CO₂-enhanced melting of biotite-bearing rocks at deep-crustal pressure–temperature conditions

Abstract

THE role of CO₂ in melting of the Earth's crust has been a topic of considerable debate. Experimental studies in quartz-feldspar systems^1,2 have shown that CO₂ solubility in simple granitic liquids is less than 1 wt% at pressures below 10 kbar and that dilution of H₂O vapour with CO₂ raises the vapour-saturated solidi³, indicating that CO₂ inhibits, rather than promotes, crustal melting. On the other hand, from experiments on the system K₂O–M_gO–Al₂O₃–SiO₂–H₂O–CO₂, Wendlandt has inferred significant solubility of CO₂ in crustal melts⁴. Wendlandt observed that biotite–quartz–feldspar assemblages melt to orthopyroxene + liquid at temperatures as low as 740 °C in the presence of CO₂-rich fluids, more than 50 °C lower than equivalent orthopyroxene-producing reactions in the pure-H₂O systems⁵. This surprising result implies a fundamental melt interaction between MgO and CO₂, because neither component alone has significant solubility in granitic liquids near the H₂O-saturated solidus⁶. In view of the potentially important consequences of these results for deep-crustal processes, additional data are needed. Here we report the results of experimental melting studies on synthetic biotite–quartz and biotite–quartz–sanidine assemblages. In general agreement with Wendlandt's results, we find that these assemblages melt in the presence of CO₂-rich vapour below 760 °C at pressures <10 kbar, conditions believed to be encountered in high-grade crustal metamorphism. This opens up the possibility that melting of the deep crust may occur in the presence of CO₂-rich fluids.