UPHILL DIFFUSION OF TRACE ELEMENTS DURING MULTI-STAGE SUBDUCTION ZONE METASOMATISM

Fluid movement in subduction zones has been invoked as a driver of intermediate-depth seismicity, and these short-lived pulses of fluid can be recorded in the redistribution of trace elements around fluid pathways. Eclogites from an exhumed subduction shear zone in the Monviso Ophiolite (Italian Alps) contain geochemical and mineralogical evidence of multiple fluid pulses at peak metamorphic conditions. Analysis of the trace element geochemistry in a metasomatized eclogite reveals a significant enrichment in light rare earth elements (LREE) adjacent to the block-rind contact that we interpret as the result of uphill diffusion. We integrate whole-rock and in situ geochemical data to describe the metasomatic rind-forming process and propose a mechanism for the uphill diffusion of trace elements during subduction zone metasomatism. Distinct zoning and textural patterns between garnet crystals across the reaction boundary suggest the breakdown and regrowth of garnet as the metasomatic rind advanced further into the eclogite block. Cyclic release and re-uptake of HREEs in garnet produced oscillatory zoning with interruptions due to subsequent garnet dissolution. The periodic release of HREEs led to the formation of REE-rich apatite at the reaction boundary, providing the chemical potential for the up-gradient diffusion of LREEs. This explanation of REE movement relies on multiple pulses of fluid moving through the shear zone to refresh the chemical potential gradient needed to grow the multiple generations of garnet and apatite present in the boundary and rind layers, consistent with the cyclic fluid events noted in previous studies.