The Bulk Analysis of Silicate Rocks by Portable X-Ray Fluorescence: Effect of Sample Mineralogy in Relation to the Size of the Excited Volume

This paper examines the limitations arising from sample mineralogy when a portable X-ray fluorescence instrument is applied to the direct in situ analysis of silicate rocks. Estimates were made of the size of the excited volume from which the X-ray fluorescence signal originates by calculating the critical penetration depth for selected X-ray lines. Measurements were made of the variations in detected intensities over the area of the P-XRF analyser window and showed that, using radioisotope sources incorporated in the instrument used in this study (Spectrace TN9000), excitation intensities were six to ten times greater at the centre compared to the edge of the analyser window. These data indicated that the region of the sample at the centre of the window will make an enhanced contribution to detected spectra. Replicate measurements on slabs of rock selected to represent fine-to coarse-grain size textures indicated the magnitude of the sampling precision that can be achieved in the direct analysis of silicate rocks. Typical values were better than 5% relative standard deviation of the mean from an average of five determinations on fine- to medium-grained rocks and better than 10 % relative standard deviation in a single determination on these samples.     

Using Field-Portable XRF to Assess Geochemical Variations Within and Between Dolerite Outcrops of Preseli, South Wales

Eight dolerite outcrops in Preseli, south Wales were measured in situ using field-portable XRF analysis, in order to compare two different analysis ("sampling") strategies, and to investigate geochemical variability within and between outcrops. A sampling strategy of two (neighbouring but independent) measurements at each of a maximal number of locations dispersed over an outcrop was the more effective in indicating the overall chemical variance of that outcrop. Analysis of variance indicated that much of the observed variance within individual outcrops stems from real geochemical variability rather than from sampling and analytical factors. Standard ANOVA F tests showed that several of the studied outcrops are heterogeneous at the 5% significance level for one or more elements. Geochemical distinctions between some outcrops were demonstrated using discriminant analysis. PXRF analysis offers an alternative approach to conventional characterisation of outcrops, which is often based on laboratory analysis of small numbers of samples. However, PXRF data are affected by rock weathering and may require correction for this if they are to be compared with analyses of fresh rock.     

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. © 1999 John Wiley & Sons, Inc.