Graphite-bearing CO2-fluid inclusions in granulites: Insights on graphite precipitation and carbon isotope evolution

摘要

Graphite in deep crustal enderbitic (orthopyroxene + garnet + plagioclase + quartz) granulites (740 degrees 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 delta(13)C values of -12.7 +/- 0.4 parts per thousand (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 delta(13)C values of -11.9 +/- 0.3 parts per thousand (n = 14). Both subtypes show well-defined Raman shifts suggesting a highly crystalline nature. Cores of interstitial graphite grains have, on average, lower delta(13)C values by similar to 0.5 parts per thousand 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 mu m) that have decrepitation halos of numerous small (< 15 mu m) satellite fluid inclusions of pure CO2 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,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 CO2 fluid inclusions (1.12 g/cm(3)). Raman spectral characteristics of graphite inside fluid inclusions suggest graphite crystallization at low temperature (similar to 500 degrees C). The precipitation of graphite probably occurred during the isobaric cooling of CO2-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 CO2 (in fluid inclusions) and graphite show a fractionation (alpha(CO2-gr)) of -6 parts per thousand in Garnet, consistent with the existing theoretical estimates of alpha(CO2-gr) at 800 degrees C. A subsequent generation of CO2 inclusions trapped in matrix quartz and quartz segregation have higher values, -4 parts per thousand and -2.9 parts per thousand respectively. Graphite in quartz segregations also has higher delta(13)C values (-9.8 parts per thousand) than those in enderbite (-12.7 parts per thousand). Micro-graphite crystals included in garnet, quartz (enderbite), and quartz (segregation) have average delta(13)C values of -11.1, -10.4, and -8.7 parts per thousand respectively, indicating progressive enrichment in C-13 with a decrease in temperature of recrystallization of respective minerals. This progressive enrichment is also observed in carbon isotope compositions of fluid inclusion CO2, suggesting isotopic equilibrium during Graphite precipitation from CO2 fluids. Thus, the carbon isotope record preserved in these rocks by the interstitial graphite, CO2 fluid in enderbite, graphite inicrocrystals, graphite in quartz segregation, and CO2 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 CO2 in C-13. Copyright (c) 2005 Elsevier Ltd.