Experimental observation of deep crust fluid-NaCl aqueous solution at elevated temperatures and pressures and its significance

Abstract

In order to reveal the nature of deep crust fluids, the phase relations of NaC1-saturated solution at high temperatures and pressures in a hydrothermal diamond anvil cell (HDAC) are investigated. Salinity of the solutions observed is about 35%–50%. The temperatures for the observation range from 25 to 850°C and the pressures from 1 atm to about 1 GPa. A supercritical single phase, liquid phase (L), vapor phase (V), solid phase (H), L+H, H + V + L and the near-critical phases L + V can be observed. A two-phase (L + V) immiscibility field for the NaCl solution has been discovered to lie in a wide range of temperatures and pressures: from 250(±3) to 721°C. Within this field there are two parts, where the upper high temperature part of the two-phase regions is very unstable in character.

It is possible to observe a “critical phenomenon”. In some of our experiments an “explosion” almost constantly occurred at the interface between the liquid and vapor phases, making the interface obscure, and a continuous transition between the two phases could be found. By a visual microscope it was found that in the two-phase immiscible field near to the critical state the vapor phase and liquid phase compose a crystal structure geometry. It is a special solution structure that was found under a microscope. This discovery is important for us to understand the reason of increasing electronic conductivity of NaCl aqueous solution with respect to temperatures and pressures. And also, it will be easy to interpret the variation of electronic conductivity in the earth crust.