Chemical and Isotopic Cycling in Subduction Zones

Subduction zones are avenues for the delivery of crustal, atmospheric, and oceanic (including organic) components to the mantle and understanding of subduction zone chemical and isotopic cycling is central to many models of crust-mantle-atmosphere evolution. During subduction, diagenetic and metamorphic reactions and related geochemical effects can profoundly influence the element inventory and isotopic composition of subducting slabs to depths beneath volcanic arcs and as they enter the deeper mantle. Physical juxtaposition of chemically disparate rocks and the generation and mobility of various fluids lead to myriad metasomatic effects in subduction zones. Larger scale manifestations of such processes include the mass transfer that leads to arc magmatism and convergent margin volatile cycling. Subduction zone metasomatism is initiated at very shallow levels, as oceanic slabs entering trenches bend and are infiltrated by seawater, and as sedimentary sections experience physical compaction, fluid expulsion, and diagenetic alteration. Studies of forearc fluid geochemistry track this shallow-level metasomatic alteration, and high- and ultrahigh-pressure metamorphic rock suites provide records of fluid generation and flow, and related metasomatism, to depths exceeding those beneath volcanic fronts. Subduction zone processes act as a geochemical filter, altering the compositions of deeply subducting rocks and generating outputs such as arc magmas (and associated volcanic gases) and chemically and isotopically modified rocks. The latter enter the deeper mantle and influence its long-term geochemical evolution.