Back to the roots: boron geochemistry of mud volcanoes and its implications for mobilization depth and global B cycling

At convergent margins, fluid flux through active mud volcanoes (MVs) has been estimated to exceed that of the frontal accretionary prism, and may have done so in earlier earth history. Often, however, it is largely unknown how deep mud volcanoes root within accretionary wedges and orogenic belts. We report results from a systematic geochemical study of fluids and clays/claystones from mud volcanoes around the world using the mobile element boron as depth indicator for progressive diagenesis. Boron shows maximum enrichment in the fluid phase, owing to desorption in the mud, when faulting roots deepest and deformation is strongest. Deep-seated, B-rich fluids liquefy clay-bearing strata to facilitate extrusion, allowing the clay to re-adsorb and incorporate large quantities of B in the process. Given the abundance and high discharge rates of mud volcanoes along subduction zones, we propose that this process has to be considered a major backflux mechanism in global B cycling from the lithosphere into the hydrosphere.