SUBDUCTION OF CONTINENTAL-CRUST AND TERRIGENOUS AND PELAGIC SEDIMENTS - AN EXPERIMENTAL-STUDY

An important role for the recycling of terrigenous sediments in the mantle was proposed by Armstrong [1,2]. Aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust, over the pressure interval 6-24 GPa. This composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments, in addition to terrigenous sediments, during subduction. The subsolidus phase relationships displayed by lithologies derived from these materials are complex, with major roles being played by K-hollandite, garnet, Na-clinopyroxene and stishovite. Continentally derived lithologies achieve densities similar to, or greater than, surrounding mantle at depths greater than 200 km. The geodynamical implications of these buoyancy relationships are discussed. A reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies. During partial melting at relatively low pressures (5-10 GPa), orthoclase, wadeite and K-hollandite are eliminated near the solidus, whereas the stability field of Na-clinopyroxene extends to temperatures well above the solidus. Liquids formed by small to moderate degrees of partial melting of subducted, continentally derived lithologies in this pressure interval have high K/Na ratios and high SiO2 contents. At higher pressures (16-24 GPa), the stability fields of K-hollandite and stishovite extend towards the liquidus, whereas sodium-bearing phases are eliminated much closer to the solidus. Resultant partial melts consequently possess relatively low K/Na ratios and SiO2 contents. Lead possesses a high crystal-liquid partition coefficient in K-hollandite, whereas uranium is excluded. The broad stability field of K-hollandite during partial melting of these lithologies at pressures above 15 GPa therefore has the capacity to cause fractionation of lead from uranium. The petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle.