Raman spectra of polycrystalline microdiamond inclusions in zircons, and ultrahigh-pressure metamorphism of a quartzofeldspathic rock from the Erzgebirge terrane, Germany

Polycrystalline microdiamonds are rare in ultrahigh-pressure (UHP) rocks worldwide. Among samples collected at Erzgebirge, Germany, we found abundant polycrystalline microdiamonds as inclusions in zircons from a quartzofeldspathic rock. To illuminate their origin and forming age, we investigated morphologies and Raman spectra of 52 microdiamond inclusions, and dated the zircon host. The zircons have low Th/U values (0.03–0.07) and a concordia U/Pb age of 335.8 ± 1.9 Ma. Polycrystalline diamond (10–40 µm) consists of many fine-grained crystals (1.5–3 µm) with different orientations; discrete single diamonds (2–20 µm) are rare. All measured Raman spectra show an intense diamond band at 1332–1328 cm⁻¹ and have a negative correlation with full width at half maximum (FWHM) of 5.8–11.3 cm⁻¹. These data combined with previously reported diamond band data (1331–1337 cm⁻¹) are compatible with those of diamond inclusions in various host minerals from other UHP terranes, but are different from those of ureilite diamonds. The Erzgebirge microdiamonds in zircon do not display visible disordered sp³-carbon, but show downshifting of the Raman band from the ideal value (1332 cm⁻¹), and have a broader diamond band (FWHM >3 cm⁻¹) than those of well-ordered diamonds. These features may reflect imperfect ordering due to rapid nucleation/crystallization during UHP metamorphism and rapid exhumation of the UHP terrane. Graphite inclusions in zircon show a typical G-band at 1587 cm⁻¹. Our study together with previously reported C-isotopic compositions (δ¹³C, −17 to −27‰) of diamond and occurrences of fluid/melt inclusions in diamond and garnet indicates that Erzgebirge microdiamonds are metamorphic, have an organic carbon source, and crystallized from aqueous fluids. Limited long-range ordering suggested by the Raman spectra is a function of the P–T time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.