Basalt-water interactions at high temperatures: 1. Dissolution kinetic experiments of basalt in water and NaCl-H2O at temperatures up to 400 °C, 23 MPa and implications

Kinetic experiments of basaltic rock-water (and NaCl-H2O) interactions at temperatures from 20 to 400 °C were carried out using flow through a backed bed reactor. Experimental results indicate that all of the metal release rates vary with temperature and that the relative release rates for the various metals in the basaltic rock vary significantly at constant temperature. The rock dissolution rate, i.e., the release rate of Si, increased as the temperature increased from 20 °C to 300 °C, and then decreased as the temperature continued to increase from 300 °C to 400 °C. The release rate of Si reached a maximum at 300 °C (at 350 °C in NaCl-H2O). The maximum release rate of Al was reached at 350 °C. Release rates of Ca, Mg and Fe decreased with increasing temperature from 20 to 300 °C (or 350 °C), and generally, Ca, Mg, Fe, and Al were dissolved more quickly than Si at temperatures in the range from 25 to 300 °C, but more slowly in the range from 300 to 400 °C. Our experiments demonstrated that Si will be fixed on the rock surface at temperatures from 100 to 300 °C, Al will be fixed at 200-300 °C, or >350 °C; but Mg, Ca and Fe will be fixed on the surface at temperatures from 300 to 400 °C. The experimental results can be used to illustrate the factors controlling alteration zoning in sub-sea floor rocks at Mid-Ocean Ridges (MOR) and in continental volcanic rocks in the Luzong basin, Yangtze river valley, China. The results also demonstrate that alteration zoning in basaltic rocks in a geological profile with a temperature gradient is caused by the variable release rates for different metals with temperature. Strong variations in those release rates always occurred at 300-400 °C, which reflect changes in the properties of water as it passes from a sub-critical to a supercritical state.