The origin and migration of mud volcano fluids in Taiwan: Evidence from hydrogen, oxygen, and strontium isotopic compositions

Abstract

Mud volcanoes are important gateways for deep fluids to migrate upward and provide windows for studying fluid/sediment interaction at depth. Fluids emitted from 40 terrestrial mud volcanoes were collected in southern and eastern Taiwan to study their chemical compositions, including H, O and Sr isotopes (87Sr/86Sr and δ88/86Sr). Relative to seawater, the mud volcano fluids are depleted in Cl, Mg, SO42−, and δD and are elevated in B, Ba, Li, and δ18O, possibly due to water–rock interaction and clay dehydration (mainly smectite to illite transformation) in the source region. The distribution of Sr in mud volcano fluids shows patterns associated with their localities and geological settings. Most fluids have higher 87Sr/86S than seawater, indicating water–rock interaction at depth. The low Na, 87Sr/86S, and high Ca fluids emitted from eastern Taiwan imply intense interaction with igneous basement. Most fluids have higher Sr/Cl and lower δ88Sr than seawater except mud volcanoes in the northern Chu-kou Fault, which emit low Sr/Cl and extremely high δ88Sr (up to 0.82‰) fluids. We performed laboratory carbonate precipitation experiments that indicate that these high Sr isotope ratios are caused by co-precipitation of carbonates from high alkalinity fluids. Mud volcano fluids in Taiwan originate at depth and their chemical compositions are controlled by the host rock, degrees of water–rock interaction, and clay dehydration, but also are masked by retrograde progresses, such as carbonate precipitation during migration. Our results show that an approach combining water isotopes (δD and δ18O) and strontium isotopes (87Sr/86Sr and δ88/86Sr) provides a robust tool for tracing fluid sources and migration pathways in accretionary prisms.