Effects of weathering on in situ portable X-ray fluorescence analyses of geological outcrops: dolerite and rhyolite outcrops from the Preseli Mountains, South Wales

Laboratory experiments were undertaken to investigate the effect of weathering on dolerite and rhyolite samples from the Preseli Mountains, South Wales, an area of archaeological significance for the provenancing of archaeological artefacts. Portable XRF (PXRF) analysis was used to construct a depth profile of elemental concentrations through the weathering layer of suitable rock samples. These measurements were made by sequentially grinding away successive layers, each time measuring the newly exposed surface by PXRF. The elements studied included K, Ca, Ti, Mn, Fe, Ba, Nb, Pb, Sr, Y and Zr. In dolerite samples, Ca, Fe and Y were generally depleted in heavily weathered surface layers, whereas K and Pb were enhanced, the latter element significantly so, probably as a result of anthropogenic activities (emissions from leaded automobile fuel). Element behaviour in rhyolites was less consistent. A correction factor was derived for each element based on the ratio of the weathered surface concentration to the corresponding steady-state (fresh rock) concentration beneath the altered layer. This correction factor was evaluated as a possible means of compensating for elemental discrepancies when measuring weathered surfaces. Improved data were obtained for a significant number of elements, but further work is required to evaluate this procedure fully.