Fluid-melt trace-element partitioning behaviour between evolved melts and aqueous fluids: Experimental constraints on the magmatic-hydrothermal transport of metals

摘要

The fluid-melt partition coefficients of ten trace metals (Li, Mn, Cu, Zn, Rb, Sr, Mo, W, and Pb, along with K) between hydrous evolved melts and coexisting Cl-bearing fluid(s) have been experimentally determined at 850-1050 degrees C, 0.6-2.2 kbar, and fO(2) approximate to NNO -0.5 to NNO +2 log units. The measured compositions of the quenched fluid phases vary from low salinity (similar to 2 wt% Cl-) to > 19 wt% Cl-, with maximum starting fluid concentrations of > 21 wt% Cl. The results of 15 partitioning experiments confirm that D-metal (fluid/melt) values depend strongly on the melt and fluid composition, with the greatest metal affinities for the fluid phase shown in the experiments with high-Cl fluids, and the most fractionated melts. Five experiments conducted in Au96Cu4 capsule alloy show D-cu(fluid/melt) values ranging from similar to 40 to > 80, and a strong correlation with melt FeO wt% contents at equivalent fluid Cl- contents. In contrast, Li shows only weak partitioning in favour of coexisting fluids across all melt compositions, with D-Li(fluid/melt) ranging from 0.16 in low salinity fluids to 1.17 in the highest Cl fluid (19.15 wt% Cl-).