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Trace Element Composition of Fluids during Serpentinite Dehydration

E. Tenthorey & J. Hermann

It is currently believed that island arc magmas are generated when H 2 O migrates from subducting lithosphere into the overlying wedge. The addition of H 2 O lowers the melting temperature of the ultramafic rocks and also acts as a medium for transport of various trace elements. Although serpentinites are probably the most important reservoir for H 2 O in subduction zones, little is known concerning the role of serpentinites in magma genesis. Determining the sources of the fluids is contingent on understanding the trace element behaviour during mineral reactions and also their partitioning behaviour into the breakdown fluids.

To this end, we have conducted a series of piston cylinder experiments to characterize trace element mobility during serpentinite dehydration. These novel experiments involved capping our serpentinite specimen with a porous diamond aggregate so that the fluids liberated during dehydration were separated from the sample and trapped in the diamond (see figure below). The olivine-rich residue and the quench precipitates in the diamond were then analysed using laser ablation ICP-MS so that partition coefficients could be determined for various elements. The most surprising result was that boron, an element long thought to be highly mobile, was far less mobile than other elements such as Cs and As. Since serpentinites contain high concentrations of B, our results have significant implications for B recycling into the deep mantle and indirectly for the dynamics of fluid flow in the mantle wedge.

In addition healing and sealing of faults is a rapid process in fluid-rich systems. Thus the extended period that after-shocks occur over implies those zones that are repeatedly activated attain high cumulative fluid fluxes relative to the major through-going fault. In this way we have been able to explain the location and clustering of mineralised small displacement structures around non-mineralised major structures.