Publication details
Spectral parameters of microdiamonds from north Bohemian Saxothuringian basement
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| Year of publication | 2019 |
| Type | Conference abstract |
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| Description | Microdiamonds reaching 10–30 microns in size are enclosed in kyanite, garnet and zircon within two types of the ultra-high-pressure rocks (acid and intermediate, with mineral assemblage garnet-kyanite-feldspar-quartz and garnet-clinopyroxene-feldspar-quartz, respectively) both from boreholes and outcrops in the north Bohemian Saxothuringian basement (Kotková et al. 2011). Microdiamonds are well-preserved without graphite coating. We focused on spectral parameters of microdiamonds, located under the surface of the thin-section within various host phases, as a reflection of microdiamond structure and stress state, the latter potentially constraining the exhumation path. Single octahedral microdiamonds in the acid UHP rock exhibit perfect to slightly distorted crystals with sharp edges, closed and straight boundaries towards the host phase, and rare resorption and growth features on crystal faces. The position of the first-order Raman band of the diamond octahedra varies mostly between 1332.1 cm-1 and 1334.4 cm-1 (1336.7 cm-1 at maximum), plotting in the up-shift region in Fig. 1. FWHM (full width at half band-maximum) shows a narrow range of 6.3–7.8 (10 at maximum). The Raman spectral parameters do not change with diamond morphology (i.e. perfect octahedra vs. elongated, distorted grains, all enclosed in kyanite). Cuboid microdiamonds in the intermediate UHP rock occur both as individual grains and clusters and can be monocrystalline or polycrystalline. They show irregular boundaries towards the host phase and cavities and gaps at the diamond-host interface. Resorption and growth features are common. The Raman shift of cuboid diamonds under the surface (1331.2 cm-1–1334.4 cm-1) extends to the down-shift region in Fig. 1, and FWHM range (2.8–8.0) is large compared to diamond octahedra from the acid UHP rock. Both the down-shift and variable FWHM up to relatively high values are characteristic of diamond enclosed in zircon, whereas diamond within garnet features both up-shift and down-shift at lower and less variable FWHM. Raman spectral parameters do not depend on the diamond grain character (individual monocrystal, monocrystal in clusters, cuboid vs. elongated grain). The contrasting values and range of FWHM for the two distinct diamond morphologies relate to their variable crystallinity, FWHM of c. 6–8 for octahedral diamond being typical of these well-crystallized monocrystalline microdiamonds (comp. 1.6 for macroscopic diamonds (Solin and Ramdas 1970)). Based on experimental data, which constrain pressure dependence of the first-order Raman band in diamond (Hanfland and Syassen 1985), the microdiamonds exhibit residual overpressure of 0.8 GPa (up-shift of 1334.4 cm-1). However, according to the theory of elasticity and to a concept of isomeke, diamond enclosed in garnet should develop underpressure on exhumation as a result of the differences between the thermo-elastic properties of the diamond and garnet. A viable explanation of the documented overpressure is elastic resetting of diamond at high temperature and lower pressure (Angel et al. 2015). This is in agreement with the “hot”, adiabatic exhumation P-T path reconstructed for the UHP rocks based on thermodynamic modelling (Haifler and Kotková 2016). |