Subsolidus evolution of the magnetite-spinel-ulvöspinel solid solutions in the Kovdor Phoscorite-Carbonatite Complex

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Marco E. Ciriotti
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Subsolidus evolution of the magnetite-spinel-ulvöspinel solid solutions in the Kovdor Phoscorite-Carbonatite Complex

Messaggio da Marco E. Ciriotti » ven 10 nov, 2017 9:56

Referenza:
▪ Ivanyuk, G.Yu., Kalashnikov, A.O., Pakhomovsky, Ya.A., Bazai, A.V., Goryainov, P.M., Mikhailova, J.A., Yakovenchuk, V.N., Konopleva, N.G. (2017): Subsolidus Evolution of the Magnetite-Spinel-UlvöSpinel Solid Solutions in the Kovdor Phoscorite-Carbonatite Complex, NW Russia. Minerals, 7, 215; doi:10.3390/min7110215.

Abstract:
The Kovdor phoscorite-carbonatite ore-pipe rocks form a natural series, where apatite and magnetite first gradually increase due to the presence of earlier crystallizing forsterite in the pipe marginal zone and then decrease as a result of carbonate development in the axial zone. In all lithologies, magnetite grains contain (oxy)exsolution inclusions of comparatively earlier ilmenite group minerals and/or later spinel, and their relationship reflects the concentric zonation of the pipe. The temperature and oxygen fugacity of titanomagnetite oxy-exsolution decreases in the natural rock sequence from about 500 °C to about 300 °C and from NNO + 1 to NNO − 3 (NNO is Ni-NiO oxygen fugacity buffer), with a secondary positive maximum for vein calcite carbonatite. Exsolution spinel forms spherical grains, octahedral crystals, six-beam and eight-beam skeletal crystals co-oriented with host magnetite. The ilmenite group minerals occur as lamellae oriented along {111} and {100} planes of oxy-exsolved magnetite. The kinetics of inclusion growth depends mainly on the diffusivity of cations in magnetite: their comparatively low diffusivities in phoscorite and carbonatites of the ore-pipe internal part cause size-independent growth of exsolution inclusions; while higher diffusivities of cations in surrounding rocks, marginal forsterite-rich phoscorite and vein calcite carbonatite result in size-dependent growth of inclusions.
Marco E. Ciriotti

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