Fluid evolution and graphite genesis in the deep continental crust

Abstract

Metamorphism in deep continental crust is often accompanied by carbon-rich, H₂O-poor fluids^1–5, and is characterized by the development of granulite facies mineral assemblages^3–5. Graphite, a minor phase in many high-grade metamorphic rocks^6–17, 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 gneisses^11,15–19. The presence of supracrustal sequences in granulite facies regions^6–19 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.