Calibration of a QEM-EDS system for rapid determination of potassium concentrations of feldspar grains used in optical dating
Potassium (K)-rich feldspars are one of two mineral types typically used for optical dating. Feldspar grains can contain up to ~14 wt% K that gives rise to an internal dose rate component. This internal component can comprise a significant proportion of the total environmental dose rate to which a mineral grain is exposed. The environmental dose rate term—the denominator in the optical age equation—determines the rate at which electronic charge is transferred into the crystal lattice of a mineral grain over its period of burial. Not all feldspar grains have the same K concentration, so internal dose rates differ between individual grains. K concentrations can range from 0 to 14 wt% due to the variable compositions of feldspar phases, differing proportions of discrete feldspar phases and/or the presence of other mineral inclusions in grains. Numerous techniques are available for determining the K concentrations of individual grains, but are time-consuming, and either lack the spatial resolution to classify discrete mineral phases within multi-phase grains, or the coverage to obtain whole-of-grain average K concentrations. Quantitative evaluation of minerals using energy dispersive spectroscopy (QEM-EDS) is a time-efficient and automated mapping technique that has the spatial resolution to classify most mineral phases, as well as the coverage to determine their area proportions. QEM-EDS can also be used to determine elemental concentrations based on spectral matches to EDS reference spectra. It is, however, difficult to determine accurate elemental concentrations for minerals such as feldspars, where solid solutions exist. To overcome this, we establish a QEM-EDS calibration using EDS spectra from six feldspar reference standards to define five solid solution regions along the alkali and plagioclase feldspar series. We test this calibration through comparisons of the K concentrations of discrete phases of three feldspar varieties (orthoclase/microcline, albite and sanidine), derived using both QEM-EDS and wavelength dispersive spectroscopy (WDS). We assume that the WDS-derived K concentrations represent the ‘true’ K concentrations of the phases, and calculate, from a best-fit weighted regression of the two data sets, a correction and uncertainty estimate that can be applied to the QEM-EDS-derived K concentrations, taking into account instrument irreproducibility and any measurement bias. We also test alternative mapping step sizes to optimise efficiency, and apply this time-efficient technique to individual luminescent feldspar grains from two samples. We propose that QEM-EDS is capable of (1) classifying the range of mineral phases in grains, (2) determining the area proportions of each of these phases, and (3) obtaining accurate whole-of-grain average K concentrations of individual feldspar grains using the calibration and correction presented here.
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Australian Research Council