Fractionation of trace elements by subduction-zone metamorphism - effect of convergent-margin thermal evolution

Differential chemical/isotopic alteration during forearc devolatilization c an strongly influence the cycling of volatile components, including some tr ace elements, in subduction zones. The nature and magnitude of this devolat ilization effect are likely to be strongly dependent on the thermal structu re of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at less than or equal to 45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, a ffords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subd ucting rocks. In the Catalina Schist, the most extensive devolatilization o ccurred in metasedimentary rocks which experienced prograde P-T paths encou ntering the epidote-blueschist facies (>350 degrees C at 9 to 12 kbar) or h igher-ir conditions; such rocks are depleted in 'fluid-mobile' elements suc h as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C-reduced/N, and increases in delta(15)N and delta(13)C. The relative susc eptibilities of the 'fluid-mobile' elements to loss along increasingly high er-T P-T paths can be categorized. Boron and Ca show the greatest susceptib ility to low-T removal by fluids, showing >50% depletion in even lawsonite- blueschist-facies metasedimentary rocks which experienced relatively low-T prograde metamorphic paths. In rocks which experienced higher-T paths, As a nd Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at similar to 25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce , and Sb/Ce among arcs from differing convergent-margin thermal regimes, an d conceivably some cross-are declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zone s (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regime s similar to that which formed the low-grade units of the Catalina Schist l and blueschist-facies rocks in the Franciscan Complex), forearc devolatiliz ation is less profound, B, Cs, As, Sb, and N are more likely to be deeply s ubducted, and enriched in are lavas, and significant devolatilization occur s at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e. g., Cascadia) and back-arcs in which are lavas are depleted in B, Cs, As, a nd Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the war mer Archean subduction which resulted in greater slab melting and productio n of abundant trondhjemite-tonalite magmatic suites. (C) 1999 Elsevier Scie nce B.V. All rights reserved.