DISSOLUTION OF PRIMARY BASALTIC MINERALS IN NATURAL-WATERS - SATURATION STATE AND KINETICS

The state of saturation of olivine, orthopyroxene and plagioclase of variable composition has been assessed in various types of natural waters in Iceland including river waters, groundwater and geothermal waters with temperatures up to 250-degrees-C. The stability of olivine and orthopyroxene decreases with increasing Mg content. Similarly, the stability of plagioclase decreases with increasing anorthite content. The river waters which are representative of the weathering environment are always undersaturated with both olivine and orthopyroxene, the degree of undersaturation being dominated by the water pH. All river waters tend to dissolve olivine of the composition encountered in basalts according to the linear rate law and orthopyroxene when the pH is < 8. Groundwaters, which have a higher pH, appear to dissolve orthopyroxene and Fe-rich olivine according to the slower near-equilibrium dissolution rate law. In river waters and groundwaters Na-rich plagioclase is stable but when Ca-rich it is close to equilibrium or slightly undersaturated. The state of saturation is independent of pH in the range occurring in these waters (pH 7-10). Geothermal waters of high temperature ( > 200-degrees-C) are undersaturated with olivine, orthopyroxene and plagioclase, the first two particularly when Fe rich, and sufficient to cause olivine to dissolve according to the linear rate law. At intermediate temperatures (50-150-degrees-C) the geothermal waters are close to equilibrium with these minerals except for Mg-rich olivine. It seems likely that dissolution of glass from basalt, which is very reactive, will favor stability of the igneous minerals.