in quartz as fluid inclusions (Synthetic Fluid Inclusion Technique, SFIT) and brought to superheating state by isochoric cooling. About 40 samples were synthetized at 0.75 GPa and 530-700 °C with internally-heated autoclaves. Nine hundred and sixty-seven inclusions were studied by micro-thermometry, including measuring the temperatures of homogenization (Th: L + V → L) and vapor bubbles nucleation (Tn: L → L + V). The Th–Tn difference corresponds to the intensity of superheating that the trapped liquid can undergo and can be translated into liquid pressure (existing just before nucleation occurs at Tn) by an equation of state. Pure water (840–935 kg m −3), dilute NaOH solutions (0.1 and 0.5 mol kg −1), NaCl, CaCl 2 and CsCl solutions (1 and 5 mol kg −1) demonstrated a surprising ability to undergo tensile stress. The highest tension ever recorded to the best of our knowledge (−146 MPa, 100 °C) is attained in a 5 m CaCl 2 inclusion trapped in quartz matrix, while CsCl solutions qualitatively show still better superheating efficiency. These observations are discussed with regards to the quality of the inner surface of inclusion surfaces (high P– T synthesis conditions) and to the intrinsic cohesion of liquids (thermodynamic and kinetic spinodal). This study demonstrates that natural solutions can reach high levels of superheating, that are accompanied by strong changes of their physico-chemical properties.