Cathodoluminescence activation and zonation in carbonate rocks: an experimental approach

Abstract

Calcite crystals have been grown with the objective of studying the cathodoluminescence (C.L.) characteristics. No trace elements other than Mn2+ were needed to activate luminescence similar to that observed in natural carbonates. C.L. observations, combined with Mn and Fe analyses of synthetic crystals, natural calcites and dolomites, show that: 1. 15-30 ppm and 30-35 ppm Mn is sufficient to activate luminescence in calcites and dolomites, respectively. 2. The intensity of luminescence is controlled by the absolute amount of Mn2+ and not by the Fe2+/Mn2+ ratio. Both conclusions are valid provided Fe concentrations are low (< 200 ppm). 3. Luminescent zonation, common in synthetic and natural carbonates, reflects differential Mn2+ uptake during growth, caused by: a. changes in the Mn2+ concentration of the precipitating fluid, or b. changes in the rate of crystal growth independent of the Mn2+ concentration in the fluid. The latter process has not been considered sufficiently in former C.L. studies, but will have important consequences for interpretation of luminescence features: e.g. definition of the geochemical history of pore fluids by characterisation of the rock's C.L., may not always be justified. Specifically, the concept of cement stratigraphy (i.e. correlation of similar luminescent colours or intensities) should be handled with care and only applied when it can be demonstrated that the zones are the result of bulk geochemical changes in the pore fluids and not merely of differences in crystal growth rate. Tentative criteria for such a differentiation are discussed.