Barium isotope variation during fluid–rock interaction at forearc depths: Evidence from high-pressure fluid–metasomatized rocks in the Eastern Alps.

As a highly fluid mobile element, barium (Ba) and its isotopes are excellent tracers for fluid associated processes in subduction zones. However, the behavior of Ba isotopes during high-pressure (HP) fluid–rock interaction in subduction zones is poorly constrained. To shed light on this issue, we focus on typical HP fluid−metasomatized rocks from the Sopron area of the Eastern Alps. These fluid−metasomatized rocks comprise Mg-rich leucophyllite and transitional gneiss that occurs between leucophyllite and metagranite protolith. These rocks provide excellent targets for the exploration of Ba isotope variations during fluid−rock interactions. From unaltered metagranite to metasomatized interiors, Ba contents steadily decrease, but δ138/134Ba values increase. In the first stage of fluid−related metasomatism, the transitional gneiss shows significantly higher δ138/134Ba values but lower Ba contents compared to its metagranite protolith, which can be attributed to the dissolution of K-feldspar characterized by high Ba contents but low δ138/134Ba values. In the second stage, leucophyllite was produced, displaying remarkably high δ138/134Ba values ranging from 0.20 to 0.47‰ and extremely low Ba contents down to 29.5 μg/g. These features are interpreted to result from an infiltration of serpentinite-derived fluids with high δ138/134Ba values and relatively low Ba contents, which caused an increase in the δ138/134Ba values of leucophyllite and a removal of Ba during fluid−rock interaction. Additionally, the contribution of fluids from mélange at the slab-mantle interface composed of dragged-down forearc serpentinite, carbonate-rich sediments and altered oceanic crust can result in the heterogeneous Ba isotope compositions of mantle wedges and arc lavas. This study provides new insights into Ba isotope behavior during HP fluid−rock interactions at forearc depths, which has a great bearing on understanding Ba isotope variations in mantle wedges and arc magmas. • Negative correlation occurs between δ138/134Ba and Ba contents of fluid-metasomatized rocks. • Dissolution of K-feldspar causes high δ138/134Ba of the transitional gneiss. • Serpentinite derived fluid-rock interaction explains high δ138/134Ba of leucophyllite. [ABSTRACT FROM AUTHOR]