Magmatic graphite in dolomite carbonatite at Pogranichnoe,North Transbaikalia,Russia |
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Authors: | A G Doroshkevich F Wall G S Ripp |
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Institution: | (1) Geological Institute SB RAS, Sakhanovoy st., 6a, Ulan-Ude, 670047, Russia;(2) Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK |
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Abstract: | A recently discovered dolomite carbonatite at Pogranichnoe, North Transbaikalia, Russia, dated at 624 ± 3 Ma, contains xenoliths
of calcite-bearing dolomite carbonatite with graphite spherulites. Apatite and aegirine are the other rock-forming minerals.
Chemically the carbonatites are ferrocarbonatite and ferruginous calciocarbonatite. The graphite forms <1 mm up to 1.5 mm
diameter spherulites, with Raman spectra similar to published spectra of microcrystalline, amorphous carbon and disordered
graphite, with G and D bands at 1,580−1,600 cm−1 and at around 1,350 cm−1. Alteration has formed Fe-bearing calcite to Ca-bearing siderite compositions not previously reported in nature around the
graphite along cracks and fractures. Mineral and stable isotope geothermometers and melt inclusion measurements for the carbonatite
all give temperatures of 700°–900°. It is concluded that the graphite precipitated from the ferrocarbonatite magma. There
are three candidates to control the precipitation of graphite (a) a redox reaction with FeII in the magma, (b) potential presence of organics in the magma (c) seeding of, or dissolution in, the magma of graphite/diamond
from the mantle, and further work is required to identify the most important mechanism(s). Graphite in carbonatite is rare,
with no substantial published accounts since the 1960s but graphite at other localities seems also to have precipitated from
carbonatite magma. The precipitation of reduced carbon from carbonatite provides further evidence that diamond formation in
carbonate melts at high mantle pressures is feasible. |
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Keywords: | Dolomite carbonatite Graphite Transbaikalia |
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