THE EFFECT OF DISSOLVED WATER ON THE OXIDATION-STATE OF IRON IN NATURAL SILICATE LIQUIDS

Ferric-ferrous ratios have been measured in 22 experiments on three na tural compositions equilibrated at known temperature (950 degrees-1100 degrees C) and oxygen fugacity, and at water-saturated conditions ove r a pressure range from 0.05 to 0.2 GPa. There does not appear to be a ny reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these composi tions has also been determined at 1 bar as a function of temperature a nd oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This resu lt parallels the effect of water on sulfide speciation in basaltic mel ts, and confirms published results on experimental hydrous basalts.