Graphite-bearing CO 2-fluid inclusions in granulites: Insights on graphite precipitation and carbon isotope evolution

Abstract

Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740°C, 8.9 kb) of Nilgiri hills, southern India were investigated for their spectroscopic and isotopic characteristics. Four types of graphite crystals were identified. The first type (Gr I), which is interstitial to other mineral grains, can be grouped into two subtypes, Gr IA and Gr IB. Gr IA is either irregular in shape or deformed, and rough textured with average δ 13C values of −12.7 ± 0.4‰ ( n = 3). A later generation of interstitial graphite (Gr IB) shows polygonal crystal shapes and highly reflecting smooth surface features. These graphite grains are more common and have δ 13C values of −11.9 ± 0.3‰ ( n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower δ 13C values by ∼0.5‰ compared to that of the rim. The second type of graphite (Gr II) occurs as solid inclusions in silicate minerals, commonly forming regular hexagonal crystals with a slightly disordered structure. The third type of graphite (Gr III) is associated with solid inclusions (up to 100 μm) that have decrepitation halos of numerous small (<15 μm) satellite fluid inclusions of pure CO 2 with varying density (1.105 to 0.75 g/cm 3). The fourth type of graphite (Gr IV) is found as daughter crystals within primary type CO 2-fluid inclusions in garnet and quartz. These fluid inclusions have a range of densities (1.05 to 0.90 g/cm 3), but in general are significantly less dense than graphite-free primary, pure CO 2 fluid inclusions (1.12 g/cm 3). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (∼ 500°C). The precipitation of graphite probably occurred during the isobaric cooling of CO 2-rich peak metamorphic fluid as a result of oxyexsolution of oxide phases. The oxyexsolution process is evidenced by the magnetite-ilmenite granular exsolution textures and the systematic presence of numerous micron-sized rutile and other oxide inclusions in association with fluid inclusions within garnet, plagioclase, and quartz. The carbon isotope compositions of coexisting CO 2 (in fluid inclusions) and graphite show a fractionation ( α CO 2 − gr ) of ∼6‰ in garnet, consistent with the existing theoretical estimates of α CO 2 − gr at 800°C. A subsequent generation of CO 2 inclusions trapped in matrix quartz and quartz segregation have higher δ 13C values, −4‰ and −2.9‰ respectively. Graphite in quartz segregations also has higher δ 13C values (−9.8‰) than those in enderbite (−12.7‰). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average δ 13C values of −11.1, −10.4, and −8.7‰ respectively, indicating progressive enrichment in 13C with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO 2, suggesting isotopic equilibrium during graphite precipitation from CO 2 fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO 2 fluid in enderbite, graphite microcrystals, graphite in quartz segregation, and CO 2 fluid in quartz segregation, suggests a temperature-controlled isotopic evolution. This evolution is in accordance with a closed system Rayleigh-type graphite precipitation process which progressively enriched residual CO 2 in 13C.