title: Serpentinite phase relations : An experimental study on redox conditions and fluid migration in subduction zones
Creators: name: Eberhard, Lisa
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: serpentinite
subject: ophicarbonate
subject: subduction zone
subject: experimental phase relations
subject: permeability
subject: 550

description: 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. Mössbauer 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 relations in carbonated (CaCO3-bearing) antigorite-serpentinites, similar to ophicarbonates, were examined. Ca-Mg exchange results in the formation clinopyroxene, which replaces orthopyroxene and leads to a strong decrease in antigorite dehydration temperature. This will prevent the antigorite stability field from reaching conditions where dense hydrous magnesium silicates form. The presence of ophicarbonates will, therefore, favour the subduction of carbonate-rich but water-poor assemblages into the deep mantle. In the final part of this study a new technique was developed to measure permeabilities at high pressures and temperatures. The method was used to measure the permeability in serpentinites during dehydration. In multi-anvil experiments strongly foliated serpentine cylinders were embedded in MgO sleeves. Fluids, formed upon dehydration, migrate outward and react with MgO to produce brucite. The fluid flux is calculated from the amount of brucite formed. Using equations of state to determine the fluid overpressure, the permeability is subsequently calculated with Darcy’s law. A slightly modified setup, using Al(OH)3 as the fluid source, was used to analyse the permeability prior to dehydration of antigorite. The results indicate a large increase in permeability of about 2 log units upon serpentine dehydration to near 1e18 m2 at 3 GPa, whereas serpentinites are found to be impermeable below the dehydration temperature. Although previous studies performed at near room pressure and temperature indicate that foliated serpentinites exhibit strong permeability anisotropy, the results reported here indicate that all anisotropy is lost once dehydration commences. An anomalously low fluid flux measured at 5 GPa provides the first experimental evidence for pore fluid underpressure upon antigorite dehydration at pressures above 3 GPa, that may prevent fluids from leaving the slab. Below this pressure, however, the large increase in permeability and the lack of permeability anisotropy as antigorite starts to dehydrate will favour pervasive rather than channelised fluid flow, which will promote the decarbonatisation of the slab by dissolution of carbonates in H2O.