Relationship between structure, morphology, and carbon isotopic composition of graphite in marbles: Implications for calcite-graphite carbon isotope thermometry

Abstract

Carbon isotope exchange between calcite and graphite is a useful and reliable geothermometer for medium-to high-grade marbles. However, in rare instances, such as at Naxos, Greece, apparent disequilibrium carbon isotope fractionation between calcite and graphite has been previously reported. In this study, new results are presented on the morphological features, X-ray diffraction studies, Raman spectroscopic studies, and carbon isotope studies of graphite. Three morphologically distinct graphite types are identified. The first type is fine grained with distinct polygonal crystal aggregates having smooth pinacoid faces. The second type consists of large botryoidal aggregates with typical crystal overgrowth features, and the third type is "normal" large platy crystals associated with phlogopite. Graphite morphological features using an FE-SEM suggests that the botryoidal aggregates are composed of complexly intergrown and stacked graphite layers, comprised of cone-helix structures. Different types of graphite display distinct Lc (002) and DG values as well as a sharp first-order Raman peak at ∼1580 cm -1 with disordered bands. The presence of Raman disordered bands in fine-grained well-crystallized graphite is attributed to edge effects. Carbon isotope analysis of graphite reveals that marble with fine-grained well-crystallized graphite show a consistent isotope fractionation with the calcite, which perfectly match temperature estimates based on the mineral isograds. In contrast, coarse botryoidal graphite gives heterogeneous carbon isotope values (about a few per mill) and show anomalous carbon isotope fractionation between the calcite and graphite. The graphite associated with phlogopite also shows variations in the carbon isotope values. Combining morphologic, crystallographic, and carbon isotopic data we conclude that the variations in carbon isotopic composition were caused by the overgrowth of graphite on the preexisting grain and that morphological observations and structural characterizations of graphite crystals is a key for predicting isotopic equilibration during metamorphism.

Carbon isotopes of graphite: Implications on fluid history

“…Moreover, the fractionation during carbon-exchange between two mineral species, such as an isotopically heavier carbonate and isotopically lighter graphite, is a function of temperature. As temperature increases, there is greater carbon isotopic exchange and less fractionation of 13 C between the two phases (Dunn and Valley, 1992;Kitchen and Valley, 1995;Satish-Kumar et al, 2011a). This means that more of the available 13 C will enter graphite at higher than at lower temperatures, and that highergrade metamorphic graphite is therefore likely to be isotopically heavier than lower-grade graphite, in rocks containing carbonate.…”

“…This situation appears to be unlikely in most cases since graphite precipitation from fluids proceeds through nucleation and crystal growth and because the sluggish diffusion kinetics of carbon in graphite once graphite is formed. The mechanism involved in the growth of graphite flakes is that of spiral growth (Santosh et al, 2003;Barrenechea et al, 2009;Satish-Kumar et al, 2011b). Thus, the Rayleigh precipitation process, in which every carbon atom incorporated to the graphite structure is isolated from the fluid, appears to be more appropriate.…”

“…In addition, heterogeneity of carbon isotope ratios within populations of graphite grains in a rock can be recognized in many fluid-deposited occurrences as well as carbon isotope zoning within single graphite crystals (e.g. Santosh and Wada, 1993a;Farquhar et al, 1999;Santosh et al, 2003;Binu-Lal et al, 2003;Satish-Kumar et al, 2011b). Considering the very sluggish kinetics of carbon diffusion in graphite (i.e., graphite can be considered isotopically inert to further alteration once crystallized; Wada, 1988), the isotopic composition of graphite grains is a very useful geochemical feature for detecting changing fluid regimes.…”

Metastable equilibrium in the C-H-O system: Graphite deposition in crustal fluids

“…Thus, the reaction rates associated with establishing equilibrium among C-H-O fluids and graphite could be considered an important part of understanding complex natural processes such as hydrothermal deposition of graphitic carbon, the phase stability along the pathways of carbon subduction, and the extent of 13 C/ 12 C isotopic fractionation along redox and thermal gradients relevant to metasomatic and/or metamorphic events (Duke and Rumble 1986;Rumble and Hoering 1986;Satish-Kumar et al 2011). Furthermore, constraining the thermodynamic stability field of graphitic carbon along the subduction geotherm has important implications for the cycling of carbon in the Earth's interior.…”

“…It is, therefore, of a great interest to relate graphite formation to the speciation of the coexisting C-H-O fluids, and thus, to constrain the redox and thermal conditions of graphitic carbon deposition in hydrothermal veins and metasomatic contacts (Brasier et al 2002;McKeegan et al 2007;Papineau et al 2011;Satish-Kumar et al 2011;van Zuilen et al 2003). Accordingly, graphite-C-H-O geothermometers have been developed by correlating the degree of mineral crystallinity with the Raman spectral characteristics of the ordered (G) and disordered (D) bands of graphitic carbon [i.e., integrated peak areas, full-width half height (FWHH)] (Beyssac et al 2002;Busemann et al 2007;Cody et al 2008;Wopenka and Pasteris 1993).…”