Composition of the Fluid in Carbonate- and Chlorine-Bearing Pelite near the Second Critical Point: Results of Diamond Trap Experiments

composition of the fluid in carbonate-and chlorine-bearing pelite was experimentally studied at 3.0 GPa and 750 and 900 oC, using the diamond trap method. The results of inductively coupled plasma atomic-emission spectrometry (ICP AES) and mass balance calculations showed that a supercritical fluid formed in the studied system at 3.0 GPa and 750 & DEG;C. The fluid is Si-and Al-rich and contains 30-50 wt.% H2O + CO2 and up to 1 wt.% Cl. The contents of other major elements decrease in the order: K > Na > Ca & AP;Fe > Mg > Mn > Ti & AP;P. Compared with supercritical fluids appeared in the systems pelite-H2O and eclogite-H2O, the fluid with high CO2 and Cl contents is richer in Fe, Ca, Mg, and Mn but poorer in Si. Silicate melt generated in this system at 900 oC has a composition typical of pelitic melt. Our experiments reveal a set of fingerprints of element fractionation between a supercritical fluid and solids forming an eclogite-like association, namely, high mobility of P, Sr, and B and low mobility of Li and S. Thus, a supercritical fluid compositionally similar to the pelitic melts generated in subduction zones can transfer significant amounts of both volatiles (H2O, CO2 , Cl, and P) and major components to the regions of arc magma generation. It is important that supercritical fluids should have trace element signatures of diluted low-temperature fluids.