Carbon isotopes of graphite: Implications on fluid history

Abstract

Stable carbon isotope geochemistry provides important information for the recognition of fundamental isotope exchange processes related to the movement of carbon in the lithosphere and permits the elaboration of models for the global carbon cycle. Carbon isotope ratios in fluid-deposited graphite are powerful tools for unravelling the ultimate origin of carbon (organic matter, mantle, or carbonates) and help to constrain the fluid history and the mechanisms involved in graphite deposition. Graphite precipitation in fluid-deposited occurrences results from CO 2 -and/or CH 4 -bearing aqueous fluids. Fluid flow can be considered as both a closed (without replenishment of the fluid) or an open system (with renewal of the fluid by successive fluid batches). In closed systems, carbon isotope systematics in graphite is mainly governed by Rayleigh precipitation and/or by changes in temperature affecting the fractionation factor between fluid and graphite. Such processes result in zoned graphite crystals or in successive graphite generations showing, in both cases, isotopic variation towards progressive 13 C or 12 C enrichment (depending upon the dominant carbon phase in the fluid, CO 2 or CH 4 , respectively). In open systems, in which carbon is episodically introduced along the fracture systems, the carbon systematics is more complex and individual graphite crystals may display oscillatory zoning because of Rayleigh precipitation or heterogeneous variations of d 13 C values when mixing of fluids or changes in the composition of the fluids are the mechanisms responsible for graphite precipitation. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Vein graphite deposits: geological settings, origin, and economic significance

“…In addition, Duke et al (1990) Detailed isotopic studies at the microscale in graphite from vein occurrences in granulite terranes demonstrate isotopic variations within single graphite crystals Luque et al 2012b, and references therein).…”

“…However, in syngenetic graphite occurrences this straightforward relation of the carbon isotopic signature in terms of the carbon source is usually modified by the fractionation effects depending upon the temperature and the presence of other mineral phases (carbonates). The recognition of the origin of carbon in fluid-deposited graphite is even more complicated because of the possibility of mixing different carbon reservoirs before or while graphite precipitated from the fluid and by the fractionation of carbon between the species in the fluid and graphite during the evolution of the fluid (see Luque et al 2012b for a comprehensive discussion).…”

The geochemistry of methane isotopologues

“…Furthermore, CH 4 / 3 He ratios in vent fluids (see Keir, 2010) indicate less-than-quantitative conversion (~0.2% to 50%) of mantle carbon to CH 4 (C/ 3 He~1×10 9 , Marty and Tolstikhin, 1998). Precipitation of graphite from a CH 4 -rich fluid entrapped in plutonic rocks may explain both the missing carbon and the observed δ 13 C values (Luque et al, 2012). Fig.…”

Correlación entre parámetros químicos, cristalográficos y espectroscópicos en la termometría de grafito aplicada a una aureola de contacto del monzogranito de La Soledad (Andes venezolanos)

“…El incremento en este parámetro en las cercanías del contacto es ocasionado por el intercambio isotópico entre el grafito y las fases fluidas, concomitante con la devolatilización que tiene lugar en las condiciones imperantes durante la intrusión (Luque et al, 2012).…”

“…Los gases que emergen y contienen carbono, principalmente CH 4 , deben ser ricos en el isótopo ligero, derivando en un enriquecimiento del isótopo pesado en el residuo (Morikiyo, 1986;Wada et al, 1994). Después que el grafito de origen singenético se ha formado, la composición isotópica de éste puede variar bajo condiciones de equilibrio a través de reacciones de intercambio con compuestos de carbono (principalmente CH 4 y CO 2 ) en fase fluida, lo que implica la posible aparición de grafito epigenético (Hoefs & Frey, 1976;Crespo et al, 2005;Luque et al, 2012).…”