Traverses across metamorphic reaction zones at contacts between mafic rocks and ultramafic mélange matrix were analyzed in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate a mechanically mixed contribution to subduction-related reaction zones. Elements enriched in peridotite relative to mafic crust such as Os, Ir, Ru, Ni and Cr were used to assess the addition of a peridotitic component to reaction zones. Results showed co-varying concentrations of Ni, Cr, Os, Ir, and Ru in reaction zones, suggesting these components were added via mixing with mantle peridotite. 187Os/188Os ratios were lower in reaction zones relative to block cores, consistent with the addition of peridotite to mafic rock. Reaction zone samples plot along mixing trends between peridotite and basalt, indicating that mechanical mixing, along with fluid-mediated mass transfer of fluid-mobile elements, contributes to reaction zone formation in the Catalina Schist mélange zone.