Copper partitioning between granitic silicate melt and coexisting aqueous fluid at 850 ℃ and 100 Mpa

摘要

Experiments on the partitioning of Cu between different granitic silicate melts and the respective coexisting aqueous fluids have been performed under conditions of 850 °C, 100 MPa and oxygen fugacity(f O_2) buffered at approaching Ni–Ni O(NNO). Partition coefficients of Cu(DCu= cfluid/cmelt) were varied with different alumina/alkali mole ratios [Al_2O_3/(Na_2O·K_2O), abbreviated as Al/Alk], Na/K mole ratios, and Si O_2 mole contents. The DCu increased from 1.28 ± 0.01 to 22.18 ± 0.22 with the increase of Al/Alk mole ratios(ranging from 0.64 to 1.20)and Na/K mole ratios(ranging from 0.58 to 2.56). The experimental results also showed that DCuwas positively correlated with the HCl concentration of the starting fluid.The DCuwas independent of the Si O_2 mole content in the range of Si O_2 content considered. No DCuvalue was less than 1 in our experiments at 850 °C and 100 MPa, indicating that Cu preferred to enter the fluid phase rather than the coexisting melt phase under most conditions in the melt-fluid system, and thus a significant amount of Cu could be transported in the fluid phase in the magmatichydrothermal environment. The results indicated that Cu favored partitioning into the aqueous fluid rather than themelt phase if there was a high Na/K ratio, Na-rich, peraluminous granitic melt coexisting with the high Cl-fluid.