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

Abstract

Abstract—The 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 °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 > Сa ≈ Fe > Mg > Mn > Ti ≈ 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 ºС 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.