Abstract

Most carbonate petrologists consider Mn²⁺ and Fe²⁺ to be the only trace elements responsible for cathodoluminescence in carbonates. However, luminescence in carbonates is caused or inhibited by a number of trace elements. The main activators in calcite and dolomite are Mn²⁺ , Pb²⁺ and several rare earth elements. For Mn-activated luminescence in these minerals, the main sensitizers are Pb²⁺and Ce²⁺, and the main quenchers are Fe²⁺, Ni²⁺, and Co²⁺. Non-sensitized Mn-activated luminescence in calcite and dolomite occurs at a minimum concentration of 20-40 ppm, perhaps as little as 5 pm. Pb²⁺ and Ce²⁺ can sensitize Mn-activated luminescence at levels as low as 10 and 20 ppm, respectively. Fe²⁺, the most abundant quencher, begins to quench Mn-activated luminescence at about 35 ppm. Ni²⁺, Co²⁺ and Fe³⁺ quench at even lower concentrations. The quencher concentrations necessary for extinction of Mn-activated luminescence have not been determined with sufficient accuracy, and probably depend on the quencher/activator ratios. It may be misleading to assign a specific activator or quencher element to a carbonate crystal on the basis of the luminescence colour without spectroscopic measurement. Several elements can interact to produce a certain luminescence colour that is a mixture of different emission peaks. These peaks are distinctive for certain activators and/or sensitizers and may be used to identify these elements. The variable cathodoluminescence of diagenetic carbonates is commonly used to infer the pH and redox potential of diagenetic environments by means of pH/E, diagrams that contain only Mn and Fe as cations. This is permissible only if elements other than Mn and Fe are insufficiently abundant to be effective. Other important processes that lead to significant luminescence variations in diagenetic carbonates are closed- and open-system partitioning, clay mineral and organic matter diagenesis and variations in trace element supply. Considering the multitude of parameters that determine and influence the luminescence of carbonates, environmental and stratigraphie interpretations of diagenetic carbonates on the basis of their cathodoluminescence should be undertaken with extreme caution.