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

Abstract

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.

Non-destructive pXRF of mafic stone tools

“…For this type of matrix the detection depths of lower energy X-rays for elements in the range SieCa are effectively restricted from surface to very shallow (tens of microns) and are highly susceptible to surface related epi-phenomena (e.g., morphology, organic skins, contamination, weathering) (Forster et al, 2011;Ge et al, 1998;Lundblad et al, 2007;Potts et al, 2006;Potts and West, 2008). For low abundance elements in this region of the X-ray spectrum (P, S, Cl, Sc, Ti, V, Cr, Mn) reliable measurement is further compromised by the combined effects of lower counting statistics and detection limitations of current generation pXRF.…”

Evaluation of portable X-ray fluorescence (pXRF) in exploration and mining: Phase 1, control reference materials

“…This work was published in: Potts et al (1995), preliminary assessment using 70 international CRMs (control reference materials); Potts et al (1997a), development of a procedure to correct for surface irregularity effects in a rock sample; Potts et al (1997b), examination of the limitations of pXRF caused by near-surface chemistry and mineralogy in the direct analysis of in-situ silicate rocks; and Argyraki et al (1997), evaluation of the application to mapping contaminated land (soils) for Pb. In more recent work, this group measured geochemical variations within and between dolerite outcrops, compensating for surface relief and employing a range of reference materials (Jones et al 2005) and studied the effects of weathering on in-situ pXRF analyses on such outcrops (Potts et al 2006). Potts & West (2008) edited a multi-authored book, 'Portable X-Ray Fluorescence Spectrometry.…”

Geo-material provenance and technological properties investigation in Copper Age menhirs production at Allai (central-western Sardinia, Italy)

“…The samples were labelled as MIA (Monte Ironi Allai). In order to obtain the best representativeness of the geological unit, the Monte Ironi specimens were sampled to cover the entire extension of the outcrop ( Potts et al 2006;Ruge, Barber, and Magee 2007), including samples from the unmodified bedrock (MIA_C1-6) and from the quarry site of Arasseda (MIA_C7-8) ( Figure 5). The topographic position of each sample was recorded by means of a GPS system (Fornaseri, Malpieri, and Tolomeo 1975) and shown in the map of Figure 4.…”