Fluid-Driven Mass Transfer During Retrograde Metamorphism and Exhumation of the UHP Western Gneiss Region Terrane, Norway

Dehydration reactions within subducted oceanic crust are important for fluid-mediated element transfer within the subducting plate and potentially to the mantle wedge. The effects of metamorphic reactions and fluid flow on element recycling that occur during retrogression and exhumation of subducted continental crust from mantle depths are poorly understood. We study two metabasite pods with fresh eclogite cores and retrogressed amphibolite-facies rims and surrounding host gneiss within the Western Gneiss Region (WGR), Norway, to better understand element mobility and mass transfer during exhumation of subducted continental crust. Bulk-rock data were collected from samples taken across the pod and into the host gneiss. Phengite breakdown in eclogite and epidote recrystallization in veins and/or gneiss within pod cores contributed large ion lithophile elements and REE to retrogressed eclogite closest to the pod cores. In gneiss hosting the pods, phengite and epidote breakdown provided fluid that mediated elemental transfer and redistribution to the pod rim or tail. Compared to the studied pod in the southern WGR, the pod in the northern WGR underwent higher P-T conditions, partial melting and higher strain rates. This resulted in the infiltration of external fluid farther into the pod interior from the rim and facilitated larger mass gain in trace elements in the amphibolite tail of the pod relative to fresh eclogite in the core. The results show clear evidence for retrogression dehydration reactions driving significant fluid-mediated element redistribution as observed on the outcrop scale during exhumation following ultrahigh-pressure metamorphism, which directly impacts element signatures within the exhuming crust. A lot is not known about how fluids and metamorphic reactions recycle elements in continental crust as it is brought to the surface after being subducted to similar to 100 km. We study two eclogite pods that are surrounded by gneiss in the Western Gneiss Region, Norway to study how different pressure-temperatures (P-T), deformation, and fluids control element movement in deep crust as it is exhumed. The pod cores have eclogite that record the rock conditions at similar to 100 km, but the exteriors were altered during exhumation. We sampled transects from the pod cores to similar to 20 m into the surrounding gneiss. Bulk-rock and mineral trace-element data show two situations of mineral breakdown that generated trace-element-rich fluid that reacted with and enriched metabasite in Rb, Sr, Ba, and light rare earth elements: (a) veins or gneiss in pod cores enriched adjacent eclogite, and (b) surrounding gneiss enriched the pod exteriors. As shown by the pod that experienced higher P-T conditions and deformation than the second pod, larger amounts of elements were transferred into the pod rim, and fluid penetrated the pod, enriching eclogite near the pod rim. Clearly, exhumation conditions and mineral break-down reactions are important for elemental recycling in crust during exhumation. In the Western Gneiss Region, Norway, fluids redistribute elements during retrogression after ultrahigh-pressure metamorphism Recrystallization of epidote-group minerals and phengite in metabasite pod interiors facilitate elemental transfer within the pods Host gneiss-derived fluids infiltrate amphibolite-facies retrogressed pod rims, driving significant element redistribution and mass transfer