Boron isotopic composition of mud volcano fluids: Implications for fluid migration in shallow subduction zones

Mud volcanoes are important conduits for deep fluids to migrate upward, providing an important window to study fluid/sediment interactions at depth in accretionary prisms. Fluids emitting from mud volcanoes in southern and eastern Taiwan were analyzed for dissolved chemical composition and B isotopes to delineate the sources and chemical alteration occurring during fluid migration. Concentrations of dissolved major elements (Cl, Na, K, Ca, Mg, and SO₄) in most of the mud volcano fluids were lower than seawater concentrations, except for the high Ca samples in eastern Taiwan, which were probably affected by fluids/igneous rock interactions. The high concentrations of B, Ba, and Li in the mud volcano fluids are consistent with a scenario that involves intense sediment alteration and clay dehydration at depth. The reaction temperatures estimated using chemical thermometers fall within the range of 70-170°C, with some variation by location. In nearby mud volcano sites, the concentration of dissolved major elements varied little, but the B stable isotope compositions varied by as much as 20%, supporting the argument that dissolved B concentrations were over-printed with effects of sediment adsorption. This explains the occurrence of B enriched and heavy δ¹¹B fluids at moderate depths in subduction zones. It is probable that the fluids in mud volcanoes in Taiwan originated from 2 to 5km depth and are influenced by processes involving tectonic shortening, sediment alteration, and clay dehydration, and subsequent retrograde adsorption during fluid migration.