Korsakov Andrey V, Hutsebaut Didier, Theunissen Karel, Vandenabeele Peter, Stepanov Alexander S
Institute of Geology and Mineralogy of Siberian Branch Russian Academy of Sciences, Koptyug Pr. 3, Novosibirsk 630090, Russia.
Spectrochim Acta A Mol Biomol Spectrosc. 2007 Dec 15;68(4):1046-52. doi: 10.1016/j.saa.2007.04.005. Epub 2007 Apr 19.
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.
柯石英包裹体存在于多种岩性中,因此柯石英是一种强大的超高压(UHP)指示矿物。柯石英向石英的转变可通过三个光学上易于识别的特征来证明(例如栅栏状结构、径向裂纹模式、多晶石英假象)。在超压条件下,基于光学观察为单矿物的柯石英(缺乏上述转变特征)可能会留存下来。拉曼显微光谱被应用于来自巴尔奇 - 科尔地区(哈萨克斯坦北部科克切塔夫地块)含金刚石的石榴石 - 斜黝帘石 - 黑云母片麻岩中石榴石斑晶内的单矿物柯石英包裹体。这些柯石英包裹体呈自形,具有特征性的各向异性晕圈。然而,这些包裹体的拉曼图谱和单独光谱显示柯石英和石英的谱带发生了位移。在显微镜下无法检测到的石英,在拉曼图谱上显示为柯石英包裹体周围的薄壳。柯石英主谱带的位移有助于估算其超压:石榴石和锆石中的柯石英包裹体记录的超压为0 - 2.4吉帕。石英壳层处于较低压力0 - 1.6吉帕。文中还讨论了柯石英和石英拉曼地质压力计在超高压变质岩中的可能应用。