Substitution-induced internal strain and high disorder in weakly radiation damaged hydrothermal zircon from Mt. Malosa, Malawi

Characteristics of two types of zircon from a single rock sample from the Mt. Malosa Massif, Malawi, are presented. According to Raman spectroscopy, the structure of a yellow hydrothermal zircon grown within a miarolitic cavity seems to be highly disordered. However, the extent of radiation damage calculated from the Raman data exceeds the degree of disorder that can be ascribed to the maximum possible internal radiation dose received by the crystals since their formation. Optical birefringence suggests an ordering degree close to that of well-ordered zircon for most areas in the crystals as may be also concluded from the always sharp electron backscattered diffraction (EBSD) patterns. Brownish zircon crystals from the wall-rock of the cavity of the same sample are most probably magmatic in origin. These crystals display a high degree of order and common low contents of radioactive and other substituting elements (<0.5 wt% in total). The absolute ages and U and Th contents of both zircon types were verified by U–Pb sensitive high-resolution ion microprobe (SHRIMP) analysis and the magmatic zircon is found to be significantly older (292 ± 10 Ma) than the hydrothermal one (121 ± 9 Ma). The apparent discrepancies between the data obtained by various methods for the hydrothermal zircon may be explained taking into account the unusual chemical composition and structural particularities established as well as (re-)interpreting the physical meaning of the variations in band positions and band widths in the Raman spectra. Electron probe microanalysis (EPMA) shows relatively high contents of yttrium (up to 4.8 wt% Y₂O₃) and the rare earth elements (REE, up to 1.5 wt% oxides in total), occasionally also of thorium (up to 0.8 wt% ThO₂). The charge unbalance caused by the substitution of trivalent elements is not compensated by coupled substitution involving other elements like phosphorus, niobium or uranium. The unusual composition and substitution is reflected by the related cathodoluminescence (CL) spectra consisting of sharp and intense line emissions from Dy³⁺, Sm³⁺, Nd³⁺ and Tb³⁺ centers in zircon. The broad band emissions normally observed in well-ordered zircon are quenched. Orientation contrast imaging (OCI) and EBSD reveal textures consisting of relatively large strained orientation domains showing a distinct distribution pattern. We suggest that internal tensional strain induced by substitution, vacancy formation and related domain formation may explain the data. Possible consequences of these findings for the understanding of the formation of metamict zircon should be considered in future research.