The Redox State of Iron: A Powerful Indicator of Hydrothermal Alteration

The redox state of iron, (Fe +2/Fet), in rocks is a useful Indicator of hydrothermal alteration. In general, rocks are resistant to shifts in (Fe +2/Fet) unless large volumes of fluid or high concentrations of exotic reactants such as H2 or O2 are present. The earth's surface in equilibrium with the atmosphere is an oxidizing environment; surface waters carry in solution O2 plus SO4-2. Laboratory experiments conducted at high temperatures reveal that water in the presence of ferrous minerals tends to dissociate and yield free hydrogen. At depths exceeding a few kilometres, waters are hot and reducing with H2 H20 = 1 30 to 1/60. In a hydrothermal convection cell, rocks along the descending limbs will be oxidized whereas rocks along the ascending limbs will be reduced by the circulating water