Microscale Variations in Eclogite‐Facies Trace Element Redistribution Controlled by Mineral Banding

ABSTRACT Microanalysis of trace elements in garnet can yield new and important insights into the kinetics of garnet growth, metamorphic reactions and fluid–rock interaction in subduction zones; however, differentiating the effects of these processes on a garnet‐by‐garnet basis can be challenging. In this study, we couple microanalyses of trace elements and stable oxygen isotopes in garnet to isolate the causes of trace element variation in mineralogically banded blueschists and eclogites from the Franciscan Complex. Trace element variations in garnet are not apparently driven by interaction with externally derived fluids as recorded by rimward decreases in δ 18 O and can be best explained by local mineral reactions. Intrasample heterogeneity in garnet trace element zoning patterns can be attributed to localized differences in mineral assemblages, which impact the availability of trace elements to growing garnets. These mineralogical heterogeneities also control the extent of reaction with externally derived fluid and resulting δ 18 O zoning patterns through differences in reaction‐induced porosity. Our study highlights the importance of linking petrography to advanced microanalysis and has important implications for understanding the chemical composition and physical pathways of fluids in the eclogitized downgoing slab.