Conditions of Formation of Iron-Carbon Melt Inclusions in Garnet and Orthopyroxene under P-T Conditions of Lithospheric Mantle

Bataleva, Yu. V.; Palyanov, Yu. N.; Borzdov, Yu. M.; Novoselov, I. D.; Bayukov, O. A.; Sobolev, N. V. Source: PETROLOGY, 26 (6):565-574; 10.1134/S0869591118060024 NOV 2018

Of great importance in the problem of redox evolution of mantle rocks is the reconstruction of scenarios of alteration of Fe0- or Fe3C-bearing rocks by oxidizing mantle metasomatic agents and the evaluation of stability of these phases under the influence of fluids and melts of different compositions. Original results of high-temperature high-pressure experiments (P = 6.3 GPa, T = 1300–1500°С) in the carbide–oxide–carbonate systems (Fe3C–SiO2–(Mg,Ca)CO3 and Fe3C–SiO2–Al2O3–(Mg,Ca)CO3) are reported. Conditions of formation of mantle silicates with metallic or metal–carbon melt inclusions are determined and their stability in the presence of CO2-fluid representing the potential mantle oxidizing metasomatic agent are estimated. It is established that garnet or orthopyroxene and CO2-fluid are formed in the carbide–oxide–carbonate system through decarbonation, with subsequent redox interaction between CO2 and iron carbide. This results in the formation of assemblage of Fe-rich silicates and graphite. Garnet and orthopyroxene contain inclusions of a Fe–C melt, as well as graphite, fayalite, and ferrosilite. It is experimentally demonstrated that the presence of CO2-fluid in interstices does not affect on the preservation of metallic inclusions, as well as graphite inclusions in silicates. Selective capture of Fe–C melt inclusions by mantle silicates is one of the potential scenarios for the conservation of metallic iron in mantle domains altered by mantle oxidizing metasomatic agents.