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.

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

“…Coesite-to-quartz transformation is accomplished with significant volume changes; very high residual pressures can be preserved in ''monomineralic'' (following the definition of Parkinson, 2000) coesite inclusions, as well as in partly transformed coesite (Parkinson & Katayama, 1999;Sobolev et al, 2000;Ye et al, 2001;Korsakov et al, 2007). The Raman shift of quartz was recently tested on natural samples in order to estimate the effect of peak metamorphic pressure on residual pressure within the quartz inclusions (Enami et al, 2007).…”

“…They have homogeneous cores whereas slight variations in their composition occur in narrow ($100 mm) external zones (Shatsky et al, 1995;Korsakov et al, 1998;Hermann et al, 2001;Korsakov et al, 2002;Korsakov & Hermann, 2006). Diamond and coesite inclusions identified in the garnet porphyroblasts point to peak metamorphic conditions of P$4-6 GPa and T$950-1000 C (Sobolev & Shatsky, 1990;Hermann et al, 2001;Korsakov et al, 2007). Higher peak metamorphic conditions of P$6-8 GPa and T ¼ 1100 C have been reported by Ota et al (2000), Massonne (2003) and Dobrzhinetskaya et al (2006).…”

“…Mineral assemblages, P-T estimates and characteristic features of garnet porphyroblasts from coesite-and diamond-grade metamorphic rocks from the Kokchetav Massif (Northern Kazakhstan) and eclogite xenolith from the Mir kimberlite pipe (Yakutiya). Massonne (2003), Shatsky et al (2005), Korsakov et al (2005Korsakov et al ( , 2007. small garnet grains and the mantles and the rims of the large garnet porphyroblasts contain numerous microscopic rutile rods.…”

“…BSE images of garnet porphyroblasts and P-T paths for coesite-(a-d) and diamond-grade (e-f) metamorphic rocks from the Kokchetav massif plotted on metamorphic facies diagram (after Liou et al, 1998;Okamoto & Maruyama, 1999). The P-T stages indicated along the path of the coesite-and diamond-grade metamorphic rocks are based on petrologic data of Parkinson (2000), Theunissen et al (2007) and Hermann et al (2001), Korsakov et al (2007), respectively. Thick arrows indicate the different stages of garnet growth.…”

Pressure-induced phase transitions in coesite

“…Shock-induced formation of coesite has been also observed in meteorites, where it can be found along with other shock minerals (e.g., Ohtani et al 2011). Terrestrial occurrence of coesite was reported from deeply (>100 km) subducted crustal rocks (ultrahigh-pressure metamorphic rocks) or mantle derived rocks (kimberlites), which have been exhumed under conditions that prevent retrograde transition to quartz (e.g., Smyth and Hatton 1977;Chopin 1984;Mosenfelder and Bohlen 1997;Parkinson 2000;Korsakov et al 2007;Ruiz-Cruz and Sanz de Galdeano 2012). In these rocks, coesite commonly occurs as an inclusion within clinopyroxene, olivine, garnet, or diamond.…”