Graphite as an Internal Source of CО2 During Crustal Anatexis: Experimental Study on Melting of Graphite-Bearing Garnet–Two Mica Schist at 500 MPa and 900°C

Abstract

In addition to CO₂ coming from external mantle sources, CO₂ generated by the transformation of carbonaceous material of the protolith (internal sources) is actively involved in the processes of high-grade metamorphism in the crust. The paper presents results of experiments at 500 MPa and 900°C on the partial melting of plagioclase-free garnet–two mica (+ quartz, apatite, ilmenite) schist containing 0, 4.2, 10.1, 14.6, and 18.6 wt % graphite. The melting of graphite-free rock results in peraluminous melts corresponding to alkali-calcic ultrapotassic granites. As the graphite content is increased, the A/CNK and A/NK indices decrease, the MALI index of the melts increases, and their compositions shift toward alkalic granites. The estimated content of H₂O + CO₂ in the melts decreases with an increase in the graphite content in the starting system. The peritectic phases are hercynite–magnetite spinel, orthoamphibole (gedrite), sillimanite, and potassium feldspar. The decrease in the Fe³⁺/ΣFe ratio in Fe–Mg minerals with an increase in the graphite content in the starting mixtures suggests more reducing conditions. This conclusion is confirmed by the logfO₂ values calculated from the equilibrium of spinel, sillimanite, and quartz in the experimental products, which range from ~NNO + 0.5 for experiments in the absence of graphite to lower than ~NNO − 1.5 for experiments in the presence of more than 14 wt % graphite. The interaction of Fe₂O₃ and, possibly, H₂O, released as a result of peritectic melting reactions of the initial schist minerals (primarily micas) with graphite facilitates the formation of CO₂. Modeling of phase relations showed that, along with oxygen fugacity, water activity could be an additional factor influencing phase compositions in the presence of graphite. The Raman spectroscopy of the quenched melts and bubbles in them demonstrates that CO₂ is not only the predominant component of the free fluid phase accompanying the melts but is also partially dissolved in the melt as molecular CO₂ and \({\text{CO}}_{3}^{{2 - }}\) complexes with alkali and alkali-earth cations. Experiments demonstrate that under conditions of high-grade metamorphism, graphite-bearing metapelites can serve as an effective internal source for CO₂ accompanying granite melts during anatexis.