Boron and lead isotope signatures of subduction-zone mélange formation: Hybridization and fractionation along the slab-mantle interface beneath volcanic arcs

The B and Pb isotope systems are widely applied tracers of recycling processes occurring during subduction. Studies examining these complementary systems as a pair enjoy considerable success, where B primarily records the thermal and fluid evolution of the subducting slab, whereas the tripartite Pb system constrains the source of subducted material returned to volcanic arcs. However, interpretations derived from the arc volcanic record critically depend upon assumptions regarding compositions of unmetamorphosed inputs to subduction zones. Few studies have directly addressed potential fractionation of B isotopes and U-Th-Pb by analysis of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic suites, despite that fractionation in these systems during subduction-zone metamorphism has been inferred in many studies of volcanic arcs and ocean-island basalts. Here, we address the metamorphic evolution of subducted material with B and Pb isotope determinations for the mélange matrix of the Catalina Schist, CA. Within the Catalina Schist, mélange matrix formed through the synergistic effects of metasomatism and deformation, affecting basalts and sediments derived from the subducting Farallon plate with peridotites derived from the overlying mantle wedge. Models of simple mechanical mixing among these end-members broadly predict both B and Pb concentrations within hybridized schistose mélange matrix, but an explanation of isotope ratios for both systems requires significant fractionation during metamorphism. The B isotope results are compatible with the previously presented model for sources and transport of fluid within the Catalina Schist subduction zone based on O and H isotope data: δ¹¹B values for the amphibolite facies mélange matrix are consistent with infiltration by B-bearing fluid produced in lower-T metasediment-rich domains, whereas the lower-grade lawsonite-albite and lawsonite-blueschist tectonometamorphic units represent possible analogs for the sources of this B-bearing fluid. Overall, Pb isotope ratios are indistinguishable as a function of metamorphic grade and are highly radiogenic. We constrained the potential influence of radiogenic continental detritus to the Catalina subduction zone by estimation of the continental input component from detrital zircon U-Pb age spectra. This zircon-based sedimentation proxy demonstrates that the potential influence of the Mesozoic California Andean-type convergent margin cannot in all cases explain the radiogenic Pb signature of the Catalina mélange matrix, seemingly requiring some fractionation of the U-Th-Pb system during formation of the lawsonite-albite and lawsonite-blueschist mélange units. Pb isotope signatures of the lower-grade mélange matrix can be explained by a two-stage metamorphic fractionation model involving early loss of Pb by desulfidation reactions, followed by deeper loss of silicate U, during subduction. Pb signatures of the amphibolite facies mélange matrix suggest either efficient retention of protolith Pb signatures during metamorphism or faithful transfer of the fractionated Pb signature by metamorphic fluid flow. Contamination of the mantle wedge by Catalina Schist B and Pb isotope fluid signatures can explain B-Pb isotope anomalies observed for modern arcs, indicating that the effects of mélange mixing should be considered in models of subduction-zone mass transfer.