MANTLE-CRUST INTERACTION DURING DEEP SUBDUCTION: RECORDS IN MICRODIAMONDS FROM UHPM TERRANES

UHPM rocks contain mineralogical and micro-nanoscale features - witnesses of their deep subduction to 210-280 km. The geochemical interactions of the slab with mantle wedge and fluids circulating in subduction channels are important for geosciences. Diamond from UHPM terranes, is the only mineral that due to its chemical inertness is capable to retain “unchanged” fluids and solids that were trapped during its crystallization. The diamonds from UHP metasedimentary rocks were crystallized from a COH-fluid/melt which contained both crustal and mantle components (Al, K, Ba, Ca, Mg, Fe, Si, Ti, Ni, V, Zn, Co, Fe, F, Cl, S, P). Our recent finding of polycrystalline diamonds in both the Kokchetav and the Erzgebirge areas extends our knowledge related to fluid nature and composition. The polycrystalline diamonds consist of 5-15 single crystals of 0.3-5 micron size with a typical “zig-zag” grain boundaries and triangle voids filled with a C-O-H fluid with traces of Al, Na, Co, F, V, Zn, Si, Cl, S, Ca, Mg, Fe, K, P in different combinations. Presence of the carbonate inclusions (CaCO₃ and MgCO₃) in diamonds suggest an oxidizing environment of the fluids to be close to the CCO buffer. Carbon isotopes speciation (δ¹³C = -10 to -27 ‰ for Kokchetav diamonds (e.g.Cartigny et al., 2001), and δ¹³C = -17 to -25 ‰ for the Erzgebirge ones) suggests organic carbon reservoir. Whereas UHP diamond-bearing gneisses have clear sedimentary protolith (based on both their litostratigraphy, and carbon and oxygen isotopes characteristic), traces of deep mantle plume were detected recently in microdiamonds from the Kokchetav massif. These diamonds also contain Th_xO_y and other nanoinclusions which preserve the noble-gas signature trapped at the time of diamond formation. The inclusion-hosted ³He/⁴He content is close to the maximal value observed in OIBs ( Sumino et al., 2011), what suggests that noble gases were enriched in a primordial component. We hypothesized that the primordial gases were delivered to the place of UHPM diamond crystallization by mantle plumes, or the continental slab was interacted with a fragment of the very deep mantle “contaminated” by deep plumes. The deep-mantle-derived fragment might have been delivered to the mantle wedge of the subduction channel by large-scale mantle convection originating from a deeper lower mantle source.