Olwen Williams-Thorpe

Analysis of silicate rocks using field-portable X-ray fluorescence instrumentation incorporating a mercury(II) iodide detector: a preliminary assessment of analytical performance

An assessment is reported of the performance of a field-portable energy dispersive X-ray fluorescence analysis system using 55Fe, 109Cd and 241Am excitation sources and a high resolution mercury(II) iodide detector. Seventy international reference materials, mainly of silicate composition, were analysed as compressed powder pellets. Results from spectra recorded for live times of 200 s (per source) showed that the major elements K, Ca, Ti, Mn and Fe and the trace elements Ba, Nb, Rb, Sr, Y and Zr could be determined routinely in ‘normal’ silicate rocks and in addition Co, Cr, Cu, Ga, La, Nd, Ni, V and Zn at higher concentrations or in mineralized samples. Detection limits for the most sensitively measured trace elements (Rb, Sr, Y, Zr, Nb) were found to be in the range 6–14 µg g–1 and for Ba (K-line), 21 µg g–1. Precision in the determination of the major elements was generally in the range 0.45–2%(relative standard deviation) and a high degree of accuracy was achieved when judged from a self-consistent comparison of analysed values with the expected compositions of the 70 reference materials. Having characterized performance in the laboratory with ‘ideal’ control over sample presentation, work is now in progress to evaluate additional discrepancies that will be encountered in the analysis of geological and archaeological samples in the field.

Investigation of a Correction Procedure for Surface IrregularityEffects Based on Scatter Peak Intensities in the Field Analysis ofGeological and Archaeological Rock Samples by Portable X-ray FluorescenceSpectrometry

Discrepancies arise in the analysis by portable XRF of geological and archaeological rock samples that have irregular shaped surfaces, because the instrument is calibrated for the quantitative analysis of flat samples. A simple correction procedure was investigated to overcome these discrepancies in which the measured intensity is normalised by the ratio of the scatter peak intensity from a compositionally similar flat reference sample to the scatter peak intensity measured from the sample itself. The scatter peak data were obtained from the 55 Fe, 109 Cd and 241 Am sources incorporated in the instrument used for this investigation. Under controlled conditions, this correction has proved to be successful in compensating for effective air gaps of up to 3 mm in the analysis of the K lines of higher atomic number elements (Rb, Sr, Y, Zr, Nb, Ba) and up to 1 mm for the Fe K line. Low energy K lines are affected by air attenuation in the air gap, which is not accounted for in this simple model. The scatter peak from the 55 Fe source is preferred for the correction because its intensity is least dependent on sample composition, but the 109 Cd scatter peak can be used instead with more careful matching of the composition of the flat sample used to derive the reference scatter peak intensity. Apart from additional air attenuation, the principle limitations to the application of this method to larger air gaps were ( i ) the change in scatter angle and, therefore, relative scatter intensity as the air gap is increased and ( ii ) the increasing contribution from scatter in air, particularly to the measured 55 Fe scatter peak at larger air gaps between sample and analyser.

The Geological Sources and Transport of the Bluestones of Stonehenge, Wiltshire, UK.

Stonehenge on Salisbury Plain is one of the most impressive British prehistoric (c. 3000–1500 BC) monuments. It is dominated by large upright sarsen stones, some of which are joined by lintels. While these stones are of relatively local derivation, some of the stone settings, termed bluestones, are composed of igneous and minor sedimentary rocks which are foreign to the solid geology of Salisbury Plain and must have been transported to their present location. Following the proposal of an origin in south-west Wales, debate has focused on hypotheses of natural transport by glacial processes, or transport by human agency. This paper reports the results of a programme of sampling and chemical analysis of Stonehenge bluestones and proposed source outcrops in Wales . Analysis by X-ray-fluorescence of fifteen monolith samples and twenty-two excavated fragments from Stonehenge indicate that the dolerites originated at three sources in a small area in the eastern Preseli Hills, and that the rhyolite monoliths derive from four sources including northern Preseli and other (unidentified) locations in Pembrokeshire, perhaps on the north Pembrokeshire coast. Rhyolite fragments derive from four outcrops (including only one of the monolith sources) over a distance of at least 10 km within Preseli. The Altar Stone and a sandstone fragment (excavated at Stonehenge) are from two sources within the Palaeozoic of south-west Wales. This variety of source suggests that the monoliths were taken from a glacially-mixed deposit, not carefully selected from an in situ source. We then consider whether prehistoric man collected the bluestones from such a deposit in south Wales or whether glacial action could have transported bluestone boulders onto Salisbury Plain. Glacial erratics deposited in south Dyfed (dolerites chemically identical to Stonehenge dolerite monoliths), near Cardiff, on Flatholm and near Bristol indicate glacial action at least as far as the Avon area. There is an apparent absence of erratics east of here, with the possible exception of the Boles Barrow boulder, which may predate the Stonehenge bluestones by as much as 1000 years, and which derived from the same Preseli source as two of the Stonehenge monoliths. However, 18th-century geological accounts describe intensive agricultural clearance of glacial boulders, including igneous rocks, on Salisbury Plain, and contemporary practice was of burial of such boulders in pits. Such erratics could have been transported as ‘free boulders’ from ‘nunataks’ on the top of an extensive, perhaps Anglian or earlier, glacier some 400,000 years ago or more, leaving no trace of fine glacial material in present river gravels. Erratics may be deposited at the margins of ice-sheets in small groups at irregular intervals and with gaps of several kilometres between individual boulders . ‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.

Mons Claudianus and the problem of the granito del foro': a geological and geochemical approach

Granito del foro is a distinctive igneous rock, in fact a granodiorite rather than a granite, long known and named for its use in buildings of the Roman Forum. Exactly what is it? Where does it come from? Where else was it used and not used? What does the granito del foro say about ownership and empire?

Non-destructive geochemical and magnetic characterisation of Group XVIII dolerite stone axes and shaft-hole implements from England

Thirty-five polished stone axes and shaft-hole implements which had been previously assigned by petrographic (partially destructive) study to Group XVIII (assumed to be from the Whin Sill dolerite source in northern England), were characterised non-destructively using a combination of portable X-ray fluorescence analysis (PXRF) and magnetic susceptibility measurements. All the implements were compared with the Whin Sill, and with other petrologically and/or visually similar rock sources in the north of England and in Scotland, using a database of chemical elements constructed from the published literature, and statistical atypicality testing. Twenty-nine of the implements are good matches for the Whin Sill as expected, but three do not match Whin Sill rocks and may therefore need to be re-assessed as members of Group XVIII. A further three implements have ambiguous characteristics, perhaps as a result of surface weathering. One axe, an axe-hammer and an adze that had not previously been assigned to implement groups were also analysed, as a test of the robustness of the non-destructive methods. All three implements could be distinguished from Group XVIII on the basis of chemical and magnetic characteristics. Most of the distribution of Group XVIII implements is best explained by the opportunistic use of glacial erratics rather than trade from the primary source. Possible alternative sources for the non-Group XVIII implements include the Scottish Old Red Sandstone age lavas, which could also have been obtained as erratics. The chemical and magnetic characteristics established in this paper provide a non-destructive alternative to thin section study of potential Group XVIII artefacts.

Stone Axe-head Manufacture: New Evidence from the Preseli Hills, West Wales

Fieldwork by the Dyfed Archaeological Trust during 1989–92 has identified clear evidence for the manufacture of stone axeheads at two locations on the eastern flanks of the Preseli Mountains, Dyfed: at Glyn-y-Fran, Llanfyrnach (SN 186 307) and near Glandy Cross (SN 143 266). At both sites, small quantities of lithic debris were collected from field surfaces after cultivation; unfortunately, no contemporaneous features were found by subsequent, very limited, trial trenching. In this report we describe the fieldwork at these two sites, and the resulting lithic collection, concluding that the latter represents evidence for small-scale and opportunistic exploitation of locally abundant erratics during the Neolithic. The Glandy Cross area was later a focus for the construction of ritual monuments during the Bronze Age, and there is also some evidence for continuing activity at Glyn-y-Fran at this time. Petrological thin section analysis of some of the artefacts is reported and demonstrates a probable identity with petrological Group VIII; geochemical analysis of some of the same artefacts places the likely geological origin of these at local igneous exposures also in the Preselis. These conclusions are reviewed in the light of current discussion on the usage and origins of raw materials in later prehistory.

Chlorine-36 dating and the bluestones of Stonehenge

The bluestones of Stonehenge on Salisbury Plain continue to attract controversy. New dates on Stonehenge material using the Chlorine-36 method have been reported as evidence that the bluestones were moved from their Welsh sources by human transport, not by glaciation. Here Olwen Williams-Thorpe and colleagues, who have argued for the glacial transport theory, show that the Chlorine-36 dates have been misinterpreted.

Geochemical provenancing of igneous glacial erratics from Southern Britain, and implications for prehistoric stone implement distributions

Sixteen basic and intermediate composition igneous glacial erratics from Anglian (pre-423,000 years) deposits in Hertfordshire and Buckinghamshire, southern Britain, were selected for chemical and petrographic analysis in order to determine their original source outcrops. Major and trace element compositions suggest that seven samples (plus two uncertain) originated in the Lower Carboniferous volcanics of the Scottish Midland Valley (SMV), four came from the Upper Carboniferous quartz dolerite association which crops out in Scotland, northern England (Whin Sill) and extends to Norway, and one came from the northern England Cleveland Dyke. One sample of altered dolerite is ambiguous but has some similarity to the Old Red Sandstone (Devonian) age lavas of the SMV, and one meta-basalt sample may be from southwest Scotland or Scandinavia. These results identify specific outcrops which provided glacial erratics within currently accepted ice trails in the United Kingdom, and provide the first supporting evidence based on geochemistry, rather than petrography, for these ice movements. The distribution and provenance of glacial erratics are of importance in archaeological studies, because erratics provided a potential source of raw material for stone implement production. There is a marked geographical correlation between the distribution of prehistoric stone implements of quartz dolerite in the United Kingdom, and directions of ice movements from quartz dolerite outcrops within Britain. This correlation lends support to the hypothesis that prehistoric man made extensive use of glacial erratics for implement manufacture, as an alternative to quarrying at outcrops and subsequent long-distance trade.

Radioelement distribution in the Tertiary Lundy Granite (Bristol Channel, UK)

The radioelement distribution and content of the Lundy granite, a coarse-grained megacrystic granite of Tertiary age, has been measured using a portable gamma-ray spectrometer in order to assess fractionation and alteration processes in the granite. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. The plateau region of the island (particularly the southern half) is relatively depleted in all radioelements. Over the island, measurements of K vary from 1.3–4.9 wt %, Th varies from 5.0–20.3 ppm and U varies from 2.0–12.5 ppm. A petrographic, electron microprobe and autoradiography examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbite (U) and uraninite (U) are the most important. Uraninite is rare, being preserved only in fresh samples which come mainly from abandoned quarries. Mass balance modelling indicates that up to 76.6% of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the present radioelement content no longer reflects the original rock composition. Fission track evidence is presented to show the pathways along which uranium has been mobilized from or within the granite. Secondary sites of radioelements include fractures cross-cutting all major minerals (but especially quartz), grain boundaries, altered cores of plagioclase feldspar and occasionally yellowy brown mixed chlorite/smectite replacement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces. Combined effects of crystal fractionation (primary variation) and secondary alteration best explain the distribution of radioelements, with K controlled by fractionation of the major phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (±xenotime and uraninite) and U contents by uraninite and tungsteniferous columbite. Secondary processes have removed much of the uraninite leaving behind indeterminate Fe—U material along fractures and residual U (and Th) enrichment within altered major minerals. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures during the waning stages of magmatic activity.

Geochemical and Magnetic Provenancing of Roman Granite Columns from Andalucía and Extremadura, Spain

One hundred and fifty two granite columns were examined in eight towns and archaeological sites in Andalucia and Extremadura, Spain, in order to determine the geological provenance of the columns. Three non-destructive methods of characterization were used: mineralogical features, magnetic susceptibility, and concentrations of radioelements (K, U, Th) determined by portable gamma ray spectrometry. Columns were compared with potential sources within Spain and in the Mediterranean area using analogous data previously published and also new data obtained for this work. The majority of the columns are made of Spanish granites, some of which were probably quarried near Merida. Different chemical types of Spanish granites were used in the northern part of the area studied (Extremadura) and in the southern part (Andalucia). Twenty five columns are not of Spanish granites, but were imported from other sources, namely the Troad and Kozak Dag areas of western Turkey, the Italian islands of Elba and/or Giglio, and Sardinia (confirming a column previously identified in the literature). The imported columns are found in Italica, Hispalis (Seville) and Astigi (Ecija), and were probably carried along the River Guadalquivir and its tributaries.