Microscale isotopic zonation in graphite crystals: Evidence for channelled CO influx in granulites

Abstract

Coarse graphite in association with cordierite and quartz within a vein in a metapelite locality at Chittikara (CHT) in southern India illustrates precipitation of graphite from CO 2-rich fluids. Carbon isotope traverses within individual graphite crystals using a microsampling technique reveal strong isotopic zonations in graphite crystal from the vein, with δ 13C values oscillating between −13‰ in the core and −8.5‰ in the rim. CO 2 trapped within fluid inclusions in the cordierite shows δ 13C= −5‰, suggesting isotopic equilbrium with the core composition of the graphite. A graphite crystal from the metapelite immediately adjacent to the vein is also zoned, with a smooth rimward increase in 13C. Disseminated graphite flakes in the host metapelites ca. 1 m from the vein are, however, unzoned, and possess strongly negative δ 13C values (−34.4‰). The rimward increase in 13C, the absence of other carbon-bearing phases and the sluggish exchange rates for carbon in graphites preclude diffusive re-equilibration, and suggest that the different zoning patterns in the coarse graphite flakes within and adjacent to the vein assemblage at CHT have resulted from precipitation in different parts of a highly channelled fluid regime, where fluids are non-wetting and move episodically by a hydraulic fracture mechanism. Our study provides the first unambiguous insight into graphite precipitation mechanisms, and has potential implications for the nature of fluid interconnectivity and CO 2 transport and equilibration length scales within the crust.