X-ray fluorescence microanalysis Instrumental capabilities and application to Archaean cherts

Abstract

This thesis describes the results of an analytical study into µ-XRF spectroscopy and capabilities of the technique, aimed at applications in geological research. Background, principles and a brief history of XRF spectrometry are explained, a theoretical account is given of the interaction between X-rays and target material, and the components needed for XRF application on micro-scale are described.

Tests were performed to explore parameter settings of the instrument that would yield optimum results for various analytical purposes. The results indicate that (1) an amplifier time of 6.4 µs produces optimum peak resolution for qualitative analysis, (2) voltages of 20-30 keV and currents up to 600 µA are most suitable for qualitative analysis, (3) reproducible data can be obtained for quantitative analysis, (4) software provided by the manufacturer and a standardless approach are inadequate for obtaining quantitative results with an accuracy that many geological research questions demand. Tests on international geological reference materials with known compositions showed deviations from accepted values between 0.5 and 22% for major and minor elements.

Using the optimized parameter settings, µ-XRF analyses were performed on two different types of color-banded Archaean chert. One was supposedly deposited as a chemical precipitate in a marine-hydrothermal environment. Individual, up to several cm thick layers showed clear compositional contrasts. Red and black layers were iron-rich, while white, pink or orange layers contained abundant silicon. Quantitative analyses, carried out with a 2-mm spot size, yielded ca. 60 wt% SiO2 and 30 wt% Fe2O3 for the dark group of layers, and ca. 88 wt% SiO2 and virtually no iron for the pale group. The measurements showed MnO concentrations up to ~0.15 wt% in the red layers and the existence of a clear stratigraphic trend in Fe/Mn ratios of individual layers. Findings support the hypothesis that hydrothermal activity played a major role in the supply of Si, Fe and Mn. Hence, it is inferred that color-banding reflects changes in the vigor of hydrothermal activity.

A second analyzed sample is a color-banded “silicified precursor chert”, which represents a volcaniclastic sediment that became silicified by interaction with silica-saturated seawater. Qualitative µ-XRF results pointed to the presence of quartz, Fe-bearing sericite and some rutile. In this case, color differences were attributable to subtle mineralogical variations, possibly reflecting original changes in sediment supply.

This thesis includes a recommended work routine as manual for µ-XRF analysis, meant to serve future users.