Behavior Of Barium Isotopes During High-Pressure Metamorphism And Fluid Evolution

Barium (Ba) and its stable isotopes are powerful geochemical tracers for the recycling of subducted materials into the convective mantle. However, this requires a good understanding of the behavior of Ba isotopes during subduction-zone fluid processes. Here we present the first integrated study on Ba isotopes for two eclogite-vein systems (Ganghe and Hualiangting) that formed at high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic conditions from the Dabie orogen, central China. The Ganghe system displays constant delta Ba-138/134 (-0.01 parts per thousand to 0.03 parts per thousand) in the UHP eclogites but relatively high delta Ba-138/134 (0.17 parts per thousand) in the HP vein. In contrast, in the Hualiangting system, delta Ba-138/134 of the UHP eclogites vary from -0.14 parts per thousand to 0.05 parts per thousand, while delta Ba-138/134 of the multiple HP veins and the retrograded amphibolites range from -0.17 parts per thousand to 0.34 parts per thousand and from -0.11 parts per thousand to 0.46 parts per thousand, respectively.The combined results suggest that Ba isotopes can be considerably fractionated during subduction-zone fluid processes. HP-UHP metamorphic dehydration of the eclogites during the early stages of exhumation can produce an isotopically heavy initial vein-forming fluid due to the preferential leaching of epidote enriched with heavy Ba isotopes from the host eclogites. However, such processes have limited effects on the bulk eclogites because the Ba-rich phengite exists as a residual phase. During vein formation, light Ba isotopes are preferentially enriched in minerals relative to the fluids, resulting in the residual fluids with high delta Ba-138/134. In addition, the fluids released from the gneisses have negative delta Ba-138/134 which is different from the eclogites, indicating that Ba isotopes can be used to trace the source of the metasomatic fluids. This study demonstrates that the UHP metamorphic rocks and HP veins can record Ba isotope fractionation during subduction-zone metamorphism and fluid-rock interaction, and recycling of the metamorphic rocks could modify Ba isotopic composition of the mantle wedge and arc volcanics. (C) 2021 Elsevier B.V. All rights reserved.