Trace Element Behavior during Shock Transformation of Zircon to Reidite

Abstract

Large single crystals of natural zircon were shock-loaded at 13.6 and 51.3 GPa in planar geometry. No structural changes were observed in zircon after loading at 13.6 GPa. Loading to 51.3 GPa resulted in zircon transformation to a denser scheelite-structured phase, reidite. The investigation of reidite samples by X-ray diffraction, Raman, photo- and cathodoluminescence spectroscopies revealed segregation of some trace cations (e.g., REE) on planar defects during the transformation. Importantly, the segregation occurred in a laboratory experiment without long-term annealing after shock loading. A possible mechanism of segregation of trivalent trace cations in zircon includes local violation of charge balance during the zircon–reidite reconstructive transformation, which is accompanied by considerable changes in the topology of polyhedra and second coordination spheres (Si–Zr). This results in expulsion of a fraction of the trace element into energetically expensive interstitial positions with high diffusivity even at relatively low temperatures.