Fluid activity in continental subduction zones: Insights from barium isotopes of ultrahigh-pressure jadeite quartzites in Dabie orogen

Abstract

Fluid activity during deep continental subduction and exhumation plays a critical role in crust-mantle geochemical recycling. However, the origins and scales of fluid activity in continental subduction zones remain poorly constrained. Barium (Ba) isotopes exhibit significant fractionation during fluid-related processes, providing a powerful tool for tracing fluid sources and quantifying fluid-rock interactions. Coesite-bearing jadeite quartzites from the Dabie orogen in east-central China are unique ultrahigh-pressure metamorphic rocks recording extensive fluid-rock interactions during deep continental subduction. The protolith of the jadeite quartzites is a suite of ca. 2.0 Ga granitic rocks that underwent simple physical weathering and proximal accumulation. In this study, we analyzed Ba isotope compositions of these coesite-bearing jadeite quartzites and compared them with ca. 2.0 Ga granitic rocks to investigate fluid activity in continental subduction zones. The jadeite quartzites display a wide range of Ba isotope compositions (δ^138/134Ba = −0.50 to 0.11 ‰), generally lower than those of the ca. 2.0 Ga granitic rocks (−0.07 to 0.29 ‰), albeit with minor overlap. Ba isotope compositions of the jadeite quartzites correlate well with Ba contents and other fluid-mobile elements (e.g., K, Rb, Cs, and Sr), indicating significant fluid activity during their formation. Furthermore, using a Rayleigh fractionation model, we constrained the Ba isotope fractionation factor to a range of 0.10 to 0.15 ‰ (Δ^138/134Ba_fluid-rock) during fluid-rock interaction. Considering the widespread occurrence of jadeite quartzites, fluid activity is quantitatively significant in continental subduction zones. Such fluids critically facilitate partial melting and crust-mantle interactions in subduction zones.