Fluid evolution and graphite genesis in the deep continental crust

Abstract

Metamorphism in deep continental crust is often accompanied by carbon-rich, H2O-poor fluids1–5, and is characterized by the development of granulite facies mineral assemblages3–5. Graphite, a minor phase in many high-grade metamorphic rocks6–17, has also been recognized in recent xenoliths of deep crustal material18, demonstrating its presence in modern deep continental crust. Although graphite is most common in biotite schists, it has also been recorded in pyroxene gneisses, two feldspar-quartz pegmatites, marbles and quartzo-feldspathic gneisses11,15–19. The presence of supracrustal sequences in granulite facies regions6–19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material. Much of this material must have experienced a metamorphic recrystallization in the amphibolite facies, accompanied by the development of hornblende. Although graphite in the high-grade rock may develop from oxidation of organic material, this may not be the most common means for graphite genesis. In much of West Greenland14 and in southern Norway12 the low-grade, hornblende-bearing material does not contain significant quantities of organic compounds, although graphite is found in higher-grade metamorphic equivalents. This strongly suggests that introducing CO2 into deep crustal material leads to fluid evolution and graphite formation. I outline here the evolution of fluid composition in the C–O–H system which is in equilibrium with common crustal mineral assemblages. Thermodynamic data for CO2–H2O mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of CO2 into various types of rock. The results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a CO2-rich fluid phase. In addition, the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane-rich gas.