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 <i>sp</i>³-carbon, but show downshifting of the Raman band from the ideal value (1332 cm⁻¹), and have a broader diamond band (FWHM &gt;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 <i>P</i>–<i>T</i> time of crystallization and subsequent thermal annealing on decompression. Combined with regional geology, our work further constrains the tectonic evolution of the Erzgebirge terrane.

全球范围内的超高压（UHP）岩石中，多晶微金刚石（Polycrystalline microdiamonds）极为罕见。本次研究在德国厄尔士山脉（Erzgebirge）采集的样品中，于长英质岩石的锆石包裹体中发现了大量多晶微金刚石。为阐明其成因与形成时代，我们对52颗微金刚石包裹体的形貌与拉曼光谱（Raman spectra）开展了分析，并对寄主锆石进行了定年。寄主锆石的Th/U比值较低（0.03~0.07），谐和U-Pb年龄为335.8 ± 1.9 Ma。多晶金刚石粒径为10~40 μm，由众多取向各异的细粒晶体（1.5~3 μm）组成；孤立的单颗金刚石（2~20 μm）则较为稀少。所有测得的拉曼光谱均在1332~1328 cm⁻¹处呈现出强金刚石特征峰，且峰强与半高宽（FWHM）呈负相关关系，半高宽范围为5.8~11.3 cm⁻¹。结合此前报道的金刚石特征峰数据（1331~1337 cm⁻¹），本次所得结果与其他超高压地体中不同寄主矿物内的金刚石包裹体拉曼特征相符，但与顽火辉石陨石（ureilite）中的金刚石拉曼特征存在差异。厄尔士山脉锆石中的微金刚石未表现出可见的无序sp³碳（sp³-carbon），但拉曼峰相较于理想值（1332 cm⁻¹）发生了红移，且其金刚石特征峰的半高宽（>3 cm⁻¹）大于有序金刚石的半高宽。这些特征可能反映了超高压变质作用及超高压地体快速折返过程中，因快速成核/结晶导致的有序度不完善。锆石中的石墨包裹体在1587 cm⁻¹处呈现典型的G峰（G-band）。结合此前报道的金刚石碳同位素组成（δ¹³C为-17‰~-27‰），以及金刚石与石榴石中流体/熔体包裹体的产出特征，本次研究表明厄尔士山脉微金刚石为变质成因，其碳源为有机碳，并从水溶液流体中结晶析出。拉曼光谱所揭示的有限长程有序性，取决于结晶过程中的温压与时间条件，以及减压过程中发生的后续热退火（thermal annealing）作用。结合区域地质背景，本次研究进一步约束了厄尔士山脉地体的构造演化历程。