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

摘要

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 radiationdamage 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 inthe 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 contentsof radioactive and other substituting elements (2 O3) and the rare earth elements (REE, up to 1.5 wt% oxides in total), occasionallyalso 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 refl ected by the relatedcathodoluminescence (CL) spectra consisting of sharp and intense line emissions from Dy3t, Sm3t, Nd3t and Tb3t centers in zircon. The broad band emissions normally observed in well-ordered zircon are quenched. Orientation contrast imaging (OCI) and EBSD reveal texturesconsisting 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 thesefi ndings for the understandingof the formation of metamict zircon should be considered in future research.