FIRST RESULTS FROM A NEW EXPERIMENTAL-TECHNIQUE TO DETERMINE FLUID SOLID TRACE-ELEMENT PARTITION-COEFFICIENTS USING DIAMOND AGGREGATE EXTRACTION TRAPS/

Partitioning of representative members of the LFSE, LREE, HREE and HFS E trace element groups between fluid (H2O and 1.5 M HCl) and garnet ha ve been determined experimentally at 50 kbar and 1000 degrees C, corre sponding to the depth of the upper part of a subducting plate beneath active island are volcanoes. Partition coefficients were determined by direct analysis of solid residues and the fluid solute which was trap ped in a diamond aggregate during quenching of the experiment. Each ca psule contained 50 percent of diamond crystals (grain size similar to 50 mu m) in the upper half of the capsule; the pore space between the diamonds aggregates is preserved even at 50 kbar, so that the fluid wa s able to communicate with the solid residual phases throughout the du ration of the experiment. Trace element concentrations in the fluid tr ap and crystalline residues were determined by laser ablation micropro be (LAM). Results indicate that equilibrium is reached within two days . REE are fractionated by a fluid effectively, D-(fluid/gt)(Sm)/D-(flu id/gt)(Yb) is around 50. Neither in the presence of pure water nor aft er addition of hydrochloric acid do partition coefficients for Nb and Ta seem to be decoupled from La and Ce. Therefore, we, conclude that a Cl-solution cannot produce negative HFSE anomalies during selective e nrichment in subduction zone infiltration processes.