Field-Portable Non-Destructive Analysis of Lithic Archaeological Samples by X-Ray Fluorescence Instrumentation using a Mercury Iodide Detector: Comparison with Wavelength-Dispersive XRF and a Case Study in British Stone Axe Provenancing

Journal of Archaeological Science (1999) 26, 215–237 Article No. jas.1998.0323, available online at http://www.idealibrary.com on

Field-Portable Non-Destructive Analysis of Lithic Archaeological Samples by X-Ray Fluorescence Instrumentation using a Mercury Iodide Detector: Comparison with Wavelength-Dispersive XRF and a Case Study in British Stone Axe Provenancing Olwen Williams-Thorpe, Philip J. Potts and Peter C. Webb Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, U.K. (Received 31 October 1997, revised manuscript accepted 8 June 1998) Field-portable X-ray fluorescence (PXRF) instrumentation incorporating three radioisotope sources and a mercury (II) iodide detector has been evaluated in the non-destructive quantitative chemical analysis of lithic artefacts of archaeological interest. The method was tested by comparing PXRF analyses of 19 archaeological samples of fine- to medium-grained igneous rocks with laboratory wavelength-dispersive whole-rock XRF analyses of the same samples. Elements determined were K, Ca, Ti, Mn, Fe, Rb, Sr, Y, Zr, Nb, Ba, Cu, Zn and Pb, with detection limits for the most sensitively-determined elements of 6–14 ppm. PXRF analyses on fresh rock or artefact surfaces compare very well with the bulk WDXRF determinations, but weathered surfaces can give results significantly different from the corresponding bulk composition, especially for K, Ca, Ti and Fe. Seven British prehistoric stone implements (axes, axe-hammers and a mace-head) in the National Museum and Gallery, Cardiff, were analysed non-destructively by PXRF to assess the potential of this method in the provenancing of stone implements. Of four implements which had previously been assigned on petrographic grounds to Implement Petrology Committee (IPC) Group XIII (Preseli dolerite), two were confirmed as Group XIII and two have chemistry (and mineralogy) unlike Group XIII dolerite. Two implements previously assigned to IPC Group VIII (a rhyolite Group with suggested sources mainly in south Wales) have chemical characteristics which suggest sources respectively at an axe-manufacturing site near Carnalw in south-west Wales, and on the far west or north Pembrokeshire coast. The final implement, an axe, could not be assigned to any source with certainty either on petrographic or chemical grounds.  1999 Academic Press Keywords: PORTABLE X-RAY FLUORESCENCE, RADIOISOTOPES, MERCURY (II) IODIDE DETECTOR, MAJOR AND TRACE ELEMENTS, RHYOLITES, DOLERITES, BRITISH STONE AXES, PROVENANCING, NON-DESTRUCTIVE ANALYSIS.

Introduction

combination, only four parameters for discrimination of sources and of artefacts (namely, magnetic susceptibility, and from gamma ray spectrometry, the abundances of K, U and Th). Interest has therefore turned to portable X-ray fluorescence as a non-destructive, field-portable technique capable of multi-element analysis. Earlier generations of field-portable XRF instrumentation generally used radioisotope X-ray sources for sample excitation, coupled either to gas proportional counters or to Si(Li) energy-dispersive detectors. For multi-element samples, however, proportional counters are limited by their relatively poor spectral resolution. Si(Li) detectors require either liquid nitrogen cooling or Peltier cooling which can significantly restrict their convenience for field use, needing either regular supplies of liquid nitrogen, or higher power for Peltier devices, limiting the endurance of rechargeable batteries. New opportunities have, however, arisen

T

he development of non-destructive fieldportable methods for the analytical characterization of artefacts is of key importance to the progress of archaeological provenancing studies. The use of such techniques increases the number and types of artefacts available to the researcher, and enables the study of rare and valuable objects for which destructive sampling is not appropriate. Work by the principal author and others has demonstrated the potential of the non-destructive techniques of magnetic susceptibility (Williams-Thorpe & Thorpe, 1993; WilliamsThorpe et al., 1996; Williams-Thorpe, Tindle & Jones, 1997) and portable gamma ray spectrometry (work in progress and compare Thorpe, Tindle & WilliamsThorpe, 1995) in archaeology. However, although these two techniques have been shown to be highly effective in specific applications, they provide, in 215

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through the introduction of mercury (II) iodide detectors which require no external cooling. Although not offering quite as good spectral resolution as Si(Li) devices, an evaluation of this instrument has already shown that it is capable of determining key geochemical trace elements such as Rb, Sr, Y, Nb and Zr to detection limits of 6–14 ppm in silicate rocks (Potts et al., 1995). Field-portable XRF using mercury (II) iodide detectors is now finding increasing environmental application, including the analysis of hazardous waste (Bernick et al., 1992). Cesareo et al. (1992) were the first to suggest its use in archaeometric studies. For this paper, 19 archaeological samples of fineand medium-grained igneous rock with both fresh and weathered surfaces were analysed using the Spectrace TN9000 portable XRF instrument, and the results compared with whole-rock analysis of the same samples using established laboratory wavelengthdispersive XRF techniques. These data are used to demonstrate the effectiveness of the instrument in the analysis of lithic artefacts and to comment on problems associated with surface alteration and weathering of samples. Seven British prehistoric artefacts (stone axes, axe-hammers and a mace-head) in a collection at the National Museum and Gallery, Cardiff, which had previously been assigned to axe groups and sources on the basis of petrographic examination, were also analysed non-destructively using portable XRF. The data are used to identify the geological sources of the axes, re-assessing and in some cases rejecting previous source assignments, and to assess the potential of the portable XRF method in provenancing. The overall aim of this paper is to show that modern portable XRF instrumentation is capable of the in situ quantitative analysis of rock samples of archaeological interest, providing data with the potential to extend considerably the scope of provenancing studies. In undertaking this evaluation, we identify both the capabilities and limitations of the technique. Although our work so far is limited to the application of portable XRF in lithic analysis, the method has potential application in other areas of archaeology, including studies of ceramics and metals.

Instrumentation and Analytical Performance Portable XRF: general description Portable XRF (PXRF) measurements were made with a Spectrace TN9000 instrument, manufactured by Tracor Northern Inc. (TN Technologies, Round Rock, Texas, U.S.A.) and hired from Thermo FI, Crawley, Sussex, U.K. The instrument comprises a hand-held analyser unit connected to a portable spectrum acquisition and data processing unit. The analyser unit incorporates three radioisotope excitation sources (55Fe, 109Cd and 241Am) which are used in sequence to

excite samples so that characteristic X-rays can be measured in three different energy ranges. Fluorescence spectra are detected by a solid-state mercury (II) iodide detector, which requires no external cooling and has a spectral resolution of about 260 eV full width at half maximum peak height (FWHM) at 5·9 keV. The instrument is powered by rechargeable batteries which have a field endurance of 4–5 h. Small artefacts are analysed by placing them over the 25 mm diameter analyser window with the analyser unit clamped vertically in a laboratory stand. For analysis of large objects in the field, the analyser unit is held against the sample surface. When an analysis sequence is initiated, each source is exposed automatically in turn behind the analyser window to excite an area approximately 25 mm in diameter on the surface of the sample. Count times for individual sources may be optimized according to the application, but total count times selected for this work were less than 6 min. At the end of each analysis sequence, spectra from each source are deconvoluted by the data processing software to determine fluorescence intensities for a pre-selected range of elements. These intensity data are then quantified automatically using a fundamental parameter correction procedure, incorporating sample matrix corrections, based on factors that are pre-programmed for the appropriate application (in this case, silicate rocks). Results can be viewed on the integral display screen and are stored for subsequent down-loading to a PC and/or printer through an RS232 port. Radiation dose rates from the instrument used in this work with sources retracted and the safety cover in position were up to 0·25 ìGy h
1 (measuring adjacent to the end cover), rising to a maximum of 0·6 ìGy h
1 when the sources were exposed behind the cover. This indicates that an operator holding the analyser unit against a flat sample surface does not receive any significant dose. Maximum dose rates measured at 100 mm distance from the analyser unit aperture with the sources exposed and no safety cover or sample over the aperture were 2 ìGy h
1 (55Fe source), 50 ìGy h
1 (241Am source) and 267 ìGy h
1 (109Cd source). These dose rates reduce to <0·15 ìGy h
1, 0·3 ìGy h
1 and 0·7 ìGy h
1, respectively, at a distance of 2 m from the analyser unit. For a particular instrument, dose rates will decrease over time owing to the radioactive decay of the sources. However, these measurements indicate that a person 2 m away from the analyser unit is unlikely to receive a significant radiation dose, even when standing in direct line of sight of the analyser with a source exposed (i.e., in the analysis position) and no safety cover or sample obscuring the analyser window. This situation should not arise in normal work, during which sources are exposed only when a sample is obscuring the analyser window, and safety interlocks prevent accidental exposure of the sources. Nonetheless, use of this equipment must comply with national regulations governing instruments containing radioactive sources.

Portable Non-Destructive X-Ray Fluorescence Analysis

Portable XRF: laboratory performance The performance of the PXRF instrumentation in the laboratory has already been reported in detail (Potts et al., 1995). Briefly, a wide range of silicate rock reference materials were analysed as powder pellets to evaluate accuracy, precision and detection limits. The study showed that the instrument is capable of the routine determination of a range of major and minor elements (K, Ca, Ti, Mn and Fe) and selected trace elements (Rb, Sr, Y, Nb, Zr, Ba) in typical silicate rocks, using K-line characteristic X-rays for all these elements. The lighter major elements (below K in the Periodic Table) cannot be determined because their characteristic X-rays are low energy and significantly absorbed in the air path between sample surface and detector. Other trace elements, including Co, V, Cr, Cu, Ni, Zn, Pb, Ga, La and Ce can also be determined, but their lower counting sensitivities mean that the concentrations found in many silicate rocks are near to or below detection limits. However, measurement of these elements is possible at higher concentrations, for example in mineralized samples. Using a count time of 200 s per source, 3 sigma detection limits for key geochemical trace elements are in the range 6 ppm (Nb) and 9 ppm (Zr), to 14 ppm (Sr) and 21 ppm (Ba). Precision in the analysis of a Whin Sill dolerite is given by Potts et al. (1995) as between 1 and 10% rsd for 200 s per source count times, for those elements with higher counting sensitivities including K, Ca, Ti, Fe, Rb, Sr, Zr and Ba.

Corrections and Other Considerations in Portable XRF of Rocks and Artefacts In laboratory XRF analysis of silicate rocks using WDXRF or non-portable EDXRF instrumentation, typical sample preparation involves crushing and grinding of rock or artefact samples to a fine homogeneous powder which can be compressed (with binders) into pellets or fused with a flux and quenched to form glass discs. In both cases, the sample presented for analysis has a reproducible, flat, uniform surface and is representative of the bulk composition of the sample. This is not the case for field portable analysis of archaeological samples. Sample preparation is not possible, only the selection of the sample and sample surfaces for measurement. This limitation leads to a number of factors which must be considered in producing quantitative results. These arise from: (i) surface irregularity effects, (ii) the different critical penetration depths and volumes of sample from which X-rays of different elements can be detected, (iii) effects of sample grain size and mineralogy, and (iv) effects of weathered surfaces on sample analysis. All these effects require attention, both in terms of applying additional correction to data and in terms of limiting the interpretation of results. They are considered in detail in other publications (Potts, Webb & Williams-Thorpe, 1997;

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Potts, Williams-Thorpe & Webb, 1997) and, because of their importance to the results presented in this paper, are summarized below. Surface irregularity effects PXRF instrumentation is calibrated for flat samples (e.g., compressed powder pellets) mounted so that the surface to be analysed lies in the analytical reference plane of the analyser. This analytical reference plane is defined by the flat end of the analyser unit surrounding the analyser window. The analysis of unprepared or field samples can only be quantitative and reproducible (without the application of correction factors) if the sample has a flat surface. Any rough or irregular surface will not lie in the analytical reference plane of the instrument. In these circumstances, both the excitation and detection efficiency (for fluorescence X-rays) will be affected by the inverse square law, which reflects the fact that intensities from a point source vary in inverse proportion to the square of the X-ray path length. A detailed investigation of this problem has been undertaken by Potts, Webb & Williams-Thorpe (1997), with a view to devising a simple correction procedure. The recommended correction is based on measuring the intensity of the scatter peaks present in X-ray spectra produced by excitation of samples with the 55 Fe source. As well as producing fluorescence photons from atoms in the sample, the Mn Ká/Kâ radiation (derived from radioactive decay of 55Fe) is scattered in the sample by one of two processes. Elastic or Rayleigh scatter, in which the exciting X-rays are re-irradiated from the sample with their energy unmodified, is dominant at this energy. The other process is inelastic or Compton scatter in which part of the energy of the exciting X-rays is transferred to the sample and the scattered photons have reduced energy. This latter process is less important for low energy Mn Ká/Kâ radiation. Recorded X-ray spectra, therefore, generally contain not only characteristic X-ray lines derived from atoms of the sample, but also Rayleigh and Compton scatter peaks, derived by the scattering of source photons within the sample. The investigation by Potts, Webb & WilliamsThorpe (1997) showed that when measurements are made from rough or broken sample surfaces, the change in intensity of fluorescence X-rays compared with that observed from a flat surface of the sample is reflected by a change in the intensity of the Rayleigh scatter peak produced during excitation by the 55Fe source. Thus, an effective method of correcting for surface irregularities is to normalize measured fluorescence intensities to the ratio of the Rayleigh scatter peak intensity measured from a ‘‘reference’’ sample surface to the Rayleigh scatter peak intensity measured from the sample surface under study. The reference sample should have a flat (polished or sawn), smooth surface and a composition which matches approximately that of the unknown sample. A suitable

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reference sample may be an artefact of similar lithic type and composition which has a naturally flat and smooth surface, or an artefact which can be studied semi-destructively to produce such a surface. Alternatively, a pellet of known composition (for example in Potts, Tindle & Webb, 1992), of similar rock type to the studied artefacts, may be used. The reference surface should be measured within a few days of measuring the unknown samples. This correction procedure was shown by Potts, Webb & Williams-Thorpe (1997) to be applicable to samples with a surface roughness equivalent to an air gap of up to 3 mm. For larger air gaps, the correction becomes progressively less reliable, mainly because there is an increasing proportion of scatter in air contributing to the measured Rayleigh scatter peak intensity. Within the 3 mm constraint, the correction procedure gives good results for elements analysed above Fe in atomic number, and for Fe, corrected determinations are within 10% of values derived from a flat, smooth surface. For elements lighter than Fe there is an additional error (even after the scatter peak correction is applied), probably because fluorescence intensities of these lower atomic number elements suffer additional attenuation in air (up to about 6% for K, with a 3 mm air gap), as well as surface irregularity effects. The stone artefacts measured in the current work were found to have surface relief of generally c. 0·5–1 mm, so the correction procedure was applicable, ameliorating surface irregularity effects but incorporating the limitations noted in this section. This correction procedure should ideally be carried out on measured fluorescence intensities before the application of matrix corrections. However, this was not possible because the on-line spectrum analysis and matrix correction procedures in the instrument used for this work are automatic and not accessible to the operator. As a compromise, therefore, the backscatter peak intensity was recorded manually after each measurement, and the analysis results (concentrations) were multiplied by the correction ratio. This compromise is likely to contribute slightly to overall errors. The investigation by Potts, Webb & WilliamsThorpe (1997) was carried out using artificially prepared rough but fresh rock surfaces in order to ensure that changes in measured scatter peak intensities reflected only varying surface relief and not weathering effects. In artefacts and natural rocks, the presence of surface dirt, weathered surfaces and organic materials such as lichen may further affect scatter peak intensities. It is generally possible to avoid dirty or lichencovered surfaces (or gentle cleaning may be possible) but weathering products such as clays concentrated close to the rock surface may not always be avoidable. This underlines the importance of carefully selecting the surface for measurement. Anomalous scatter peak intensities within an assemblage of similar

samples are readily identified during measurement sequences, and may be an indication of an unreliable surface. Critical penetration depths and volumes of sample from which X-rays are detected Fluorescence X-rays are attenuated within a sample by an amount which varies according to their energy and the mean mass attenuation coefficient of the sample. When considering, therefore, the volume of sample from which a fluorescence X-ray signal is detected, account must be taken of the critical penetration depth, which is normally defined as the depth within the sample from which 99% of the fluorescence signal originates. Examples of critical penetration depths for a silicate rock of rhyolitic composition are as follows: Ca Ká: 0·03 mm; Fe Ká: 0·17 mm; Rb Ká: 1·1 mm; Zr Ká: 1·6 mm; Ba Ká: 10·8 mm. The depths which contribute 50% of the fluorescence signal from the same sample are much shallower: Ca Ká: 0·005 mm; Fe Ká: 0·03 mm; Rb Ká 0·2 mm; Zr Ká: 0·2 mm; Ba Ká: 1·6 mm (Potts, Williams-Thorpe & Webb, 1997). As a general rule, the critical penetration depth for fluorescence lines of a particular X-ray line series (K or L) increases with the atomic number of the element. When interpreting results of X-ray analysis, therefore, it is important to be aware of the fact that proportionately more of the analytical signal originates from the surface of the sample, and that the signal from different elements represents different thicknesses beneath the surface. A further complicating factor in assessing the area and volume of sample from which the analytical signal originates is that, although the surface of the sample is excited through a window which is 25 mm in diameter, this area is not uniformly excited. Preliminary evaluation of this problem indicates that the efficiency of excitation and detection of photons from the sample is about 6 times greater per unit area near the centre of the window than at the edges (although this bias is offset by the very limited size of this area of preferential excitation compared with the whole excited area). These factors are described and their implications discussed in more detail by Potts, Williams-Thorpe & Webb (1997). Effects of sample grain size and mineralogy Since destructive sample preparation is often not possible or acceptable in the application of field-portable XRF to artefacts, it is only the surface ‘‘seen’’ by the instrument, a relatively thin layer over a small area, that is analysed. This volume and, hence, the analytical results, may not always be representative of the bulk composition of rocks or artefacts. When the mineral grains comprising, or contained within, a rock are distributed randomly, the precision of determinations by PXRF is affected by

Portable Non-Destructive X-Ray Fluorescence Analysis

grain size—that is, there are statistical variations in the number of discrete mineral grains of a particular composition within the analysed volume. In an investigation reported in detail elsewhere (Potts, Williams-Thorpe & Webb, 1997), a series of measurements was undertaken on test samples of five different rock types and grain sizes, in order to determine empirically the number of measurements which should be averaged to give a representative composition at a pre-determined level of precision (so compensating for statistical effects of grain size). Results showed that, for a fine-grained dolerite (e.g., Whin Sill dolerite, U.K., with a grain size generally less than 1 mm), between one and three measurements (depending on the element) are sufficient to give an average composition with a relative standard deviation of the mean better than 5% for Ca, Ti, Fe, Sr, Zr and Ba. For a slightly coarser rock of granitic composition (e.g., medium-grained granite, grain size 1–3 mm) between one and five measurements (again, depending on the element) are needed to give an average composition with a relative standard deviation of the mean better than 5% for K, Ca, Fe, Rb, Sr, Zr and Ba. For a very coarse-grained rock such as Shap granite containing phenocrysts up to 35 mm long, the number of measurements required to give a relative standard deviation of the mean of better than 5% rises sharply, to between 11 and over 300 measurements. However, a relative standard deviation of the mean of better than 20% on Shap granite can be obtained for most elements of interest by averaging just six measurements. Precise analysis of very coarse-grained rocks is not practical by the PXRF method (although analysis of individual mineral grains may be possible in exceptional cases). These results are, of course, specific to the samples used in this investigation, but give general guidance in assessing the effect of grain size. The results presented by Potts, Williams-Thorpe & Webb (1997) indicate significant variations in the number of determinations that must be averaged based on data for individual elements. This effect was still evident after some of the counting uncertainty was removed by restricting the assessment to elements present at concentrations exceeding 10 times the spectrum analysis error. No systematic relationship was apparent between the atomic number of elements and the number of measurements by PXRF required, as might be expected if critical excitation depth was the dominant factor. However, contributing factors in addition to differences in sampling volumes for different elements (dependent on X-ray energy), are likely to include the abundance of different elements in the rock, their distribution between different types of minerals, and the distribution of those minerals. In practice, a fine-grained rock with relatively little variation in mineral distribution or contrast in mineral composition is far more likely to give a representative analysis than a coarse-grained rock, especially one with greatly contrasting mineral compositions.

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Effects of weathered surfaces on sample analysis Accepting that the analytical signal in PXRF measurements originates from the surface layer, which varies in thickness with critical penetration depth of the element from about 0·03 mm to greater than 10 mm for the range for the elements determined in this work, care must be taken in interpreting results to account for weathering which may have changed the chemical composition of the surface in comparison with the bulk of the sample. Published studies of weathering in rocks show that the compositional effects can vary according to rock type, mineralogy, and the weathering environment. For example, weathered surfaces on granites may be relatively depleted in Si, Na, Ca, Rb and Sr (Fritz, 1988). Pye (1986) reported that K-rich granitoid rocks are relatively more resistant to weathering than less potassic granitoids, and noted the importance of rock permeability in weathering processes. A comparative study of granite and gabbro weathering (Fritz, 1988) showed less marked effects in the gabbro, except for changes in relative amounts of FeO and Fe2O3. Price et al. (1991) reported enrichment of Y, Ba and the REE in tholeiitic basalt, these elements being retained within the rock (partly in clay minerals) during weathering. Nesbitt & Wilson (1992), in a study of basalt weathering, noted that bulk rock and mineral composition affected the weathering trends observed, and reported preferential leaching of Na, K and Ca, with Fe and Al susceptible to leaching in extreme chemical weathering conditions. The same authors also described different weathering rates for individual minerals, with olivine most susceptible to weathering and Fe-Ti oxides most resistant (Nesbitt & Wilson, 1992). It is clear that the effects of weathering are unique to a particular rock, its mineralogy and the conditions to which it has been subjected, and consequently it is difficult to predict or quantify these effects. Great care is therefore required in interpreting PXRF results from weathered surfaces. Where an artefact assemblage contains surfaces of original (archaeological) date and more recently broken surfaces, some indication of weathering effects may be derived by comparing analyses of both surfaces. Where such a comparison is not possible, data on weathered surfaces may still be of use; either for elements that are shown to be immobile, or in qualitative work, for example to broadly identify rock type. Weathering effects are further discussed below in our comparison of PXRF and WDXRF analyses.

Comparison of Portable XRF and Wavelength-Dispersive XRF Analysis on Archaeological Samples The aim of this section of the paper is to compare results of PXRF analysis of 19 archaeological samples with those of bulk analysis by conventional

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laboratory-based wavelength-dispersive XRF of the same samples. The samples comprise a range of artefacts and unworked excavated material made available to the Open University over the last 10 years for destructive analysis. The samples have been sawn during earlier projects at the Open University, providing fresh, flat surfaces for PXRF analysis as well as crushed and homogenized powder for bulk analysis by WDXRF. The samples include rhyolites, dolerites and one microdiorite. We also assess PXRF data from weathered surfaces (still present on several samples) compared with the bulk analysis (of unweathered material) by WDXRF. Interpretation of these samples in terms of geological provenance has been discussed in earlier papers (Thorpe et al., 1991; Kelley, Williams-Thorpe & Thorpe, 1994; Richards, 1990; Williams-Thorpe in David & Williams, 1995) and it is not the intention here to add to or change those interpretations; the samples are used here only to investigate the validity of the method of PXRF. Description of the archaeological samples The archaeological samples are listed in Table 1 with details of artefact and rock types, and the scatter peak intensities and normalizing factors relating to the weathered surfaces (discussed below). The rhyolites are grey or blue-grey and mainly aphyric; OU4 and OU5 appear banded through mineral segregation, and OU17 is an ignimbrite. The dolerites are mainly the spotted dolerite known as ‘‘preselite’’ (cf., e.g., Thorpe et al., 1991), containing crystals of feldspar, pyroxene and whitish metamorphic spots. OU6 and OU12 are dolerites of similar mineralogy but contain no metamorphic spots. The microdiorite (R198) contains altered feldspar crystals and pyroxene in a chloritized matrix (Roe in Richards, 1990). The rhyolites and microdiorite are all fine-grained, and the dolerites are medium-grained with crystals of up to 2–3 mm. Many of the samples are unworked fragments from surface surveys or excavations. Sample locations include the Stonehenge environs (R prefixes; see Richards, 1990), Stonehenge (OU prefixes; see Thorpe et al., 1991), and the stone axe manufacturing sites in South Wales at Glandy Cross and Glyn-y-Fran (M10, N11, F4H5; see David & Williams, 1995). The published references contain further mineralogical descriptions. Sample sizes range from a few cm to about 10 cm across. Portable XRF procedures The samples were measured on the Spectrace TN9000 instrument between 1994 and 1996. The number of measurements per sample (listed in Table 1) varied depending on sample size. PXRF measurements were made on independent parts of sample surfaces where possible, overlapping where not. Individual measure-

ments on each sample are therefore not replicates, but mainly represent overlapping rather than independent sample volumes. Count times were mainly 100 s for the 109 Cd source, 40 or 50 s for the 55Fe source, and 20 s for the 241Am source. It was possible to reduce the 200 s count times of Potts et al. (1995) without significantly degrading the quality of the analytical data because the abundances of many of the elements of interest in these samples are significantly above their detection limits for a 200 s count time. For the analysis of these relatively small samples, the PXRF analyser unit was clamped in the vertical position (‘‘lab-stand mode’’) and the sample surface to be analysed was positioned over the analyser window. Most surfaces measured were large enough to completely cover the analyser window. However, fresh surfaces measured on stone axe fragments R198, R200 and R201, and one measurement of a fresh surface of OU11, were slightly smaller than the window, leaving mm-sized gaps at the edges. All samples were thicker than 10 mm (and therefore sufficiently thick to satisfy infinite thickness criteria for all fluorescence lines measured) except for N11 (8–12 mm) and R201 (one edge of the fresh surface had a depth of c. 5 mm). ‘‘Fresh surfaces’’ are sawn or lapped surfaces and all are smooth and flat with zero surface relief (in terms of pits to peaks). The amount of material removed from the original (archaeological) surface of the sample to provide a fresh surface varied, but was typically between 5 and 10 mm. ‘‘Weathered surfaces’’ are mainly the archaeologicalage weathered surfaces, and the features and depths of these are noted in Table 1. Unworked samples could retain weathered surfaces from older (geological time scale) processes. In two cases, N11 and F4H5, the surfaces revealed when the top c. 10 mm of sample was removed still appeared weathered, with whitish speckles and patches visible, similar to the weathered crust surrounding the remainder of the samples. Consequently, these surfaces were treated as ‘‘weathered’’ and no PXRF of fresh surfaces were available for N11 and F4H5. However, the bulk analysis by WDXRF of these two samples was based on material which did not include any whitish, obviously weathered material. Wavelength-dispersive XRF procedures The laboratory WDXRF analyses were carried out following principles and procedures described by Potts & Webb (1992) and Ramsey et al. (1995). The data have good precision, with standard deviations typically less than 1% relative (1 sigma) for major elements and between 1 and 6% relative (1 sigma) for trace elements (see XRF results for the Open University listed in Govindaraju et al., 1994). Fe2O3 in N11, F4H5 and M10, and TiO2 in N11, F4H5, M10, R198, R200, R201 and R202 were determined from powder pellets and have poorer precision, estimated at about 5% relative (1 sigma). The WDXRF analyses of crushed rock are

ditto ditto ditto ditto ditto ditto ditto ditto

Core

Flake

Flake

Axe fragment Axe fragment Flake from axe Axe fragment

OU6 OU10 OU11 OU12 OU14 OU16 OU17 OU18

M10

N11

F4H5

R200 R201 R198 R202

Rhyolite Rhyolite Microdiorite Dolerite

Rhyolite

Rhyolite

Rhyolite

Dolerite Dolerite Dolerite Dolerite Dolerite Rhyolite Rhyolite Rhyolite

Rhyolite Rhyolite Rhyolite

Dolerite

Rock type

1 1 1 0

0

0

4

2 2 2 2 2 1 2 4

1 1 4

1

Fresh surfaces

1 1 1 1

2

2

1

0 0 0 0 0 0 0 2

0 0 2

0

Weathered surfaces

Whitish speckles in sawn surface Whitish speckles in sawn surface << 1 mm << 1 mm, white << 1 mm, whitish << 1 mm, whitish

Up to 5 mm, pale coloured layer

c.0·5 mm, whitish layer

c.0·5 mm, whitish layer

Depth (mm) and description of weathered surface

192·2

188·0 185·1

195·9 197·8

Intensity of (55Fe source) Rayleigh scatter peak from sample surface (s
1)

*Number of measurements on each sample by PXRF; analysis by WDXRF was based on one pellet per sample.

OU2 OU4 OU5

Unworked fragment ditto ditto ditto

Sample type

OU1

Sample reference

No. of PXRF analyses*

Table 1. Archaeological samples analysed for comparison of wavelength-dispersive and portable XRF

218 (mean of 2 measurements on fresh flat surfaces) 223 (mean of 2 measurements on fresh flat surfaces)

222 (mean of 2 measurements on fresh flat surfaces)

Intensity of (55Fe source) Rayleigh scatter peak from reference surface (s
1)

1·16

1·16 1·18

1·13 1·12

Scatter peak normalization factor

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Table 2. Portable XRF and WDXRF analyses of representative samples Reference and rock type

Element/oxide K2O% CaO TiO2 MnO Fe2O3 Rb ppm Sr Y Zr Nb Ba

OU5 – rhyolite

OU6 – dolerite

Portable XRF fresh surfaces

WDXRF whole rock bulk sample

Portable XRF weathered surfaces

Portable XRF fresh surfaces

WDXRF whole rock bulk sample

Portable XRF fresh surfaces

WDXRF whole rock bulk sample

3·02 0·3 0·12 nd 0·74 52 69 71 281 22 586

3·02 0·28 0·17 0·01 0·92 54 63 71 292 14 608

1·64 0·53 0·09 nd 1·28 53 77 54 280 25 516

0·99 1·04 0·57 0·18 1·06 12 416 12 64 nd 169

0·76 1·02 1·11 0·17 0·99 16 328 20 66 4 184

0·14 0·99 0·91 0·15 9·96 nd 280 24 58 nd 94

0·1 1·12 1·14 0·15 9·22 8 235 20 68 3 104

OU18 – rhyolite

Element/oxide K2O% CaO TiO2 MnO Fe2O3 Rb ppm Sr Y Zr Nb Ba

M10 – rhyolite

Portable XRF fresh surfaces

WDXRF whole rock bulk sample

Portable XRF weathered surfaces

Portable XRF fresh surfaces

3·18 0·23 0·15 nd 1·06 57 88 72 327 19 466

3·02 0·32 0·26 0·01 1·42 52 84 83 320 16 455

0·96 5·99 0·09 nd 1·19 48 102 69 311 26 557

nd 1·24 0·69 nd 4·21 nd 188 33 172 24 49

R201 – rhyolite

Element/oxide K2O% CaO TiO2 MnO Fe2O3 Rb ppm Sr Y Zr Nb Ba

OU10 – dolerite

Portable XRF fresh surfaces 1·42 0·80 0·49 0·13 4·38 18 143 111 659 28 692

WDXRF whole rock bulk sample

0·43 18 107 108 520 23 614

WDXRF whole rock bulk sample

0·89 3·59 nd 165 32 150 16 55

Portable XRF weathered surfaces 0·31 0·58 0·45 nd 7·89 nd 75 13 169 23 73

R198 – microdiorite Portable XRF weathered surfaces 1·56 1·17 0·62 nd 1·96 26 127 111 601 21 695

Portable XRF fresh surface 3·04 4·38 0·85 0·13 5·38 104 118 40 177 343

WDXRF whole rock bulk sample

0·86 99 121 49 195 14 383

Portable XRF weathered surfaces 3·13 4·71 1·00 0·15 4·71 109 125 35 197 10 423

Portable XRF data are means of measurements on fresh and weathered surfaces, respectively. nd=not detected (<3the spectrum analysis error calculated by the PXRF software). Missing values=not determined.

assumed to represent the bulk composition of the samples. Results and discussion for fresh surfaces Table 2 gives PXRF data for five major and minor elements and six trace elements from fresh, sawn

surfaces of representative samples, together with comparable whole-rock (bulk) WDXRF results for the same samples and the same range of elements. Cu, Zn and Pb, less sensitively-determined by PXRF than most of the elements reported here, were recorded but are not included in this comparison. PXRF data on

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Figure 1. Graphs showing individual PXRF determinations (filled squares) and means (open squares) for fresh surfaces of archaeological samples of rhyolite, dolerite and microdiorite composition, plotted against WDXRF bulk analyses: (a) K2O, (b) Fe2O3, (c) Rb, (d) Zr, (e) TiO2. Error bars shown on representative concentrations are spectrum analysis uncertainties (1 standard deviation) for PXRF data. For Fe2O3 and Zr, PXRF uncertainties are within the size of the symbols. Uncertainties on WDXRF (precision, 1 standard deviation, from XRF results quoted in Govindaraju et al., 1994) are all within the size of the symbols on the graphs.

weathered surfaces (discussed below) are also given in Table 2. The table includes rhyolites, dolerites and a microdiorite sample. Full data on all the samples are available from the first author on request. Concentrations of below 3 the spectrum analysis uncertainty (that is, the instrumental uncertainty relating to X-ray counting statistics, given as output by the manufacturer’s software as 1 standard deviation on the concentration) are omitted from Table 2 and from the following discussion. Since the fresh surfaces measured by PXRF are all flat (sawn smooth or lapped), they do not require any correction for surface relief or non-standard presentation to the analyser. In each case, the surface analysed appeared to be representative of the sample rock, and free from obvious weathering features such as white speckles or patches. Figure 1 shows the comparison between PXRF data for fresh surfaces, and the WDXRF bulk sample

analyses, for the selected elements: K2O, Fe2O3, Rb, Zr and TiO2. The number of samples included in each graph varies depending on how many concentrations are greater than 3 the spectrum analysis uncertainty. In addition, insufficient powdered material was available for some samples to permit glass discs as well as powder pellets to be prepared for WDXRF analysis, so some major element data are missing. The spread on repeat analyses of individual samples shown in Figure 1 can be attributed in part to X-ray counting statistical uncertainty (reflecting analytical uncertainty) and to variations in the distribution of minerals in the analysed volume (i.e., sampling uncertainty). Results for most of the elements given in Table 2, including K2O, CaO, Fe2O3, Rb, Sr, Y, Zr and Ba show good agreement between PXRF and WDXRF. These are illustrated by the examples in Figure 1(a) to (d) where the data tend to straddle the 1:1 line of

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equivalence. Mn data are difficult to assess because too few WDXRF results are available, but agreement between the limited data sets for MnO is satisfactory (cf. Table 2). Nb concentrations are mainly less than three times the detection limits for Nb by PXRF, and incorporate large relative uncertainties, hence the large differences between PXRF and WDXRF in Table 2. Data in Figure 1(a) to (d) show that when the means of repeat PXRF analyses on individual samples are compared with bulk WDXRF results, the correlation is even better. TiO2 data (Figure 1(e)), however, do not follow a 1:1 equivalence, even when means for samples are taken into account. The PXRF data for many samples, especially those of doleritic composition, are lower than the WDXRF data. Such bias in Ti was not observed in an earlier assessment of the potential of the instrument using powder pellets prepared from materials of known composition (cf. Potts et al., 1995). One possible cause is that Ti measurements in uncrushed rocks by PXRF may be affected by preferential absorption of Ti Ká X-rays within the Fe-Ti oxides, which have a relatively high density compared to the silicate rock matrix, and X-ray absorption corrections may be underestimated. Much of the Ti may reside in these oxides, especially in mafic rock (dolerite and gabbro). Further work on the cause of this problem is in progress, but we recommend that Ti data should be treated with particular caution in PXRF of rocks. It is interesting that the few samples which did not quite cover the analyser window (R198, R200, R201, and one analysis of OU11) show no systematic error (e.g., lower concentrations) in comparison with the other samples analysed, or with the WDXRF data; indeed, Zr by PXRF in R201 is in fact higher than by WDXRF (Figure 1(d)). This lack of any systematic difference may be due to the lower efficiency of excitation and detection of X-rays from near the edge of the window (cf. above and Potts, Williams-Thorpe & Webb, 1997). For the majority of elements considered, these samples show encouraging results when comparing fresh artefact surface analysis by PXRF with bulk analysis by WDXRF. However, in many cases of archaeometric study, sawing the artefacts is not likely to be an option and fresh surfaces may not always be available. In the next section, therefore, we consider a comparison of PXRF analysis on weathered surfaces. mainly of archaeological age, with WDXRF bulk analysis of the same samples. Results and discussion for weathered surfaces Table 2 gives PXRF data on weathered surfaces of representative samples together with the corresponding fresh surface PXRF data and WDXRF bulk analysis. Several of the weathered surfaces, on samples OU5, OU18 and M10 (cf. Table 1) have a mean surface relief

on measured areas estimated at c. 0·5 mm or greater, necessitating a correction to concentrations for relief based on the scatter peak from 55Fe source excitation, as described above. Reference scatter peak intensities were derived from measurements on the sawn surfaces of each sample. Where more than one count had been done on the sawn surface of a sample, the scatter peak intensities were averaged (scatter peaks from sawn surfaces of the same sample agreed within 1–3%). All scatter peak intensities used and normalization factors derived from them are listed in Table 1. Samples R198, R200, R201 and R202 (now returned to the excavator) were measured before the introduction of the scatter peak correction and scatter peak data for these samples are not available. Surface relief on the areas measured, noted at the time of measurement, was very low (c. 0·5 mm or less) and therefore, judging by comparison with those samples which were scatter peak corrected, the application of the scatter peak correction would be expected to change the measured concentrations by less than 10%. The absence of the correction is therefore unlikely to mask serious weathering effects reflected in elemental abundances, and the samples are retained in the data set considered below. In Figure 2, PXRF data for weathered surfaces are plotted against the corresponding bulk analysis data by WDXRF for Fe2O3, Rb, Sr, Zr, Y, Ba and TiO2. Elements not plotted may be compared in Table 2. As in Figure 1, data below 3 the spectrum analysis error are omitted, and some samples were not analysed for major elements by WDXRF. There is a good match generally between PXRF surface analyses and bulk rock WDXRF analyses for Rb, Sr, Zr and Ba (Figure 2(b), (c), (d) and (f)). These elements generally appear to be more robust in resisting the chemical effects of weathering in these samples. However, M10 is an exception, showing a large depletion of Sr in the weathered surface analysis compared with bulk analysis. This may be because M10 has the deepest visible weathered layer of the samples considered: up to 5 mm in depth. It is relevant to note that there are fewer Rb data points on Figure 2 because some are below PXRF detection limits. The Rb graph therefore gives a less reliable picture than the more complete data sets for Sr, Zr and Ba. Y data in Figure 2(e) are systematically slightly low for PXRF measurements on weathered surfaces, yet Y has similar analytical characteristics to Zr in terms of critical penetration depth, PXRF detection limits and sensitivity (i.e., X-ray intensity per unit concentration). Relative instrumental precision for Y is worse than that for Zr because of the generally lower abundances of Y. In comparison with Zr, therefore, the slightly low Y data from weathered surfaces plotted in Figure 2(e) are unexpected, but could indicate that Y is slightly susceptible to weathering in these rocks. Major elements appear to be affected by weathering more severely than most of the trace elements

Portable Non-Destructive X-Ray Fluorescence Analysis

225

Figure 2. Graphs showing mean values of PXRF analysis of weathered surfaces of archaeological samples of rhyolite, dolerite and microdiorite composition, plotted against WDXRF bulk analyses: (a) Fe2O3, (b) Rb, (c) Sr, (d) Zr, (e) Y, (f) Ba, (g) TiO2.

considered. Fe2O3 (Figure 2(a)) shows significant differences in three samples, especially the more deeply weathered M10. TiO2 (Figure 2(g)) shows differences only in M10. K2O and CaO are not plotted because very little data are available, but examination of Table 2 shows that K2O is depleted in weathered surfaces of two samples (OU5, OU18), to approximately one third and half of its bulk analysis, respectively, while CaO is increased in the weathered surface of OU5 from 0·28% to 0·53%. The high CaO result for weathered surfaces on OU18 (5·99%) is anomalous; the reason for this is uncertain but it may reflect partial

analysis of a joint surface or calcite vein in the sample, possibly hidden within the weathered surface. It is notable that the elements showing the largest differences between weathered and bulk compositions are, as may be expected, among those with the lowest critical penetration depths (ie., K, Ca, Ti, Fe). Elements with critical penetration depths of about 1 mm or greater (e.g., Rb, Sr, Zr, Ba) generally show less difference between weathered surface and bulk composition. Y is an exception to this observation, but is not affected to the extent that K, Ca and Fe are in some weathered surfaces.

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Most of the samples for which weathered surface data were obtained are rhyolites. Two samples differ in rock type: R198 is a microdiorite, and R202 is a dolerite. These two samples show no systematic differences in behaviour from the rhyolitic samples. It may be that grain size, and the compactness and hardness of rocks, are equally or more important factors than chemical composition in influencing the extent of chemical changes seen in weathered layers. M10, with its deeply weathered layer, stands out on several graphs in Figure 2. Analysis of this deeply weathered surface by PXRF does not give a reasonable estimate of the bulk composition, and indeed careful sample preparation was necessary for bulk analysis by conventional WDXRF in order to avoid discrepancies due to weathered portions of the sample. Samples R198, R200, R201 and R202, which have very slight surface relief but were not scatter peakcorrected, do not show systematic differences from the remaining samples. In all, these data suggest that many trace elements, in particular Sr, Zr and Ba, and sometimes Rb, generally reflect the bulk composition of the thinly weathered rhyolites and also the dolerite and microdiorite considered here. Analyses of major elements with low critical penetration depths, especially K, Ca, Fe and Ti, must be interpreted with caution. More deeply weathered rhyolites are less suitable if PXRF measurements are to be used to estimate bulk composition.

Assessment of PXRF for Provenancing Using a Case Study of Seven Stone Implements from Wales In order to assess the usefulness of PXRF in a provenancing case study, seven prehistoric implements from Wales, including axes, axe-hammers and a macehead, were analysed by this method. Since the comparison of PXRF and WDXRF data described above is based mainly on rhyolites and dolerites, artefacts of these lithological types were chosen for the case study. The artefacts chosen are all held by the National Museum and Gallery, Cardiff. Their analysis forms part of a wider programme of stone axe and other implement characterization being undertaken at the Open University in collaboration with members of the Implement Petrology Committee (IPC) and with the Universities of Birmingham, Belfast and Dublin. British prehistoric stone axes have been the subject of a long-standing petrographic study by the IPC, and 34 axe Groups and their sub-Groups are summarized by Clough & Cummins (1988). The very extensive and detailed work of IPC members and their colleagues has led to the establishment of an invaluable database of stone axes, including over 4000 artefacts of which over 53% have been assigned to Groups. However, it has been recognized that some Groups may comprise implements from more than one source locality (e.g.,

Group VIII; Fell & Davis, 1988: 73), and not all axes have petrographic characteristics sufficiently unique to ensure unambiguous assignment to a source, as witnessed by the proportion of ungrouped axes. Therefore, chemical analysis of axes should contribute to their characterization, and complement, or in some cases replace, petrographic study. The aim of this case study is to compare the chemistry (from PXRF analysis) of the selected axes and other implements with that of the sources which have been proposed on the basis of petrographic thin section and hand specimen examination, and determine whether the chemistry supports the source proposals. The seven implements selected for this study, therefore, include artefacts which have been previously studied and assigned to Groups XIII (a dolerite Group) and VIII (a rhyolite Group). For some implements, which we suggest below do not match their proposed IPC Groups, comment is made on alternative likely sources, but a full comparison with all potential source outcrops is not within the scope of this paper. The National Museum and Gallery, Cardiff, holds some 30 artefacts of Groups VIII and XIII. The selection of the seven described here was dictated partly by the limited time available for the analysis at Cardiff, and partly by the extent of weathering of some of the artefacts. Those affected by deep weathering, particularly some rhyolite implements, were not suitable for PXRF bearing in mind the effects of weathered surfaces on bulk analysis noted in the preceding section of this paper. This highlights a significant shortcoming of the PXRF method. Implements analysed, analytical procedures and scatter peak correction Table 3 lists the implements studied, and their find locations in Wales are shown in Figure 3. All the objects are finished, polished implements. The macehead from Nevern, the axe from Netherwent and two axe-hammers from Llanfaethlu and Arthog have all been assigned to Group XIII (references in Table 3, summary in Clough & Cummins, 1988), with a thin section examination for the Nevern implement reported by Savory (1962–4). Axes from Barry and Llangasty have been assigned to Group VIII by thin section examination (Clough & Cummins, 1988). The Coygan implement is described as ‘‘somewhat similar mineralogically to Group VIII’’ (Wainwright, 1967: 14), but not actually belonging to that Group (cf. thin section descriptions by Phillips in Wainwright, 1967: 189). Destructive sampling for chemical analysis of these implements would be undesirable because of their relatively small size (the largest is about 20 cm long and 10 cm wide) and polished finish. We note that the coring method used by the Irish Stone Axe Project (e.g., Cooney & Mandal, 1995 and references therein) results in little obvious damage to implements,

Cilgwyn Mawr Nr NEVERN SN 089363

St Brides NETHERWENT ST 4390

Bwlch-Gwyn Farm, Nr ARTHOG & Llangelynin SH 623133

LLANFAETHLU Anglesey SH3186

61.52 Monmouth 8 Group XIII

30.147 Merioneth 8 Group XIII

33.499 Anglesey 10 Group XIII

Find locality G.R.

62.39 Pembroke 13 Group XIII

Museum ref. IPC county number and assigned Group

Axe-hammer Type 1a (Roe, 1979 : 41)

Axe-hammer Type 1a (Roe, 1979 : 43)

Axe

Mace Thames pestle type (Roe, 1979) Isolated find 100 yds E of Cilgwyn Cromlech

Object, and context if known

Table 3. Stone implements analysed by portable XRF

Dolerite? black and fine-grained, rare mafics to 1 mm; no spots or feldspar crystals visible

Dolerite, dark greenish grey, with pinkish sub-angular feldspar or spots c. 5–10 mm

Light coloured dolerite with whitish spots or crystals up to 15 mm; mafics — pyroxene? Matrix is greenish-grey

Medium-grained igneous rock with crystals of feldspar & mafics, probably pyroxene or hornblende Diorite?

Rock description and petrography

Smooth fresh surface measured (rest of artefact has weathered surface depth c. 1 mm); flat

Fairly smooth, flat not very weathered

Slight relief <1 mm slight curvature, brownish patches suggest some weathering but mainly not very weathered where measured

Fairly smooth and flat; slight brownish tinge may indicate weathering

Description of surfaces measured and weathering

194·1

158·1 (low scatter peak, may be from uneven crystal/spot surfaces)

181·2 173·2 178·5 181·3 177·9

203·6 208·8 207·1

Intensity of Fe Rayleigh scatter peak (sample) (s
1)

186 (WS-E)

186 (WS-E)

186 186 186 186 186 (WS-E)

180 180 180 (average Fe scatter peak in high-Fe reference materials)

Intensity of Fe Rayleigh scatter peak (reference surface) (s
1)

Not XIII Fine dolerite

Probably XIII Preseli spotted dolerite?

XIII Preseli spotted dolerite

Not XIII High Zr diorite?

Group and probable source or rock type from this work

Clough & Cummins (1988) Roe (1979) Grimes (1951: catalogue no. 351)

Clough & Cummins (1988) Roe (1979) Grimes (1951: catalogue no. 345)

Clough & Cummins (1988) Evens et al. (1962: 219)

Clough & Cummins (1988) Roe (1979) Savory (1962–4)

Literature references

Portable Non-Destructive X-Ray Fluorescence Analysis 227

Trebinshun Farm, LLANGASTYTalyllyn SO 137242

COYGAN Camp Llanddowror SN 284093

42.24 Brecon 4 Group VIII

67.514 Carmarthen 41 Unassigned

Axe Found near a Neolithic pit

Axe

Axe fragment

Object, and context if known

Grey/blue finegrained rock with white feldspar to 2 mm Thin section shows feldspar and pyroxene in fine-grained matrix of felds., mica, calcite and leucoxene

Black aphyric rock very fine-grained Rhyolitic tuff?

Bluish fine-grained, Some feldspar? to 1 mm fabric visible. Rhyolitic tuff?

Rock description and petrography

Polished and little curve to max. 1 mm, some weathering up to c. 1 mm depth whitish in colour; high scatter peaks: due to slightly convex surfaces?

Polished and flat, weathering greyish less than 1 mm depth

Smooth and flat; both measurements on same side of axe; slightly weathered grey/white

Description of surfaces measured and weathering

212·2 213·0

212·9 not noted on one analysis and not corrected

217·1 205·1

Intensity of Fe Rayleigh scatter peak (sample) (s
1)

197 197 (AGV-1)

202 n/a (GH-1)

211 211 (mean of this axe scatter peaks used — see text)

Intensity of Fe Rayleigh scatter peak (reference surface) (s
1)

Not Group VIII Glyn-y-Fran site; not Group VII

VIII? Chemical similarity with Ramsey Is and Abereiddy (Pembs.)

VIII Glyn-y-Fran or Glandy Cross axe manufacturing site

Group and probable source or rock type from this work

Wainwright (1967 : espec. 188–189, 161–162) Clough & Cummins (1988)

Clough & Cummins (1988) Grimes (1951: catalogue no. 118)

Clough & Cummins (1988) Grimes (1951: catalogue no. 101)

Literature references

G.R. from IPC records except for Nevern (Savory, 1962–4). 61.520/M8: this axe is listed by Clough & Cummins (1988: 252) as ‘‘The Glen Caerwent’’ but as St Brides Netherwent by the National Museum and Gallery, Cardiff and by Evens et al. (1962); the owner of the St Brides Netherwent axe lived at The Glen, Caerwent (E. Walker, pers. comm.). Short names of implements used in this paper are in upper case. WS-E, GH-1 and AGV-1 are materials of known composition from which reference scatter peaks are taken (Potts et al., 1992).

Peiro’s Abbey, BARRY ST 114665

Find locality G.R.

36.202/33 Glamorgan 14 Group VIII

Museum ref. IPC county number and assigned Group

Table 3. continued

228 O. Williams-Thorpe et al.

Portable Non-Destructive X-Ray Fluorescence Analysis

Figure 3. Map of Wales showing the find locations of the stone axes and other implements analysed (dots), the Preseli Hills, and sources outside the Preseli area discussed in the text (stars).

and so offers the most acceptable alternative to non-destructive analysis available at present. Implements were analysed using the TN9000 Spectrace instrument with total count times of either 170 or 340 s. Following the guidelines in Potts, Williams-Thorpe & Webb (1997), the aim was to obtain 1–2 analyses on each fine-grained artefact, and 2–5 on coarser-grained artefacts. This was achieved for most of the implements analysed, but shortage of time meant that only one analysis could be done on one of the coarser-grained dolerite implements (the Arthog axe-hammer). Pellets of known composition were measured three times during the analysis in order to monitor data quality. Surfaces for measurement were carefully chosen to include the smoothest and least weathered surfaces. Surface relief (roughness) was mainly zero — that is, the surface measured was smooth, having retained its polish — and where surfaces were not polished, relief was estimated to be 1 mm maximum (cf. Table 3). Some measured surfaces were slightly concave or convex relative to the analytical plane of the instrument because of the shape of the axes. Though it is difficult to measure the difference in sample position relative to the analytical reference plane, this difference was estimated at about 1 mm maximum. The measured areas of the Nevern mace-head, chosen for smoothness and freshness, were slightly smaller than the measurement aperture, leaving a small gap at one edge of the aperture of about 1 mm for the first two measurements and of 1–2 mm for the third measurement. As noted

229

above, the significance of this gap is reduced because the part of the sample at the edge of the analysis aperture contributes proportionately less to the analysed signal than parts of the sample nearer the centre of the aperture. Slightly weathered surfaces were analysed where fresher surfaces were not available. Weathering could be identified as lighter coloured layers of rock, up to about 1 mm thick, where the depth could be seen (for example on a broken surface of an implement). For each measurement, the intensity of the Rayleigh scatter peak produced during excitation by the 55Fe source was recorded. The concentrations determined for the implements were corrected for surface irregularities following the guidelines set out in Potts, Webb & Williams-Thorpe (1997) and summarized above. Reference scatter peak intensities were measured from pellets of known compositions, selected to parallel the composition of each sample, except for the Barry axe which has a total concentration of less than 2% for the major elements which were determined by the PXRF, and for which an averaged scatter peak value from two smooth surfaces on this axe was used. The scatter peak recorded from the Arthog axe is unexpectedly low at 158·1 s
1. The reason for this is uncertain but may be related to small-scale surface irregularities due to the porphyritic nature of this sample. The magnitude of the scatter peak normalization is typically 3–10% and exceptionally 18% (the Arthog axe). The resulting normalized concentrations were averaged for each implement to give mean compositions. Results for implements analysed Compositions determined by PXRF for seven stone implements, for five major and minor element oxides and nine trace elements are given in Table 4. For each implement, individual analyses, as well as their mean and relative standard deviation (rsd) are given. The rsds for measurements of the fine-grained implements (Barry, Llangasty, Coygan) vary greatly by element (mostly dependent on level of concentration and on the instrumental spectrum analysis uncertainty), for example, between 3 and 59% in the Barry axe. Relative standard deviations for the two coarsergrained implements (Nevern and Netherwent; omitting data for Cu, Zn and Pb which are generally less well determined by the PXRF method, cf. above) show less variation by element than the fine-grained implements, but all are greater than 9%. The spread of analyses from each implement for Sr, Y and Zr is illustrated in Figures 4 and 5. Typical instrumental spectrum analysis uncertainties are shown as 1 sigma error bars. The range of Sr and Zr determinations for individual samples (Figure 4) is within or near the error bars for several of the implements. For the coarser-grained Nevern and Netherwent implements, a greater range is seen, especially for Zr in the Nevern mace. Zr

Short implement ref. 4·54 6·10 5·52 5·39 14·7 0·09 7·61 9·12 12·37 8·60 10·68 9·68 19·4 9·31 5·91 0·32 0·79 0·56 59·0 0·48 0·45 0·46 3·7 0·04 3·73 3·43 3·58 6·1

0·06 1·00 0·91 0·93 1·15 0·94 0·99 10·0 0·99 0·36 0·22 0·14 0·18 32·2 3·34 2·88 3·11 10·4 0·04 1·43 1·38 1·41 2·7

CaO%

0·68 1·08 1·37 1·04 33·2

K2O%

0·05 0·72 0·69 0·70 2·8

0·08 0·54 1·07 0·54 0·77 0·75 0·73 29·8 0·70 0·96 0·16 0·07 0·11 59·1

2·03 2·75 2·63 2·47 15·6

TiO2%

0·03 0·12 0·08 0·10 30·2

0·14 0·10

0·16

0·06 0·23 0·18

0·29 0·34 0·31 0·31 8·9

MnO%

0·09 4·35 4·17 4·26 3·1

0·27 14·37 10·43 10·22 10·22 9·48 10·94 17·8 8·75 8·54 0·85 0·70 0·77 14·1 1·42 1·33 1·38 4·8

16·33 21·17 17·56 18·35 13·7

Fe2O3%

7 17 35 26 49

47

32

65

Rb ppm

9 164 167 166 1

21 263 344 392 296 348 329 15 518 246 49 53 51 5 150 125 138 13

474 352 402 409 15

Sr ppm

4 31 28 30 8

60 49 46 47 4 62 63 62 1

7 15

28 33 31 11

Y ppm

8 165 173 169 3

11 75 77 56 77 72 72 12 94 98 265 253 259 3 527 457 492 10

831 523 849 734 25

Zr ppm

3 13 6 9 48

7 9 8 19 14 13 13 3

14

12 17

Nb ppm

18 328 298 313 7

53 76 65 25 4820 3980 4400 13

23 150 146 116 114 133 132 13 192

145 271 272 229 32

Ba ppm

38 122 86 82 42

232

87

233 206

229

186 342

442 879 391 571 47

Cu ppm

37 192 117 154 35

167 174 170 3

87

81 377 339 217 368 245 309 24 307 341

1110 1110 1080 1100 2

Zn ppm

126 7890 44 329 186 108 80 43 61 43

299 353 456 369 22

Pb ppm

Data are>3 spectrum analysis uncertainty except values in italics (2–3 spectrum analysis uncertainty); missing values are<2 spectrum analysis uncertainty. Means and sd are given where all values for that sample are present. Count times: 100, 50, 20 s (Cd, Fe and Am sources, respectively), except Barry and Coygan (200, 100, 40 s). Data for Cu, Zn and Pb typically have poor precision (cf. Potts et al., 1995). Example spectrum analysis uncertainty (1 sd) for Pb is 107 +/
28 1 sigma (sample 30.147, un-normalized concentration). All concentrations have been corrected using the Fe scatter peaks as described in the text.

Nevern Nevern Nevern mean % or ppm rsd % Spectrum analysis uncertainty on the first listed Netherwent analysis (1 sd) 61.520; M8 Netherwent 61.520; M8 Netherwent 61.520; M8 Netherwent 61.520; M8 Netherwent 61.520; M8 Netherwent mean % or ppm rsd % 30.147; ME8 Arthog 33.499; AN10 Llanfaethlu 36.202/33; GL14 Barry 36.202/33; GL14 Barry mean % or ppm rsd % 42.24;BR4 Llangasty 42.24;BR4 Llangasty mean % or ppm rsd % Spectrum analysis uncertainty on the first listed Coygan analysis (1 sd) 67.514; CAR41 Coygan Camp 67.514; CAR41 Coygan Camp mean % or ppm rsd %

62.390;P13 62.390;P13 62.390;P13

Reference

Table 4. Portable XRF analyses of stone axes and other implements

230 O. Williams-Thorpe et al.

Portable Non-Destructive X-Ray Fluorescence Analysis

Figure 4. Graph of Sr versus Zr ppm, showing individual PXRF determinations on seven stone axes and other implements from Wales (based on data in Table 4). The error bars are spectrum analysis uncertainties (1 standard deviation) for a selection of implement concentrations, before the application of scatter peak corrections. Shaded areas enclose measurements of the same sample.

Figure 5. Graph of Y versus Zr ppm, showing individual PXRF determinations, for the same implements as on Figure 4. Error bars and shaded areas are as for Figure 4. Note that the Arthog implement is missing from this graph because the Y concentration is only c. 1·5 the spectrum analysis uncertainty (i.e., below the quantitative detection limit). Y for the Nevern implement is based on 2 determinations, and for the Netherwent implement on 1 determination (cf. Table 4).

concentration in the Nevern sample is very high and though variable (cf. Table 4) is high in all surfaces measured. Y determination ranges for individual samples (Figure 5) are well within the spectrum analysis error bars which, for Y, are relatively high. Group XIII and Group VIII source locations and descriptions The locations of the source areas which have been previously proposed for Groups XIII and VIII are shown on Figure 6 and 3. Group XIII comprises implements of ‘‘spotted’’ dolerite or ‘‘preselite’’ with source(s) (but no identified manufacturing site) in south-west Wales within the eastern part of the Preseli Hills (Clough & Cummins,

231

Figure 6. Map of part of Pembrokeshire, Wales, (cf. Figure 3) showing the locations of sources within the Preseli area discussed in the text (stars), and of Group VIII axe-manufacturing sites (crosses). The dotted line shows the approximate extent of spotted dolerite outcrops, adapted from Thorpe et al. (1991).

1988: 8 and references therein). The rock is a grey-blue dolerite often with ophitic texture, composed of altered clinopyroxene, plagioclase feldspar with opaque Fe-Ti oxides and sulphide accessory minerals (Thorpe et al., 1991). It is characterized by distinctive whitish or pinkish spots up to 10–20 mm in diameter composed of plagioclase with alteration products and resulting from low-grade metamorphic processes (Thorpe et al., 1991: 127; Bevins, Lees & Roach, 1989). Chemical analyses of these Preseli rocks are in Bevins, Lees & Roach (1989) and Thorpe et al. (1991). Group VIII is defined by the IPC as silicified tuff (Clough & Cummins, 1988: 8) but implements assigned to this Group show some petrographic variation (David & Williams, 1995: 452). Group VIII implements are typically blue or dark coloured, very finegrained silicic tuffs or lavas, sometimes devitrified or recrystallized. Thin sections show microcrystalline quartz, feldspar and chlorite together with clusters of, probably, titanite (from Jenkins and Rigby, both in David & Williams, 1995, and IPC record cards). The most important source localities reported for Group VIII are two recently identified axe-head manufacturing sites at Glyn-y-Fran and Glandy Cross in Pembrokeshire (David & Williams, 1995). At these sites, rhyolitic material was worked, probably glacial debris which it is thought originated mainly from nearby Carnalw, but which included one rhyolite piece from another (unidentified) outcrop. A number of other areas (without rhyolite axe manufacturing sites) have been suggested as Group VIII sources, including the Pembrokeshire localities of Mountjoy, Gorsefach, Foeldrygarn and Ramsey Island (Stone & Wallis, 1951; Morey, 1950) and the Cumbrian Lake District (Fell & Davis, 1988; Davis, 1984). Chemical analyses for the Group VIII sources at Glandy Cross and Glyn-y-Fran,

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Table 5. Representative chemical analyses of potential sources discussed in the text Area/IPC Group Sample ref. Locality Rock type G.R. SiO2% TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOl Ba ppm Cr Cu Nb Ni Rb Sr Th V Y Zn Zr

Preseli Hills, Group XIII

Carnalw, Pembrokeshire, Group VIII

CM1

CM2

CBR1

CA2

CA4

CA5

Carnmenyn

Carngyfrwy

Carnbreseb

Carnalw

Carnalw

Carnalw

Dolerite

Dolerite

Dolerite

Rhyolite

Rhyolite

Rhyolite

Axe manufacturing sites, nr. Carnalw, Group VIII GYF 226

48·44 1·30 16·84 9·94 0·17 6·12 9·66 3·99 0·24 0·13 2·57 186 123 45 4 28 9 322 1 234 25 82 89

48·05 1·27 16·29 10·38 0·17 7·34 10·92 2·33 0·79 0·14 2·75 150 295 33 5 34 17 249 2 235 28 72 99

78·7 0·19 10·06 2·74 0·02 0·37 0·22 1·15 6·04 0·03 0·72 1004 14 1 20 3 110 75 13 4 97 63 300

and the related Carnalw rocks, are in Williams-Thorpe (in David & Williams, 1995) and in Thorpe et al. (1991), respectively. Representative analyses of rocks from the outcrops mentioned in this section and other outcrops which are discussed below as potential sources for some implements are given in Table 5. Comparison of implement analyses with potential sources Since some of the implements studied appeared slightly weathered, those elements identified above as most robust in the analysis of weathered surfaces of dolerites and rhyolites (Sr, Zr, Ba and Rb) were considered first for provenancing these implements. Of these, Sr and Zr were preferred because Williams-Thorpe, in David & Williams (1995), previously identified very low concentrations of Ba and Rb in some of the worked material from Glyn-y-Fran and Glandy Cross. Y was also selected as a potential discriminator notwithstanding the (slight) discrepancy noted above between analyses of weathered and fresh samples, because Y was one of the elements used by Thorpe et al. (1991) in their

86·32 0·16 6·96 1·02 0·01 0·16 0·08 0·51 4·83 0·02 0·57 746 14 1 16 2 83 32 8 8 47 42 264

80·56 0·18 9·70 1·56 0·03 0·22 0·24 2·25 4·61 0·02 0·57 620 12 1 18 5 79 61 13 11 71 19 286

Glandy 192

Glyn-y-Fran Glandy Cross Glandy Cross Rhyolite

SN 14303250 SN 14753270 SN 13603320 SN 13903375 SN 13963375 SN 139338 SN 186307 47·61 1·02 18·93 8·40 0·15 6·49 10·29 2·71 0·83 0·10 2·82 326 213 60 3 29 19 276 1 190 20 68 68

Glandy 190

Rhyolite

Rhyolite?

SN 140266

SN 140266

0·17

0·18

0·89

1·73

2·77

3·59

26 6 4 14·5 3 1·4 24·5 9 4 82·2 8 270

36 3 2 11·9 3 0·3 41·4 8 5 88·2 31 313

55 73 10 15·8 47 nd 164·7 13 72 32·3 41 150

characterization of Pembrokeshire dolerites and rhyolites, and because Y is a well-established geochemical discriminator (Rollinson, 1993). Figures 7 and 8 (Sr versus Zr and Y versus Zr, respectively) show the means of the data for the implements analysed (or single analyses where only one was done) together with samples from the sources discussed above. The error bars on implements shown on Figures 7 and 8 are the standard deviations (1 sigma) on multiple measurements of implements (for spectrum analysis uncertainties see Figures 4 & 5). The Nevern mace-head, P13, 62.390. Although this implement has been previously assigned to Group XIII, it is clearly very different in chemistry from the Preseli dolerites shown on Figures 7 and 8 even taking into account the possible errors. In particular, the implement has very high Zr (over 700 ppm, cf. Table 4); the replication of a high Zr value on three separate parts of the implement gives confidence that this is not just a reflection of small-scale mineral inhomogeneity. This implement does not therefore belong to Group XIII as defined by the Preseli source area. In fact, the mineralogy of the Nevern mace is also clearly different

Portable Non-Destructive X-Ray Fluorescence Analysis

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Table 5. continued Area/IPC Group Sample ref.

Pembrokeshire, possible Group VIII sources

North Wales, Group VII

SP63

RIG3

FT2

JD2902

Abereiddy

Ramsey Island

Foeldrygarn

Craig Llwyd

Rock type

Volcaniclastic?

Ignimbrite

Andesite

Microdiorite

G.R.

SM 79263153

SN 15653350

SH 718752

60·51 1·34 14·91 8·58 0·17 3·98 2·97 4·57 0·90 0·36 2·48 287 36 14 15 18 14 218 3 86 93 127 453

62·62 0·84 15·10 6·10 0·15 2·61 4·87 3·41 2·59 0·19 1·56 357 67 28 12 28 94·1 122 8 87 48·7 72 195

Locality

SiO2% TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOl Ba ppm Cr Cu Nb Ni Rb Sr Th V Y Zn Zr

79·00 0·13 11·60 1·52 0·02 1·53 0·02 2·70 4·50 0·02 1·44 2602 nd

15

24 19 35 96

16 7 45 148

57 576

0·19

3·25 0·24

26 96 103 496

Carnalw samples include tuff (CA2) and probable lavas (CA4, CA5). Dolerites are all ‘‘spotted’’ with characteristic metamorphic whitish spots. Data sources: CM1, CM2, CBR1, CA2, CA4, CA5, FT2: Thorpe et al. (1991) and unpubl. data referred to therein from the Universities of Keele and Southampton; Glyn-y-Fran and Glandy samples: David & Williams (1995); SP63: Bevins (1979) reported in Thorpe et al. (1991); RIG3: R. Bevins (National Museum of Wales) unpubl. data; JD2902: unpubl. data from John Durham, Open University. nd=not detected; missing values=not determined. Fe2O3 is total iron. All data from laboratory XRF analyses.

from Group XIII/preselite, appearing more consistent with a dioritic rock. Igneous rocks, characterized by very high Zr concentrations, are found in several parts of the U.K., including both north and south Wales (Thorpe et al., 1993; Thorpe et al., 1991 and references therein). The Netherwent axe, M8, 30.147. This axe, also previously assigned to Group XIII, matches closely the chemical characteristics of the Preseli spotted dolerites (Figures 7 & 8), and its appearance and mineralogy, as far as can be assessed in hand specimen, are consistent with this source. The Arthog axe-hammer, ME8, 30.147. Only one measurement of this axe-hammer (previously assigned to Group XIII), was undertaken, so results are more difficult to interpret. The Zr concentration of 94 ppm is within the range of Preseli dolerites (Figure 7). The Sr

value is higher than the Preseli dolerite group though it would be just within the 2 sd range for Sr measurements (based on results for the mineralogically-similar Netherwent axe). Sr tends to be concentrated within the spots in spotted dolerites of Preseli (Thorpe et al., 1991: 128) and the single analysis may reflect this bias. Y in the Arthog implement is at or below the detection limit for the PXRF method, and is therefore omitted from Figure 8. Results for Arthog are unsatisfactory because only one analysis was undertaken, but the data do not give grounds to alter the petrographic assignment to Group XIII. The Llanfaethlu axe-hammer, AN10, 33.499. Sr and Zr data for the single analysis of this implement (previously assigned to Group XIII) lie within the Preseli dolerite group on Figure 7. However, the implement is significantly higher in Y (Figure 8; Y=c. 60 ppm, spectrum analysis uncertainty=15 ppm),

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(7890 ppm) and recognize the possibility of contamination of the sample, perhaps by paint used in sample identification (though none was observed on the implement by us). The mineralogy of the sample, visible in a small freshly broken part, is fine-grained and appears doleritic but is very different from the typical Preseli spotted dolerites because it has no spots, casting further doubt on the assignment of the implement to Group XIII.

Figure 7. Graph of Sr versus Zr ppm, showing the average composition (or single measurement) determined by PXRF for each of the seven implements analysed (filled squares) together with compositions of rock samples from proposed source areas of Preseli dolerites (open diamonds), Carnalw rhyolites (filled diamonds), and axe manufacturing sites at Glyn-y-Fran and Glandy Cross (open squares). Error bars shown on the implement analyses are 1 standard deviation on repeat measurements where available (not shown where the uncertainty limits fall within size of the symbols). Whole rock data for proposed sources shown are all by laboratory-based XRF. Data are taken from: Preseli dolerites: Bevins, Lees & Roach (1989) (samples of dolerite from the spotted dolerite area; samples included are within an area of G.R. SN 11–15 eastings, SN 32–33.5 northings), and Thorpe et al. (1991) and data referred to therein from the Universities of Southampton and Keele; Carnalw: Thorpe et al. (1991) and data referred to therein from the Universities of Southampton and Keele; Glyn-y-Fran and Glandy Cross: WilliamsThorpe in David & Williams (1995); Williams-Thorpe, 1992 unpubl. report from the Open University. Shaded areas enclose samples from each of the source areas or manufacturing sites. Sources and axe manufacturing sites are named in upper case, rock types in upper case italics, and implements in lower case.

Figure 8. Graph of Y versus Zr ppm, showing the average composition (or single measurement) determined by PXRF for each of the seven implements analysed, together with their proposed sources. Symbols, error bars and data sources are as for Figure 7. The Arthog implement is omitted as on Figure 5, and Y determinations for the Nevern and Netherwent implements are based on data as for Figure 5.

casting doubt on a Group XIII origin. Sampling uncertainty should be low because the implement is fine-grained. However, we also recorded extremely high apparent Pb concentrations in this implement

The Barry axe fragment, GL14, 36.202/33. The analysis of this implement showed extremely low abundances of the major elements that could be determined by PXRF (Table 4), but it was noted that the artefact surface appears slightly weathered. The more stable elements, Sr and Zr, and Y show good agreement, respectively, with material from the working sites of Glyn-y-Fran and Glandy Cross (Sr, Zr; Figure 7), and the Carnalw rocks which are believed to be their source (Y; Figure 8). This supports the IPC (thin-section based) allocation of this sample to Group VIII. The Llangasty axe, BR4, 42.24. This implement, previously thin-sectioned and assigned, like the Barry axe, to Group VIII, is very different in chemistry both from the Barry axe and from the sources and working sites of Group VIII shown on Figures 7 and 8. The Llangasty sample is characterized in particular by high Zr and Ba concentrations (492 and 4400 ppm, respectively). Of the other source areas proposed for Group VIII, chemical analyses are available for Foeldrygarn together with other acid volcanics of Pembrokeshire (Bevins, Lees & Roach, 1989; Thorpe et al., 1991), Ramsey Island (R.E. Bevins, unpubl. data), and for Lake District acid tuffs and other volcanics (Eycott and Borrowdale volcanic groups; Millward, Moseley & Soper, 1978; Sutherland, 1982). Of these, only rocks from Ramsey Island (ignimbrite) and from the north Pembrokeshire coast at Abereiddy Bay (rhyolitic volcaniclastic) show close chemical similarity with the Llangasty axe (Table 5). The Coygan Camp axe, CAR41, 67.514. This axe has also been previously thin-sectioned, but, despite some similarity with Group VIII (Wainwright, 1967: 14) and also with the north Wales microdiorite Group VII (Wainwright, 1967: 162), it was not firmly assigned to any Group (cf. Phillips in Wainwright, 1967: 188–189; Wainwright, 1967: 161–162). Chemical analysis shows that the Coygan axe does not belong to the Glyn-yFran/Glandy Cross/Carnalw Group VIII type (Figures 7 & 8). Interestingly, it is close in chemistry to one sample from Glandy Cross, but that sample lacks the phenocrysts of feldspar up to about 2 mm in length which are visible in the Coygan axe. Comparison with the other suggested Group VIII sources (data as for the Llangasty axe) yielded no close chemical parallel. The full range of Group VII

Portable Non-Destructive X-Ray Fluorescence Analysis

composition has not been established, but available analyses are chemically distinct from the Coygan axe (cf. Table 4 & 5), so that Wainwright’s rejection of a Group VII source (Wainwright, 1967: 162) appears correct. Summary and implications of the case study In summary, therefore, of the four implements previously assigned to Group XIII, two (Netherwent and Arthog) are geochemically consistent with Group XIII, one (Nevern) is definitely not Group XIII, and one (Llanfaethlu) is ambiguous in chemical interpretation but mineralogically is unlikely to be Group XIII. Of the two implements assigned to Group VIII, one (Barry) chemically matches material from Group VIII axe working sites at Glyn-y-Fran and Glandy Cross in south Wales, one (Llangasty) does not, but may be from another north Pembrokeshire source of Group VIII material. The Coygan axe is ambiguous in terms of source but is unlikely to be from the Glyn-y-Fran axe manufacturing site. The PXRF analyses of the implements are of sufficient overall precision and accuracy to be compared with laboratory-based (WDXRF) analyses of proposed sources, and show an acceptable sampling uncertainty, consistent with the study of Potts, Williams-Thorpe & Webb (1997). The analyses successfully discriminate between axes of different rock types. However, limitations were encountered in defining rock type from the geochemical analyses alone, mainly because diagnostic major elements analysed by PXRF, expecially K and Fe, were shown earlier in this paper to be subject to significant changes from bulk composition in weathered surfaces. Interpretation of data in terms of rock type and source must be limited, in weathered surface analysis, to those elements which are stable and diagnostic. In practice, the more useful elements for discrimination in the implements studied here were Zr, Y and Sr, Nb when present at higher concentrations, and Ba in some samples. In this limited exercise of comparing PXRF with a small number of proposed sources, PXRF analysis can contribute significantly to implement provenancing. The recognition that two out of the four supposed Group XIII implements are not Group XIII suggests that re-examination of other members of this Group may be worthwhile. The Group contains only 29 implements (now reduced to 27) but is of especial importance because the anthropogenic movement of Group XIII Preseli spotted dolerite axes is used as a supporting argument for the human transport of the Preseli spotted dolerite monoliths at Stonehenge (e.g., Green, 1997: 8). We therefore plan to re-examine more implements of Group XIII. Further work is also in progress on Group VIII axes, noting the potential to define the Group more precisely following the identification of the working sites (David & Williams, 1995: 453), and on selected other Groups.

235

Conclusions Portable XRF, using a mercuric iodide detector, has the potential to provide non-destructive, quantitative chemical analyses of rocks and artefacts in the field. However, PXRF results are derived from the surface layer of a rock or artefact to depths mainly between 0·03 mm and 2 mm and to a maximum depth of about 10 mm, for the elements considered in this paper. On unprepared samples, the degree to which the analytical signal is representative of bulk sample composition is likely to vary with rock type and its weathering history. Full details of the analytical method and instrumentation are in Potts et al. (1995). Investigation into surface irregularity effects and the use of scatter peaks for correcting data are in Potts, Webb & WilliamsThorpe (1997), and the effects of sample mineralogy and grain size in relation to the volume analysed by PXRF are in Potts, Williams-Thorpe & Webb (1997). The main conclusions of these three papers are relevant to this work, and are summarized below as (i) to (iii). (i) Elements in rocks that can be analysed routinely by PXRF include K, Ca, Ti, Mn, Fe, Rb, Sr, Y, Zr, Nb, Pb and Ba, with detection limits for the most sensitively determined elements in the range 6–14 ppm. Co, V, Cr, Cu, Ni, Zn, Ga, La and Ce can be determined when present at high concentrations (mainly >100 ppm, and >1000 ppm for Cr). (ii) Analytical data from measurements on irregular surfaces (i.e., rough, concave or convex surfaces) can be corrected to compensate for the effects of the varying distance between the analytical reference plane and the sample. This is done by normalizing analyte concentrations to the ratio of the intensity of the Rayleigh scatter peak from excitation by the 55Fe source in a reference surface spectrum to the intensity of the Rayleigh scatter peak from excitation by the 55 Fe source in the sample spectrum. This correction procedure is applicable for effective air gaps between analyser and sample of up to about 3 mm, but errors remain, probably as a result of preferential attenuation in air of lower energy fluorescence X-rays. (iii) Because silicate rock samples are composed of mineral grains, it is usually necessary to average multiple determinations to obtain an estimate of bulk composition to an acceptable level of precision. The number of measurements needed varies with the rock type (mainly according to grain size and mineral content), and with analyte elements. As a general guide, one to three measurements are appropriate for rocks or artefacts of grain size mainly <1 mm and up to 1–2 mm, and one to five measurements for rocks of grain size typically c. 3 mm. It is more difficult to estimate the bulk composition of coarse-grained rocks containing large proportions of minerals of greater than about 5 mm grain size using PXRF, because an average of many measurements is likely to be needed to give an analysis with a standard deviation of the mean of 10% or better.

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New data in this paper show good agreement between PXRF data of fresh sawn surfaces of samples of dolerite, rhyolite and a microdiorite, and conventional laboratory WDXRF whole rock analyses of the same samples for a range of elements. These data also illustrate the effectiveness of averaging duplicate and multiple PXRF analyses to give a better estimate of bulk composition with reduced analytical and sampling uncertainty. However, K, Ca, Fe and Ti measurements are shown to be particularly susceptible to discrepancies caused by weathering effects. Elements identified as most representative of bulk composition in weathered surfaces of the fine-grained rocks studied include Zr, Sr (except for one more deeply weathered sample), and Ba. Deeply weathered surfaces and parts of samples which appear unrepresentative in hand specimen should be avoided in PXRF analysis when an estimate of bulk composition is required. The PXRF method was tested on an archaeological problem by analysing seven stone implements (axes, axe-hammers and a mace-head in the National Museum and Gallery, Cardiff) which had previously been assigned to sources using petrographic examination in thin section and in hand specimen. PXRF was successful in discriminating between implements of different rock types, and agreement between multiple measurements on implements was good, with sampling precision consistent with the study of Potts, WilliamsThorpe & Webb (1997). Of four implements previously assigned to Group XIII (Preseli spotted dolerites), two are confirmed as Group XIII and two differ from this Group chemically (and mineralogically). Of two Group VIII (rhyolite) axes analysed, one has a chemistry consistent with Group VIII manufacturing sites in west Wales, while the other does not match the manufacturing sites but shows strong chemical similarity with other suggested source areas of Group VIII in Pembrokeshire, Wales. The final axe could not be assigned with any certainty to a source. Some important limitations in the use of PXRF were identified in this provenancing exercise. Interpretation relied on a small number of elements, with Sr, Zr and Y being most useful for the seven implements studied. Identification of rock type from the chemistry alone was very difficult because the normally diagnostic major elements analysed (K, Fe, Ca, Ti) can be affected to a significant extent by surface weathering and therefore are not necessarily representative of bulk composition. In conclusion, high resolution PXRF using a mercuric iodide detector is a truly portable XRF instrument capable of non-destructive analysis of rocks and artefacts in situ in the field or in museums for a range of elements. The method has been successfully used in characterizing stone implements in our case study, with the significant advantage that all data were obtained with no destructive sampling required. The method therefore has considerable potential for archaeological provenancing studies.

Acknowledgements This work was carried out with support from the Open University Research Committee (grants no. BR60 644 to Phil Potts, Peter Webb and Chris Jones, and BC15 1312 to Phil Potts, O. Williams-Thorpe and Peter Webb) which we gratefully acknowledge. Support for OWT from the Leverhulme Trust during earlier parts of this work (grant no. F269/P) is also gratefully acknowledged. We thank John Watson for WDXRF analyses, and John Taylor for cartography. For the loan of archaeological samples from the U.K., we are very grateful to Julian Richards, Andrew David, and to Clare Conybeare and Janet Bell (Salisbury and South Wiltshire Museum). For permission to study stone axes in the National Museum and Gallery, Cardiff, and for assistance in their examination we thank Miss Elizabeth Walker and Mr Mark Lodwick. We are grateful to Dr Richard Bevins and Mr John Durham for permission to quote and refer to unpublished analyses on Ramsey Island and Penmaenmawr, respectively. For copies of Implement Petrology Committee record cards and computer database details, we are very grateful to Dr John L1. Williams and the late Mr John Eccles. OWT would like to express her gratitude to members of the IPC, and especially Dr Vin Davis, for encouragement for the work on chemical analysis of stone axes. OWT would also like to thank Ian Rigby, Birkbeck College, for his help with PXRF work in particular at the National Museum and Gallery, Cardiff, and for comments and encouragement on several drafts of this paper. We are very grateful to Richard Kilworth and to Thermo FI, U.K., (formerly Thermo Electron, Warrington and Thermo Unicam, Cambridge) for their help and support in our work with the Spectrace TN9000 portable XRF instrument.

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