Morphology and Raman spectral parameters of Bohemian microdiamonds: implications to elastic geothermobarometry

[摘要] In this work, we combine the morphology and internal structure of northwestern Bohemian microdiamonds with theirRaman spectral parameters to describe and understand their relationship. We evaluate our data according to the theoryof elasticity and discuss implications for elastic geothermobarometry of diamond inclusions in garnet. We conclude thatmicrodiamonds enclosed in kyanite, garnet and zircon differ in morphology and internal structure depending on the typeof the host rock and host phase. Single crystal diamond octahedra in kyanite in the acidic gneiss show predominantlyRaman shift towards higher wavenumbers (upshift), while single and polycrystalline diamonds enclosed in garnet andzircon in the intermediate garnet–clinopyroxene rock yield more variable Raman shift including a shift towards lowerwavenumbers (downshift). This is consistent with closed boundaries between diamond and kyanite observed usingFIB-TEM, while interfaces between diamond and garnet or zircon are commonly open. Moreover, higher variability inthe Raman shift in diamond hosted by garnet or zircon may be caused by complex internal structure and the presenceof other phases. At the same time, a diamond in kyanite features relatively high full-width-at-half-maximum (FWHM)due to the anisotropy of thermal contraction, which is reflected by the plastic deformation of diamond mediated bydislocation glide at T ≥ 1000 °C. The entrapment pressure (Ptrap) for diamonds in garnet was calculated using elasticgeobarometry to test its compatibility with the existing peak pressure estimated by conventional thermobarometry. The“downshifted” diamonds exhibit entrapment pressures of 4.8±0.14 and 4.99±0.14 GPa at an entrapment temperature of1100 °C, using unstrained reference diamond from the literature and own measurements, respectively. This is consistentwith the earlier estimates and the elastic theory and does not require any elastic resetting suggested to account for thereported upshift in garnet. Our data suggest that the upshift in diamond hosted by garnet is related to the proximity ofother diamond grains. We conclude that the use of diamond inclusions in elastic barometry should be backed by carefulevaluation of its internal structure and associated phases and restricted to isometric monocrystalline diamond grains notoccurring in clusters as required by the method.

[学科分类] 自动化工程

[关键词] microdiamond;ultra-high-pressure rock;Raman spectroscopy;morphology;elastic geobarometry;Bohemian Massif