Serpentinite phase relations - An experimental study on redox conditions and fluid migration in subduction zones

Abstract

The oceanic lithosphere sinks into the Earth’s mantle at subduction zones, a process that provides an engine for material exchange between the surface and the Earth’s deep interior. Serpentinisation fixes H2O in the oceanic lithosphere. This process also oxidises ferrous to ferric Fe, so that the subduction of serpentinites is an important process through which oxidised material is transported into the mantle. This transport has consequences not only for the redox state of the mantle but for the oxidation state of carbon- and sulfur-bearing volatile phases and their transport into the overlying mantle wedge. The main aim of this study is to provide the first experimental data to determine the relationship between the oxidation state of Fe in serpentinites and the oxygen fugacity f(O2). Using this relationship, the effect of ferric Fe on the phase relations within subducting slabs and the speciation of volatile components can be constrained. In the first part of this study multi-anvil experiments were performed between 2.5 and 5 GPa to examine the phase relations of antigorite- and lizardite-serpentinites. The f(O2) was buffered by various metal-oxide pairs. Mossbauer spectroscopy shows that Fe(3+) is charge balanced by a coupled substitution with Al in both serpentine minerals. Thermodynamic properties are derived to describe the substitution of both elements in both minerals. Lizardite displays a higher Fe(3+)/Fe(tot) ratio than antigorite under similar conditions, whereas the phase relations of antigorite and lizardite are found to be identical. Global Gibbs free energy minimisation calculations show that Al increases the stability of serpentine, whereas ferric and ferrous Fe decrease the stability. The effects are very small, however, and cannot explain differences among previous studies. Serpentine is found to dehydrate at lower temperatures with decreasing f(O2), due to a process termed redox dehydration. Most serpentinites have compositions that result in f(O2) in the range FMQ-0.5 to FMQ+2 at 500°C. As antigorite dehydrates at temperatures above 600°C, the f(O2), regardless of the initial bulk Fe(3+)/Fe(tot) ratio, will become buffered by the coexistence of magnetite and hematite. This oxidation state cannot be communicated to the mantle wedge through transfer of sulfate-rich fluids, since the f(O2) remains below the sulfide-sulfate equilibrium. The f(O2) during serpentinite subduction will also remain in the carbonate stability field. Previous observations of carbonate reduction to graphite associated with serpentinites and the disappearance of magnetite must result from the action of external reducing agents, such as H2. Calculations for the overlying mantle wedge, where antigorite forms from H2O released by the slab, show this to be one of the most reduced regions of the upper mantle. CO2 in fluids entering the wedge would consequently be reduced to CH4 and the mantle would be oxidised. This might explain the apparent raised oxidation state of island arc magmas. In the second part of the thesis phase…