Experimental study on water rock interactions at temperatures up to 435°C and implications for geophysical features in upper mid-crust condition

摘要

Globally observed zones of high conductivity and low seismic velocity in the mid-crust remain enigmatic despite previous theoretical and experimental investigations. We have carried out a series of dissolution experiments on mineral and rock samples in aqueous solutions using flow-through reactors at temperatures as high as 435°C and at pressures of 23-36. MPa. These experiments provide useful information for evaluating the behavior of crustal fluids and their role in the formation of geophysical detectible anomalies in the mid-crust. The steady state dissolution rates of minerals or rocks are measured as functions of temperature, flow rate and pressure. The maximum release rates of Si for silicate minerals are always observed at 300°C (or at 300-400°C for silicate rocks), which results in strong leaching of Si and the break-up of silicate framework structures in minerals and rocks.From a tectonic perspective, plate motions are ultimately responsible for inducing cracking in rocks, generating porosity, decreasing pressure, and moving fluids across and through continents. These processes of decreasing pressure probably lead to the migration of aqueous fluid in the mid-crust to locations where they are close to their critical state at temperatures ranging from 300 to 435°C. Therefore, water rock interactions occurring in the crust will cause strong leaching of Si, breakage of silicate framework structures, and rock collapse. The strong water-rock interaction will further lead to the generation of increased rock porosity and also drive fluid flow. These hydrothermal events, therefore, have a significant role in enhancing the conductivity of rocks in the mid-crust.