Recycling of volatiles at subduction zones is a key step in the Earth’s geochemical cycle. Some subducted CO2 is remobilized in island-arc magmas, but most of it is recycled into the deeper mantle. Calcite-olivine-bearing veins in the mantle rocks of the Kohistan Paleo-Island Arc (North Pakistan) contain some of the best gem quality olivines worldwide. The O, C, and Sr isotopy of the vein minerals indicate that these veins formed from H2O-CO2 fluids partly equilibrated with the mantle. Fe-Mg borate inclusions in gem olivine indicate that the fluid contained substantial B. Trace element concentrations of the vein minerals show patterns relatively enriched in La, Ce, Ta, Cu, and Zn, indicating that these elements were mobile in the vein fluids. These veins provide evidence that CO2 may be mobilized by dissolution of carbonates in fore-arcs and that B- and HFSE-rich reservoirs may form through secondary deposition. Reprocessing of such veins could play an important role in C, B, and HFSE recycling, making these elements not necessarily a direct indicator of the slab devolatilization processes.

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