Fig 1
Graphite (Grph) cover carbonate (Ca). Secondary electron image
Fig 2
Graphite (black) intergrowth with phlogopite (light-grey) in calcite (grey).
Back-scattered electron image
Graphite-bearing carbonatite of Guli massif.
Zaitsev V.A. Sorokhtina N.V. Kogarko L.N.
The Guli massif is a word-largest alkaline-ultrafic complex, located in a Nord of Siberian platform. Carbonatites form two separate bodies, named Nord carbonatite massif and South carbonatite massifs. First time for this massif we have found graphite in carbonatite from the South carbonatite massif.
This carbonatite is anhimonomineral calcitic rock (sevite) with minor amounts of dolomite, phlogopite, amphibole and K-feldspar. Accessory minerals are apatite, pyrite, graphite, and Nb-bearing rutile.
Wall-rock composition was estimated by X-ray fluorescent analyse: SiO2–0.95 % Al2O3–0.14 % Fe2O3*–0.48 % MnO–0.12 % MgO–1.46 % CaO–53.72 % Na2O–0.14 % K2O–0.07 % P2O5–0.02 % S2-–0.03 % Sr–0.02 % LOI–42.9 % Total –100.05 %.
Neutron-activation analyse shows relatively-low contens Sr and Ba for carbonatites (2415 и 240 ppm, respectively). Similar concentrations of Sr и Ba are reported by Egorov for «dolomitic metacarbonatites» from Guli massif. This rock is strongly depleted by the rare-earth elements (total content of REE is 28 ppm). The REE pattern shows gentle slope from La to Lu with positive Eu-anomaly.
Graphite forms the crusts on the carbonate crystals and intergrowth with phlogopite (fig. 1-2).
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Fig 1 Graphite (Grph) cover carbonate (Ca). Secondary electron image |
Fig 2 Graphite (black) intergrowth with phlogopite (light-grey) in calcite (grey). Back-scattered electron image |
Raman spectra of graphite samples from the Guli carbonatite are largely similar to published spectra of crystalline graphite. The G band exhibited notable band broadening, with G band FWHMs in the range 24-34 cm-1 in compare with graphite form Gremiakha-Vyrmes and Pogranichnoe. This band FWHMs correspond to temperature 320-500 œC of graphite formation.
The mineral composition was estimated by the electron-microprobe analyses.
Calcite contain 2-3% of dolomite component and up to 1% of siderite component. Dolomite contain 3-4 % of calcite component. The calcite-dolomite solvus curve (Anovitz Essene 1987) shows that dolomite composition correspond to the temperature as 550-600œ C, and calcite near 300œ C.
Amphybole (richterite) composition correspond the formula (K0.1-0.3Na0.5-0.8)0.9(Na1-1.3Ca0.1-1)2(Mg4.5-4.7Fe0.1-0.3Al0.1Ti0.05)5Si8O22(OH)2, phlogopite – K(Mg2.8Fe0.06Ti0.05)Si3.2Al0.9O11(OH)2 and K-feldspar - K0.95Na0.04Ca0.01AlSi3O8.
The formations conditions of association including dolomite, calcite, K-feldspar and phlogopite can be estimated from the reaction 3 Do +1 H2O +1Kfs =3 CO2+3 Cc +1Phl. This equilibrium depends on temperature, pressure and proportion of CO2/H2O in fluid.
E.M. Epstein has estimated depth of Guly carbonatites formation by the fluid inclusions and by paleoreconstruction of erosion level as 2.1-2.7 km. It correspond the pressure approximately 1 Kbar. If fluid contents 80-95% CO2, equilibrium 3 Do +1 H2O +1Kfs =3 CO2+3 Cc +1Phl coresponds 450-500 œC. If fluid was richer in CO2, the temperature estimation might be higher.
Fig. 3
P-T and f(O2)-T conditions for dolomite+K-feldspar+calcite+phlogopite+graphite association estimated by thermochemical calculations with program complex BAYES (Chatterjee et al., 1998).
We have estimated the conditions of equilibrium 3 Do +1 H2O +1 Kfs = 3 Cc +3 Grph +3 O2 +1 Phl (fig.3). Eqilibrium line Do+H2O+Kfs=Cc+Grph+Phl crosses lines of CO2-graphite and QFM – buffers at 500-500œC. The topology of T-f(O2) diagram show that cooling in the closed system Dol+Kfs+Cc+Phl+Grph+H2O results in disappear of calcite, phlogopite or graphite, but cooling along the QFM-buffered line result in formation of graphite, calcite and phlogopite after K-feldspar and dolomite at temperature lower than 500-550œ.
It mean that graphite in this rock can be formed only due to addition of reduced fluid, including the fluid, equilibrated with QFM-buffered rock at the tempetatures lower than 500œ C.
References:
Anovitz L. M., Essene E. J. Phase Equilibria in the System CaCO3-MgCO3-FeCO3 Joumal of Petrology, Vol. 28, Put 2, pp. 389-414, 1987
Egorov, L.S. (1991) Ijolite Carbonatite Plutonism. Nedra, Leningrad, 260 pp.
Epstein E.M. Geologo-Petrological model and genetic features of ore-bearing carbonatite complexes.// M: Nedra, 1994, 256 pp. ( in Russian).
Chatterjee N.D., KrØger R., Haller G. & Olbricht W. (1998): The Bayesian approach to an internally consistent thermodynamic database: theory, database, and generation of phase diagrams. Contrib. Mineral. Petrol., 133, 149-168
Zaitsev V.A., Sorokhtina N.V., Nasdala L. Kogarko L.N Raman spectroscopic investigation of graphite from the Gremyakha Vyrmes and Pogranichnoe carbonatites, Russia Geophysical Research Abstracts, Vol. 11, EGU2009-134-3, 2009.