The impact of subduction-zone metamorphism on mantle-ocean chemical cycling

Abstract

Subduction zones represent avenues of recycling of crustal, atmospheric and oceanic (including organic) components to the mantle; metamorphism in the forearc and subarc regions of subduction zones likely dictates the extent to which elements are retained in subducted rocks into the deep mantle or participate in shallower fluid- and melt-related processes (arc magmatism, production of forearc serpentinite mud volcanoes, seafloor trench fluid venting). Knowledge of the impact of subduction-zone metamorphism on the compositions of subducting sediment and oceanic lithosphere may yield significant insight into the processes leading to arc magmatism and mantle chemical heterogeneity. Devolatilization processes in the Catalina Schist subduction-zone metamorphic terrane (California) resulted in the release of volatiles from metamafic and metasedimentary rocks and the large-scale transport of H 2O-rich COHSN fluids with relatively uniform O- and H-isotope compositions. In the metasedimentary rocks of the Catalina Schist, the concentrations of elements such as B, N, Cs, As, Sb and possibly U (and the ratios of the concentrations of these elements to the concentrations of less fluid-mobile elements such as the HFSE and REE), the B Be ratios, and the H 2O contents were decreased by progressive devolatilization. The Rb Cs and C reduced N ratios, δ 15N and δ 13C of the Catalina Schist metasedimentary rocks were increased during the high- P/T metamorphism. Metabasaltic rocks, despite their metamorphism at up to amphibolite-facies P- T conditions and resultant dramatic loss of H 2O, preserve major and trace element and isotopic compositions similar to those of hydrothermally altered seafloor basalts (e.g., enrichments in B, K, Rb, Ba, and Cs, decreases in K Rb and K Cs , elevated δ 18 O and 87Sr 86Sr i relative to unaltered MORB). Metamorphism along the forearc slab-mantle interface may prevent the deep subduction of many subducted volatile components (e.g., H 2O, Cs, B, N, perhaps As, Sb, and U) and result in their transport in fluids toward shallower reservoirs (e.g., seafloor, forearc mantle wedge). The removal, by devolatilization, and up-dip transport of such components toward the seafloor or into the forearc mantle wedge, could in part explain the imbalances between the estimated amounts of subducted volatiles (e.g., H 2O, B, N) and the amounts returned via arc magmatism. The results of this study demonstrate that subduction can deliver, to various parts of the mantle (e.g., the forearc mantle wedge, the subarc mantle wedge, and deeper parts of the mantle), COHSN fluids, melts, and residual mineral reservoirs strongly fractionated isotopically and chemically relative to initial compositions of the subducted rocks. Some compatibilities between the rock compositions and inferred fluid compositions for the Catalina Schist and the isotopic and trace element compositions of various mantle-derived materials (igneous rocks and xenoliths) indicate the exciting potential to trace the large-scale transfer of these fractionated chemical signatures.