DIAMOND FORMATION IN SUBDUCTION ZONES: KEY PROCESSES AND FEATURES
V.S. Shatsky1,2,3, A.L. Ragozin1, V.N. Reutsky1, V.V. Kalinina1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
3A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
Keywords: Diamond inclusion, diamond formation, subduction zone
Abstract
New evidence for the involvement of Fe-C-O melts in diamond formation from placers of the northeastern Siberian craton - combined with our data on iron carbide inclusions and previous research - provides a fresh perspective on diamond formation in subduction zones. Inclusions of iron carbides and oxides, along with moissanite and carbonates in polyphase inclusions, attest to a heterogeneous diamond-forming environment. Extreme variations in oxygen fugacity during diamond growth likely result from hydrogen and hydrocarbon generation via interaction between carbonated rocks of the subducting oceanic lithospheric slab and aqueous fluids. Separated hydrocarbon fluids can create localized ultra-reduced mantle regions where silicon carbide forms under conditions of nonequilibrium with the environment. A key characteristic of the studied diamonds is brittle fracture followed by crack healing, which is associated with the formation of polyphase iron carbide and oxide inclusions interpreted as trapped melts. We attribute brittle diamond fracture in the lower lithosphere to high strain rates localized in hypocenters of deep-focus earthquakes within the subducting lithospheric slab, triggered by dehydration or carbonatite melt formation.
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