The water-basalt system at 4 to 6 GPa: Phase relations and second critical endpoint in a K-free eclogite at 700 to 1400 °C

The phase diagram of a K-free mid ocean ridge basalt-H2O system was determined between 4 and 6 GPa to constrain compositions of the liquid phases as liberated from an eclogite during deep subduction and to elucidate mass transfer processes at convergent plate margins. Diamond trap and conventional multi-anvil experiments were combined with a recently developed technique, in which the liquid phases, quenched from high-pressure, high-temperature conditions, are directly laser ablated in a frozen stage and analyzed by ICP-MS. Results show that at 4 GPa a fluid containing ∼80 wt.% H2O coexists with residual eclogite up to 850 °C while a hydrous melt appears at 900 °C, indicating a solidus located between 850 and 900 °C. At 5 GPa the solidus lies between 1000 and 1050 °C but terminates at a second critical endpoint between 5 and 6 GPa. At 6 GPa a supercritical liquid, with a solute content continuously increasing with temperature, forms the volatile bearing phase. Low-temperature H2O-rich fluids and supercritical liquids have a nepheline- to quartz-normative, peralkaline character due to incongruent dissolution of clinopyroxene. The hydrous melts and high-temperature supercritical liquids evolve from rhyolitic to trachytic/andesitic with increasing liquid fraction. The melting reaction at the solidus changes from eutectic (4 GPa) to peritectic (5 GPa) with garnet changing from the reactant to the product side. With increasing melt fraction, the system becomes cotectic with about equal amounts of clinopyroxene and garnet consumed, the residual mineralogy being dominated by garnet. Most P-T trajectories calculated for subduction zone environment do not cross the hydrous K-free MORB solidus, therefore, the liquid phase released from the igneous portion of the subducting oceanic crust will change from fluid to supercritical liquid around 6 GPa. This change, however, is most probably associated with the change from fluid-like to melt-like properties of the mobile phase, with important consequences on trace element partitioning that will also change from fluid- to melt-like.