Barium isotope fractionation during slab dehydration: Records from an eclogite-quartz vein system in Dabie orogen

Abstract

Barium isotopes are a potential tracer of fluid-mediated recycling of subducted crustal materials into the mantle wedge. However, the behavior of Ba isotopes during metamorphic breakdown of Ba-rich hydrous minerals is not well understood. High-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks and associated veins in convergent orogens can provide important insight into Ba transport and Ba isotope fractionation during metasomatic fluid activities. This study reports Ba isotope data for whole rocks and mineral separates of a HP-UHP eclogite-quartz vein system from Bixiling in the Dabie orogen. The samples include epidote-phengite-bearing quartz veins and two types of eclogites. The Type I eclogites are the host of the quartz veins and were not affected by the metasomatic fluids, whereas the Type II eclogites are hosted in the veins and were strongly modified by the fluids. The results show that the Type I eclogites have relatively homogeneous δ138/134Ba ranging from −0.12 to −0.04‰. In contrast, the quartz veins have highly variable δ138/134Ba ranging from 0.16 to 0.51‰. The Type II eclogites have δ138/134Ba from 0.32 to 0.40‰, similar to those from 0.25 to 0.35‰ of the quartz separates within the veins, which may represent the average Ba isotopic composition of the fluid inclusions trapped in the quartz during vein formation.These results demonstrate that the vein-forming fluid has much higher δ138/134Ba than the host eclogites, indicating significant Ba isotope fractionation during dehydration of eclogites. Mineralogical evidence suggests that the fluid was probably derived from the breakdown of phengite at the HP eclogite-facies to amphibolite-facies stage during slab exhumation. The higher δ138/134Ba of the phengite separates (from −0.16 to −0.05‰) than those of the coexisting biotite separates (from −0.34 to −0.22‰) in the Type I eclogites indicate that the fluid released from the breakdown of phengite to biotite would be enriched in heavy Ba isotopes. This explains well the observed heavier Ba isotopic compositions of the quartz veins relative to the host eclogites. In summary, our results reveal that the breakdown of phengite during continental slab exhumation could produce Ba-rich fluids with remarkably higher δ138/134Ba than that of the host rocks, and thus might potentially influence the Ba isotopic systematics at convergent plate margins.