Cathodoluminescence and characterisation of defect structures in quartz with applications to the study of granitic rocks

Abstract

The presented study contributes in solving the causes of cathodoluminescence (CL) of quartz. For this purpose a variety of quartzes from different geological environments has been systematically studied. The results have been applied to a number of granitic systems for the understanding of the textural evolution, intrusion mechanism, and the phasing of events of felsic igneous magmas.A part of the study deals with the correlation between the CL properties and trace element contents (mainly Al, Ti, Li) determined by EPMA, LA ICP-MS, and SIMS. The spectral response of the CL of quartz in the range of visible light is characterised by emission bands between 1.7 - 2.2 eV (red) and 2.4 - 3.1 eV (blue) resulting in blue, violet, and red-brown colours. High-resolution spectroscopy of quartz allows the discrimination of 9 emission bands. The emission bands with characteristic (relative) intensities, positions, and half-widths have been mathematically captured. Some CL emission bands relate to the presence of trace elements e.g. Fe (1.73 eV), Mn (2.15 eV), Li and H (2.48 eV), Al, H, Na, Li, and K (2.79 eV), and Ti (2.96 eV). The increase of the 1.85 and 1.96 eV CL emissions during electron radiation is explained by the abundance of hydroxyl defects in the quartz lattice.Cathodoluminescence has been applied to granitic systems from 2 magmatic provinces: the Kru ne Hory/Erzgebirge (Czech Republic/Germany) and the Eastern Lachlan Fold Belt (Australia). Based on the CL properties and trace elements, magmatic quartz has been subdivided into (1) euhedral quartz phenocrysts showing stable, dominantly blue CL and growth zoning related to Ti distribution and (2) anhedral matrix quartz with unstable red- brown CL and homogeneous trace element distribution. Whereas the properties of the quartz phenocryst point at low to mid-crustal origin, the matrix quartz must have been formed during the conditions of magma emplacement. The CL properties of different quartz generations in granite reflect the change of the water content of the melt. During the early crystallisation stage the magma contained <2.5wt% H2O and produced euhedral phenocrysts with stable CL and growth zoning. During magma evolution the water content of the melt increases until about 5wt% H2O. Besides primary growth zoning magmatic quartz mostly show a large variety of secondary structures, which formed during retrograde processes. These processes include (1) micro cataclasis, (2) healing, (3) diffusion, and (4) a-radiation. Based on the CL measure- ments, we proposed a classification in (a) secondary structures with reduced defects and (b) secondary structures with induced defects.