Experimental evidence for light Ba isotopes favouring aqueous fluids over silicate melts

Barium (Ba) is a fluid mobile element and enriched in the Earth's crust, which has potential implications for constraining fluid activities during magmatic-hydrothermal 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 (DFLUID-MELT) and equilibrium isotope fractionation factor (alpha Ba-138/134(FLUID-MELT)) between aqueous fluids and silicate melts with different chemical compositions at 700-900 degrees C and 200 MPa using cold seal pressure vessels. The results show that DFLUID-MELT ranges from 0.02 to 0.20, while Delta Ba-138/134(FLUID-MELT) [approximate to 1000 x (alpha - 1)] ranges from -0.62 % to -0.14 %. Both DFLUID-MELT and Delta Ba-138/134(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 alpha Ba-138/134(FLUID-MELT) is useful for tracing fluid activities in felsic intrusion-related hydrothermal deposits and in seafloor hydrothermal systems.