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

摘要

Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at <= 45 km depths) of rock packets metamorphosed along successively lower-T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (> 350 deg C at 9 to 12 kbar) or higher-T conditions; such rocks are depleted in 'fluid-mobile' elements such 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 susceptibilities of the 'fluid-mobile' elements to loss along increasingly higher-T P-T paths can be categorized. Boron and Cs show the greatest susceptibility 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 and Sb (likely in sulfides) show that greatest depletions (> 90%); N, Cs, and B (largely in micas) occur at approx 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, and conceivable some cross-arc 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 zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs 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 arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.