A Raman spectroscopic study of diamond and disordered sp³‐carbon in the coesite‐bearing Straumen Eclogite Pod, Norway

Abstract

Raman spectral point analysis and Raman mapping have been applied to study the nature of carbonbearing microinclusions C3-1 and C5-1 within zircon in phengite-kyanite-coesite eclogite sample 62a from the Straumen Eclogite Pod in the Western Gneiss Region (WGR), S.W. Norway. Graphitic carbon is always present with a degree of structural disorder comparable to that found in other ultra high pressure metamorphic (UHPM) terranes; it is compatible with an origin by a partial retrogression of the coexisting diamond due to depressurization by exhumation. Hematite coexists with diamond in C3-1. In both microinclusions, the Raman wavenumber position of the diamond peak varies from point to point, from the ideal value of 1332 cm )1 , frequently down to 1328 cm )1 and sometimes down to 1322 cm )1 in C3-1. The bandwidth also increases proportionally. Some spectra reveal doublets with peaks at 1328 and 1322 cm )1 and even a triplet. These Raman data are discussed in terms of one or other kind of structural disorder of sp 3 -carbon, for example metamictization, overtemperature or overpressure, nitrogen impurities and especially stacking disorder, that is, with the possible existence or coexistence of 3C, 2nH and ⁄ or 3mR polytypes in a mixture in the same monocrystal. The question of a metamorphic or a contamination origin of the diamond and the disordered sp 3 -carbon in slide 62a is evaluated; numerous arguments favour the UHPM origin. These new data contribute to the ongoing debate on the various petrological and geodynamical models (including possible deviatoric stress) for the creation of diamond in UHPM rocks in general. This new locality of diamond at Straumen, the fourth in the WGR but the first in the southwestern part of the UHPM area of the WGR, provides a minimum pressure estimate for UHP metamorphism of 3.5 GPa. Taken together with other published P-T estimates, 3.75 ± 0.75 GPa covers all of the estimates of the minimum pressure for coesite-, diamond-or orthopyroxene-bearing eclogite types in the WGR.

Graphite pseudomorphs after diamonds: An experimental study of graphite morphology and the role of H2O in the graphitisation process

“…At such low temperature, the graphitisation processes were not observed in experiments, even on the contact of diamond hosts with fluid inclusions (Nechaev & Khokhryakov, 2013). Furthermore, well-ordered graphites coexist with disordered graphite, although the order-disorder patterns in most cases were verified only by Raman spectroscopy (De Corte et al, 2000;Frezzotti et al, 2011;Janak et al, 2013;Katayama et al, 2000;Korsakov et al, 2002;Massonne, 1999;Mposkos & Kostopoulos, 2001;Perraki et al, 2006;Smith & Godard, 2013;Van Roermund et al, 2002;Fig. 6.…”

“…The coexistence of diamond and graphite in UHPM rocks was documented almost for all diamond-bearing complexes (De Corte et al, 2000;Frezzotti et al, 2011;Janak et al, 2013;Katayama et al, 2000;Korsakov et al, 2002;Massonne, 1999;Mposkos & Kostopoulos, 2001;Perraki et al, 2006;Shatsky et al, 1995;Smith & Godard, 2013;Van Roermund et al, 2002;Vrijmoed et al, 2008). These findings are generally interpreted as partial graphitisation of original diamond crystals (Dobrzhinetskaya et al, 2003;Janak et al, 2013;Massonne, 1998;Ogasawara et al, 2000).…”

Microdiamond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the “vacuum-cleaner” mechanism

“…These Raman bands are downshifted and broader with respect to those of ideal diamond, which is characterized by a Raman band at 1332 ± 0.5 cm -1 with a typical FWHM of 1.65 ± 0.02 cm -1 at room temperature (Solin and Ramdas, 1970). This discrepancy can be due to several reasons such as analytical limitations and accuracy of the band position, internal stress variations, structural defects, or impurities in the analyzed grains (e.g., Perraki et al, 2009;Smith and Godard, 2013). Some Raman spectra (Fig.…”

Nanocrystalline hexagonal diamond formed from glassy carbon

“…Lonsdaleite, named in honour of the crystallographer Kathleen Lonsdale, has been reported in terrestrial sediments and its presence is used as a signature for impacts on the earth’s surface of extra-terrestrial objects123. Bundy and Kasper were the first to report the formation of a sample in the laboratory by static compression that contained diffraction signatures of lonsdaleite from well-ordered graphitic precursors at temperatures above 1000 °C and pressures above 13 GPa4.…”