The validity of optical dating based on feldspar
The potential for valid dating using feldspar minerals has recently been increasingly questioned with the recognition of a range of effects capable of giving rise to age shortfalls. The intention here was to investigate the more serious of these, and to simultaneously extend the surprisingly meagre body of knowledge on the luminescence of the feldspar minerals. A suite of feldspars representative of all major mineral species of the feldspar ternary system was assembled and mineralogically classified by XRD and SEM techniques. Characterization of some luminescence properties relevant to dating (radiation dose-sensitivities, emission spectra, thermal sensitization and bleaching by IR and green light) was conducted to enable comparison of these properties with the anomalous fading behaviour in each feldspar (IRSL and OSL intensities being subsequently measured as a function of storage duration at 10°C and 100°C). Examples of feldspars representative of each major compositional regime of the ternary system were found to fade, and similarly some were stable; no luminescence characteristics or storage duration at these temperatures permitted reliable separation of stable signals. A new method for analysis of localised transition-type anomalous fading was successfully tested on zircon; however the fading mechanism operant in feldspars was found to be tunnelling-type fading. Such fading cannot be circumvented by any feasible laboratory procedure and it was concluded that monitoring of samples for the presence of fading is essential: discovery of fading is a rejection criteria. The long-term trapping stability of the IRSL source traps was investigated by "conventional" kinetic means, and also by a combined optical and thermal approach in which the bleaching response curves of five selected feldspars were measured along with their low temperature (= 100-400 K) response. It was concluded that the IRSL signal originates from one trap type only in all feldspars, which possesses a constant ground state-to-excited state transition energy of 1.44 ± 0.01 eV. However, the excited state-to-conduction band energy varies depending on the major element composition of the feldspar. Lifetimes of microcline TL from glow curve temperatures > 300°C greatly exceed the minimum required for dating and this work and complementary experiments showed no physical basis for the existence of an age limit for coarse grain feldspars, other than dose saturation. Supplementary work supported the existence of an upper age limit of = 100 ka for fine grain materials.