Experimental evidence for light Ba isotopes favouring aqueous fluids over silicate melts
Haihao Guo, W.-Y. Li, Xiao-Yun Nan, Fang Huang
Abstract
Barium (Ba) is a fluid mobile element and enriched in the Earth's crust, which has potential implications for constraining fluid activities during magmatichydrothermal processes. However, the behaviour of Ba and its isotopes during fluid exsolution from magma is poorly known. Here we present an experimental study on determining the Ba partition coefficient (D FLUID-MELT ) and equilibrium isotope fractionation factor ( 138/134 Ba FLUID-MELT ) between aqueous fluids and silicate melts with different chemical compositions at 700-900 C and 200 MPa using cold seal pressure vessels. The results show that D FLUID-MELT ranges from 0.02 to 0.20, while 138/134 Ba FLUID-MELT [1000 ( -1)] ranges from -0.62 to -0.14 . Both D FLUID-MELT and 138/134 Ba FLUID-MELT positively correlate with temperature, the salinity of fluid and alumina saturation index (ASI) of melt. The finding that light Ba isotopes are enriched in aqueous fluids relative to silicate melts suggests that the fluid exsolution process cannot explain the observed light Ba isotopic compositions of some granites. Moreover, the experimentally determined 138/134 Ba FLUID-MELT is useful for tracing fluid activities in felsic intrusion-related hydrothermal deposits and in seafloor hydrothermal systems.