Graphite‐bearing mineral assemblages in the mantle beneath Central Aldan superterrane of North Asian craton: combined confocal micro‐Raman and electron microprobe characterization

Abstract

Mantle‐derived lherzolitic and harzburgitic Cr‐pyropes from lamprophyres of the Chompolo field (Central Aldan superterrane, North Asian Craton) were studied using micro‐Raman spectroscopy and electron microprobe microanalysis. These garnets enclose graphite coexisting with forsterite, diopside, Ba‐Cl‐phlogopite, tschermakite, rutile, magnesiochromite, Mg‐ilmenite, apatite, chalcopyrite, dolomite, magnesite and lindsleyite inclusions. The PT conditions of residence of graphite‐bearing assemblage in the mantle were estimated, using a combination of mineral thermometers and barometers, to be as high as 2.87–3.55 GPa and 710–770 °C. Generally, graphite within inclusions is well ordered; D1 and D2 disordered bands in its spectra are restricted to inclusion edges. The residual pressure up to 2.1 GPa was inferred for graphite assuming pressure dependence of the G‐band upshift (1580 cm−1 at ambient conditions vs 1588.6 cm−1 at 2.1 GPa). Disordered graphite most likely appears due to the stress‐induced distortion of the fully ordered graphite crystal structure. The distortion results from difference in the thermoelastic properties of graphite inclusions and their garnet hosts exposed to decompression during ascent to the surface with lamprophyre magma. The mineralogy of inclusions in the studied garnets strongly suggests an episode(s) of metasomatism by carbon‐rich agent(s) (COH‐fluid or carbonatitic melt) in the lithospheric mantle of the Central Aldan superterrane, which was coeval with the formation of graphite inclusions and the host pyropes. Copyright © 2017 John Wiley & Sons, Ltd.