Fluid-driven element recycling during retrograde metamorphism following UHP metamorphism in the Western Gneiss Region, Norway

Many studies have argued for efficient trace-element recycling within subduction zones, with fluids produced by dehydration reactions associated with prograde metamorphism of subducting crustal material playing a significant role. However, high temperatures, fluid flow, and partial melting during retrograde metamorphism associated with exhumation of subducted crustal material likely also result in trace-element transport. To evaluate the role of these retrograde fluids and melts in controlling the trace-element budget, we integrate field, petrologic, and geochemical studies of a combination of eclogite and its surrounding host gneiss from two localities within the ultrahigh-pressure (UHP) Western Gneiss Region, Norway. The Otnheim and Fjørtoft localities from the southern and northern UHP domains were chosen to compare element transport during exhumation in a lower temperature (~650°C) domain to a portion of the slab that achieved higher-temperature (800°C) conditions and likely underwent partial melting, respectively. Within each locality, gneisses with mostly amphibolite-facies assemblages host eclogite pods that range from fresh UHP assemblages in their core to retrogressed amphibolite-facies rims. Bulk-rock trace-element data shows that the metabasites become enriched in fluid-mobile elements, whereas the opposite trend is observed in the amphibolite-facies host gneisses. B, Li, and other trace-element analyses of in-situ biotite from both the metabasites and gneisses suggest no distinguishable trace-element zoning in single biotite grains from the Otnheim gneisses. However, there is a stark contrast between B and Li composition of the fresh UHP gneiss in comparison to the retrogressed gneisses. Overall, these preliminary results suggest that there is fluid-mobile trace-element mobility during likely multiple fluid infiltration events during retrogression and exhumation, with possibly one source of the fluid being the host gneiss. Future δ¹¹B analyses will determine the relative timing along the P-T path of infiltration events during exhumation of the UHP terrane. Future trace-element and δ¹¹B isotopic work conducted in the northern WGR will also be used to evaluate how increased temperature and partial melting affect trace-element recycling during retrogression.