Use of cathodoluminescence in heavy mineral analytics illustrated by the stable mineral group monazite-xenotime-zircon from Triassic sandstones of NE-Bavaria

摘要

An appropriate technique to distinguish the stable mineral group monazite-xenotime-zircon by means of cathodoluminescence (CL) investigations is of significance in modern heavy mineral analysis since these minerals are widely distributed in sands and sandstones, even if intrastratal dissolution has eliminated less stable minerals like apatites. Therefore the CL heavy mineral analyses may provide useful information on provenance and history of sedimentary sequences. Compared to the well-known blue and yellow CL colours of zircon, monazite and xenotime differ from this optically very similar mineral by the dark-olive and bottle-green CL colour, respectively. The CL spectra of monazite, xenotime and zircon are all dominated by characteristically narrow bands of rare earth elements: Nd3+ and Sm3+ for monazite and Dy3+, Tb3+ and Sm3+ for xenotime and zircon. High contents of rare earth elements in monazite are causal for quenching effects, resulting in a very weak CL appearance. Due to different crystal lattices of all three minerals, the splitting of the multiplet level of equally available elements - Dy3+ and Sm3+ - is different. The distribution of the heavy mineral types detected in heavy mineral separates from Triassic sandstones (Bunter and Keuper) of NE-Bavaria and characterised by CL support the directions of fluvial transport known by analyses of sedimentary structures and petrography to provenance areas with appropriate crystalline rocks (Bohemian Massif and Scandinavian Shield). The distribution of blue and yellow luminescent zircons of about 53% to 47% from the samples of the Bunter indicates source areas with I- to S-I-type granitoids (e.g. Letichtenberg Granite, east of Weiden). The samples from the Burgsandstein show a distribution of blue and yellow luminescent zircons of about 67% to 33%, which points to a stronger influence of the western part of the Fichtelgebirge and the northern Pfalz with their widely distributed S-type granitoids as source areas. In contrast, the zircons from the samples of the Schilfsandstein show a distribution of blue and yellow luminescent zircons of about 45% to 55%, which indicates crystalline source areas with I-type dominated granitoids of the Scandinavian Shield and the Russian Plate.