Boron isotopic discrimination for subduction-related serpentinites

The Guatemala Suture Zone (GSZ), Guatemala, is a region that contains two distinct suture-related serpentinite melanges straddling the Motagua fault and an ophiolitic complex paired with the northern melange. The serpentinite matrix of the melanges formed by subduction-fluid hydration of peridotite from the deep mantle wedge. The occurrence of serpentinite from both exhumed subduction channel melange and ophiolite is not uncommon in paleo-suture zones, but distinguishing them and their tectonic origin can be difficult. A new method of discrimination, based on boron isotopes in serpentine from both melanges and ophiolite, as well as on mica and pyroxene from the metamorphic and vein-rock blocks embedded within the melanges, has been developed. The metamorphic and vein samples have mainly negative delta B-11, ranging from -15.3% to +4.3%, in the same range as the serpentine from the melanges (-14.4% to +9.7%). In addition to being the most negative delta B-11 values ever measured in serpentinite, comparable values from vein minerals indicate that the same fluid serpentinized the overlying mantle. In contrast, serpentine samples from the ophiolite have positive delta B-11, in the range 0% to +18.0%, consistent with hydration by seawater-derived fluids. As the GSZ displays two melanges whose serpentinite originated from two different deep subductions and mantle hydration, we hypothesize that the negative signature of exhumed melange serpentine is the norm and that the B isotopic signature can be a useful tool to discriminate the tectonic origin of serpentinization in paleo-suture zones.