MOISSANITE AND DIAMOND FROM THE PRIMITIVE SHOSHONITE DIKE IN EASTERN FINNMARK (NORWAY): EVIDENCE FOR SUPERDEEP SLAB-MANTLE INTERACTIONS IN SUBDUCTION ZONES

Deep slab-mantle interactions and crustal recycling in subduction zones are recorded in highly reduced minerals such as diamond and moissanite in ophiolites and UHP complexes. Numerous moissanite crystals have been recovered from a primitive shoshonite dike emplaced into Archean (2.5-3.7 Ga) gneiss in Eastern Finnmark (Norway). Shoshonite has high MgO (14-20 wt .%) and K₂O (1-6 wt.%) contents, K/Na ratios of > 1 and distinct Nb-Ta-Ti depletions indicating subduction-modified mantle source. Hexagonal moissanite crystals (0.1-0.7 mm) are characterized by black, gray and blue color and characteristic Raman spectra. One composite grain of unusual yellow-colored moissanite containing octahedral diamond core has been identified by optical properties and FTIR spectra. FTIR peaks in the 1000-1750 cm^-1 range suggest that inclusion is a IaAB type diamond with strong structural defects and possible presence of C-H bonding (methane?) either in diamond itself or in moissanite coating. Other minerals in shoshonite include Ba-bearing, Mn-rich ilmenite (MnO=12.91-16.74 wt%, BaO=0.14-0.56 wt.%), Sr-rich Al-Fe-Cr spinel (SrO=2.02-2.23 wt.%), Ba-rutile (BaO=0.69-1.01 wt.%) and Al-rich titanite (Al₂O₃=4-7 wt.%). Mn-ilmenite and Al-rich titanite are compositionally similar to mineral inclusions in lower mantle diamonds from Juina, Brazil and Kankan, Guinea. Sr-spinel and Ba-rutile possibly reflect mantle metasomatism by carbonatitic melt derived from subducted slab. Carbon isotope composition of moissanite (δ¹³C = 16-22 ⁰/₀₀) suggest their derivation from C¹³-deficient lower mantle reservoir formed by superdeep subduction of biogenic carbon. This CLIPPIR mantle reservoir is believed to be the source of some largest diamonds such as Cullinan and Canstellation. Our data suggest that carbonate recycling into lower mantle followed by reaction with metals and formation of superdeep diamonds, moissanite and other carbides is an important part of the global carbon cycle.