The presence and speciation of volatile C-H-O elements in the silicate systems play an important role in the genesis of magmas on the Earth’s mantle, due to the fact that these elements, mainly in the form of H₂O, CO₂, CH₄ and CxHy, decrease the solidi temperatures of source rocks, making magmatism possible in Earth’s present day thermal conditions [1]. Among those elements, carbon is the only element that changes its valence according to the oxygen fugacity (fO₂) conditions of the environment, resulting in different speciation, as: CO₃, CO₂, C_{graphite/diamond}, CH₄ or heavier hydrocarbons. In the present work, we are determining carbon speciation in equilibrium with synthetic fertile, hydrated mantle, at 5.5 GPa and up to 1300°C, using a 1000 tonf hydraulic press coupled with toroidal chambers. Preliminary experiments performed at 1300°C and 5.5GPa (initial composition in the Olivine-Quartz-Kalsalite/Nepheline system: 40mol% Ol₉₀, 40mol% Nph₅₀Kls₅₀ and 20mol% Qz, P_H2O,CO2=P_total) resulted in the formation of forsterite (Fo₉₀) in equilibrium with phlogopite (Phl), melt and volatile phases. At 1300°C, ~2 vol.% of gas was formed, based on bubbles distribution estimative in the melt. Raman analysis on this melt shows CH₄ generation in fO₂ conditions near MMO buffer, a more oxidizing condition in comparison to that used by previous works (IW buffer conditions) [e.g. 2, 3, 4].

References:

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2. K.D. Litasov. Russian Geology and Geophysics 52, 475-492 (2011).
3. P.S. Scott, R.J. Hemley, H-k. Mao, D.R. Herchbach, L.E. Fried, W.M. Howard, S. Bastea. PNAS 101, 14023-14026.
4. E. Mukhina, A. Kolesnikov, V. Kolesnikov. Nature Scientific Reports 7, 5749.