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

In addition to CO coming from external mantle sources, CO 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. One of the mechanisms of the CO generation may be the oxidation of graphite from meta-sedimentary rocks with Fe and/or HO released during the decomposition and/ or partial melting reactions of micas, which usually contain significant amounts of Fe. Paper present the results of experiments at 500 MPa and 900°C on partial melting of plagioclase-free garnet-two mica (+ quartz, apatite, ilmenite) schist containing 0, 4.2, 10.1, 14.6 and 18.6 wt. % of graphite. Melting of graphite-free rock leads to the formation of peraluminous melts corresponding to alkali-calcic ultra-potassic granites. As the graphite content increases, the A/CNK and A/NK indices decrease and the MALI index of melts increases, and their compositions shift towards alkalic granites. The estimated content of HO + CO in the melts decreases with an increase in the graphite content in the starting system. The peritectic phases are represented by heroynite-magnetite spinel, orthoamphibole (gedrite), sillimanite, and potassium feldspar. A decrease in the Fe/ΣFe ratio in Fe-Mg minerals with an increase in the graphite content in the starting mixtures manifests an increase in reducing conditions. This conclusion is confirmed by the fO values calculated from the equilibrium of spinel, sillimanite, and quartz in the experimental products that range from ~NNO+0.5 for experiments in the absence of graphite to values less than ~NNO−1.5 for experiments in the presence of more than 14 wt. % graphite. The interaction of FeO and, possibly, HO, released as a result of peritectic melting reactions of the initial schist minerals (primarily micas) with graphite provides the formation of CO. 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. Raman spectroscopy of quenched melts and bubbles in them demonstrates that CO is not only the predominant component of the free fluid phase accompanying the melts, but is also partially dissolved in the melt as molecular CO and CO complexes with alkaline and alkaline earth cations. Experiments demonstrate that under conditions of high-grade metamorphism, graphite-bearing metaplates can serve as an effective internal source for CO accompanying granite melts during anatexis.