Redox reactions in carbonate-rich magmas

Ryabchikov I.D.*, Kogarko L.N. **

*Institute for Geology of Ore Deposits, Petrography, mineralogy and Geochemistry, Moscow, Russia

** Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia

Compositions of co-existing minerals from graphite bearing carbonatites were obtained by the EPMA analysis for beforsites from Chernigovsky complex (Ukraine)(Krivdik et al., 1997), Pogranichnoe dolomite-rich carbonatites (Doroshkevich et al., 2007), Chagaday carbonatites, Uzbekistan, Khibina alkaline igneous complex, which includes carbonatites and graphite-bearing rocks (Kola Peninsula) (Nivin et al., 2005), and carbonatites of Gremyakha-Vyrmes magmatic complex (Kola Peninsula). In all cases graphite-bearing rocks contain magnetite. Sometimes late magnetite and graphite form intimate intergrowths. Thermodynamic analysis of equilibria between magnetite, silicate minerals, carbonates and graphite permitted to estimate temperatures and oxygen fugacities prevailing during the formation of the investigated rocks.

Chagaday graphite-bearing carbonatites, in which several grains of diamond were also reported, contain calcite, apatite, magnetite, clinopyroxene, albite and K-feldspar.

‰Temperatures of equilibrium for mineral clinopyroxene+calcite+ titanomagnetite+titanite+graphite + albite + nepheline depending on titanomagnetite compositions were calculated using equilibrium constants of the following reaction:

4 CaFeSi₂O₆+3CaCO₃+7 Fe₂TiO₄+0.5NaAlSiO₄=7 CaTiSiO₅+6 Fe₃O₄+3C+0.5NaAlSi₃O₈‰‰‰‰‰‰‰‰

These calculations demonstrated that for a given magnetite composition decrease in temperature causes formation of graphite together with magnetite. Therefore, appearance of graphite in carbonatites may be caused by cooling, and graphite may crystallize from melt, it may form by solid-state reactions, or precipitate from cooling aqueous fluid.

‰f_O2 values were estimated from the equilibrium constant of the reaction

6CaFeSi₂O₆+6CaCO₃+12Fe₂TiO₄=12CaTiSiO₅+10Fe₃O₄+6C+O₂‰‰‰‰‰

Calculated ‰f_O2 values are 0.5 to 1 log units below QMF buffer. Similar values of oxygen fugacities were estimated for other investigated graphite-bearing carbonatites.

Diamond forms in subcontinental lithosphere also due to the reductions of near-solidus carbonate-rich melts arriving from asthenosphere or from rising plume. This reduction is caused by interaction of these melts with the rocks of lower lithosphere, which are characterized by very low f_O2 values.

‰‰ This study was financially supported by RFBR grant 08-05-00356-·.

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References:

Doroshkevich A. G., Wall F., and Ripp G. S. (2007) Magmatic graphite in dolomite carbonatite at Pogranichnoe, North Transbaikalia, Russia. Contributions to Mineralogy and Petrology 153(3), 339-353.

Krivdik S. G., Zagnitko V. N., and Lugovaya I. P. (1997) Isotope composition of minerals in carbonatites of the Chernigovsky massif (Azov Territory) as indication of crystallisation conditions (in Russian). Mineral Zhurn 19(6).

Nivin V. A., Treloar P. J., Konopleva N. G., and Ikorsky S. V. (2005) A review of the occurrence, form and origin of C-bearing species in the Khibiny Alkaline Igneous Complex, Kola Peninsula, NW Russia. Lithos 85, 93-112.