Rb/Cs fractionation: A link between granulite metamorphism and the S-process

High-grade metamorphism of pelitic sediments produces large Cs losses with little K and Rb depletion. Island-arc volcanics contain an enhanced Cs component, relative to K and Rb. We suggest a connection between these two processes whereby fluids or melts removed from feldspar-bearing subduction assemblages under high-grade metamorphic conditions are enriched in Cs relative to K and Rb; these Cs-enriched components infiltrate subarc mantle and lead to ubiquitous Cs enrichment in arc volcanics. Normal oceanic magmatism is not effective in fractionating Rb/Cs, therefore the canonical oceanic basalt value of 85 for Rb/Cs is distinctive relative to processes involving sediments, recycling, fluid/melt metamorphism, and arc volcanism.