Cathodoluminescence, Electron Microscopy, and Raman Spectroscopy of Experimentally Shock Metamorphosed Zircon Crystals and Naturally Shocked Zircon from the Ries Impact Crater

Abstract

Thorough understanding of the shock metamorphic signatures of zirconwill provide a basis for the application of this mineral as a powerfultool for the study or recognition of old, deeply eroded, andmetamorphically overprinted impact structures and formations. This studyof the cathodoluminescence (CL) and Raman spectroscopic signatures ofnaturally shocked (Ries Crater, Germany) zircon crystals andexperimentally (at 38, 40, 60, and 80 GPa) shock-metamorphosed singlecrystals of zircon contributes to the understanding of the formation ofmicrodeformation in zircon under very high, dynamic pressures.Unshocked samples show crosscutting, irregular fractures in thebackscattered electron (BSE) images. The 38 GPa sample exhibits a densepattern of narrow-spaced lamellar features, in CL mode, which couldrepresent the twinning effect noted in a 40 GPa sample by earlierworkers and which was ascribed to partial conversion from thezircon-structure phase to the more voluminous scheelite-structure phase.The CL images of experimentally, at 40, 60, and 80 GPa, shocked samplesshow subplanar and nearly parallel microdeformations. All experimentallyshocked zircon, as well as all investigated naturally shocked samplesfrom the Ries crater, show an inverse relationship between thebrightness of the BSE signal and the corresponding cathodoluminescenceintensity of the zonation patterns. The CL spectra of unshocked andexperimentally shock-deformed specimens and naturallyshock-metamorphosed zircon samples are characterised by narrow emissionlines and broad bands in the region of visible light and in thenear-ultraviolet range. The emission lines likely result from rare earthelement activators and the broad bands might be associated with latticedefects. Raman spectra reveal that the unshocked samples, as well asnaturally shock-deformed zircon crystals from the Ries, representzircon-structure material, whereas the 38 and 40 GPa samples yieldadditional peaks with relatively high peak intensities, which areindicative of the presence of the scheelite-type structure of zirconwith zircon-structure relics. The 60 and 80 GPa samples display a Ramansignature that is characteristic of only the scheelite-type phase.According to the Raman measurements, the naturally shock-deformedzircons might be related to the low-shock regime (<30 GPa), and do notrepresent the same shock stages indicated by whole-rock petrography.The results show a relationship between the CL and Raman properties ofzircon and shock pressure, which confirm the possible use of thesemethods as shock indicators.