title: Raman mapping of coesite inclusions in garnet from the Kokchetav Massif (Northern Kazakhstan).
abstract: Coesite inclusions occur in a wide range of lithologies and coesite is therefore a powerful ultrahigh-pressure (UHP) indicator. The transformation of coesite to quartz is evidenced by three optically well identifiable characteristics (e.g. palisade textures, radial crack patterns, polycrystalline quartz pseudomorphs). Under overpressure monomineralic coesite (on an optical basis), lacking the above transformation characteristics may survive. Raman micro-spectroscopy was applied on monomineralic coesite inclusions in garnet porphyroblasts from diamond-bearing garnet-clinozoisite-biotite gneisses of the Barchi-Kol area (Kokchetav Massif, Northern Kazakhstan). These coesite inclusions are euhedral and display a characteristic anisotropic hallo. However, Raman maps and separate spectra of these inclusions display shifted bands for coesite and quartz. Microscopically undetectable, quartz shows on the Raman map as a thin shell around coesite inclusion. Shift of the main coesite band allows to estimate their overpressure: coesite inclusions record 0-2.4 GPa in garnet and zircon. The quartz shell remains under lower pressure 0-1.6 GPa. The possible application of coesite and quartz Raman geobarometers for UHP metamorphic rocks is discussed.
authors.affiliation: Institute of Geology and Mineralogy of Siberian Branch Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia. korsakov@uiggm.nsc.ru
authors.forename: Andrey V
authors.initials: AV
authors.lastname: Korsakov
authors.forename: Didier
authors.initials: D
authors.lastname: Hutsebaut
authors.forename: Karel
authors.initials: K
authors.lastname: Theunissen
authors.forename: Peter
authors.initials: P
authors.lastname: Vandenabeele
authors.forename: Alexander S
authors.initials: AS
authors.lastname: Stepanov
mesh_terms: D007623:Kazakhstan; D008903:Minerals / Q000737:chemistry*; D011791:Quartz / Q000737:chemistry; D013059:Spectrum Analysis, Raman / Q000379:methods
chemical_list: D008903:Minerals; D011791:Quartz