The B isotopic signature of serpentine from obducted ophiolites: Mixing of fluids and tectonic implications

Obducted ophiolites expose oceanic lithosphere slivers in suture zones. The few previous studies focusing specifically on the serpentinization of the ultramafic part of ophiolites mostly concluded that serpentinization was driven by seawater. Recent developments in B isotope geochemistry have enabled the tracking of fluid source(s) in serpentinites, suggesting delta B-11 values above +10 parts per thousand mostly reflect seawater hydration and delta B-11 values below +10 parts per thousand mostly reflect hydration by subducted crust-derived metamorphic fluids. In this study, we further investigate the fluid sources recorded by ophiolitic serpentinites by presenting B isotopic geochemistry for seventeen samples from well-characterized ophiolites in Guatemala, Iran, and Brazil. The samples from Guatemala and Iran are from fossil Supra Subduction Zones (SSZ), while samples from Brazil represent a fossil Ocean-Continent Transition (OCT). Most of the samples display the pseudomorphic mesh matrix and bastite microtexture typical of serpentinites. Nine of the samples are either totally (both microtextures) or partially (one microtexture only) out of isotopic equilibrium, with delta B-11 variations of up to 18 parts per thousand within individual samples/textures. Among the samples with a narrow delta B-11 range (i.e., at isotopic equilibrium), those from a fossil OCT ophiolite have delta B-11 ranging from +3.8 to +23.2 parts per thousand, which is broadly consistent with serpentinization by seawater. In contrast, serpentinites from fossil SSZ ophiolite have delta B-11 straddling 0 parts per thousand, ranging from -7.7 to +13.5 parts per thousand. These data indicate that seawater alone cannot be responsible for serpentinization of these ultramafic rocks, but rather either subducted crust-derived metamorphic fluids, or a mixture of seawater and subducted crust-derived metamorphic fluids. This interpretation agrees with the recent suggestions that SSZ ophiolites possibly represent fossil forearc lithosphere.

Automated summary

This study investigates the fluid sources recorded in ophiolitic serpentinites using B isotopic geochemistry. The results suggest that the B isotope values in serpentinites can reflect the fluid sources, with samples from Brazil indicating serpentinization by seawater, and samples from Guatemala and Iran indicating serpentinization by subducted crust-derived metamorphic fluids.