Effect of surface weathering on portable X-Ray fluorescence analysis of artefacts: Evidence from Tasmanian hornfels

Hornfels was commonly utilised by Aboriginal people in southeastern Tasmania for stone artefact manufacture. Sourcing of hornfels artefacts in this region using pXRF analysis is potentially compromised by the weathering patina generally present on this lithology. To understand the impact of weathering on hornfels composition, the mineralogy and chemical composition of weathered and unweathered hornfels were analysed, and weathering was simulated in column experiments. The results demonstrate that the cordierite and, to a lesser extent, plagioclase present in the hornfels transform very rapidly into kaolinite, which is the dominant component of the pale, porous weathering patina. Weathering simulation experiments and isocon comparisons of major and trace element analyses show that this alteration is accompanied by a substantial loss of Mg, Na and Ca (≥50%). For the trace elements, 25%–35% are removed (except Zr, which is relatively immobile), with the remainder retained in unaltered minerals or adsorbed to the neoformed kaolinite. Rb and Nb are more stable than Sr, which demonstrates the highest absolute mobility, but even this element shows relatively little loss, provided that the weathering patina is <1 mm thick. For Rb, Sr, Y, Zn and Nb, the percentage loss during weathering is similar (25%–35%), so bivariate plots of these trace elements show a spread due to weathering, but with a more or less constant ratio. As a result, pXRF analyses of these elements can be used to successfully differentiate hornfels artefacts from different sites in southeastern Tasmania. This study showed that Rb, Sr, Y and Nb, which are commonly used to source igneous artefacts, are also effective in sourcing hornfels, even though they are mobile during weathering. This study also demonstrated the importance of isocon plots in identifying the trace elements best suited to artefact sourcing using pXRF, if the analyses are affected by chemical weathering.