Peter C. Webb

The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.

PROBE-AMPH—a spreadsheet program to classify microprobe-derived amphibole analyses

Abstract A Microsoft Excel™ spreadsheet program has been developed to calculate structural formulae of microprobe-derived amphibole analyses and determine classification parameters according to the International Mineralogical Association scheme for amphibole nomenclature. The program has been tested in the determination of formulae and classification of 138 amphibole analyses from the literature. Of these, the small percentage (14%) of discrepancies were determined mainly to be the result of (i) differences between measured (wet chemical) and estimated values for FeO and Fe2O3 and (ii) differences inherent in summing structural formula of microprobe data to 23 oxygens and wet chemical data to 24 oxygens. The program has been developed for the Apple Macintosh but a PC version also is available. Spreadsheet implementations of four published geobarometers based on the total aluminium content of certain calcic amphiboles also are presented. These geobarometers are applicable to metaluminous plutonic (and associated volcanic) rocks containing the assemblage amphibole-biotite-quartz-plagioclase-(orthoclase)-sphene-FeTi oxides and apply to a pressure range of 2.5–13 kbar.

An objective assessment of analytical method precision: comparison of ICP-AES and XRF for the analysis of silicate rocks

Abstract The precision of an analytical method has been evaluated objectively by applying the method of Thompson and Howarth (1976) to the analysis in duplicate of 55 igneous rocks covering a range of silicate matrix types and analyte concentrations. Results were analysed 1.0 characterise the change in precision ( s c ) of the analytical method with concentration ( c ) according to the equation s c = s o + kc , where the k parameter represents the limiting high-level precision and s o , the precision at zero concentration, which is related to the method detection limit (MDL). Test materials were analysed using four analytical methods based on two analytical techniques, inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray fluorescence spectrometry (XRF), as operated under routine working conditions in the two participating laboratories. The two XRF methods were major elements on fused glass discs and trace elements on powder pellets, and the two ICP-AES methods were major elements after a fusion decomposition technique and trace elements together with selected major elements, after an acid attack. Statistical evaluation of the data showed that significant changes in precision as a function of concentration (i.e. the k factor) were determined in 34 cases out of 78 analyte-method combinations. In cases where no significant change in precision could be detected, a grand mean precision, representative of the concentration range analysed was calculated. The s o parameter was found to be significantly different from zero in 36 cases out of 72. To allow evaluation of the detection limit performance of all data, a maximum method detection limit (MMDL) was calculated, which was estimated to be on average 1.62 times greater than the MDL derived from significant values of s o In terms of the four methods studied, median high-level precision of the techniques used to determine major elements were found to be 0.23% relative (XRF/glass discs), 0.43% (ICP-AES/fusion decomposition) and 0.70% (ICP-AES/acid attack). Typical precision values in the determination of trace elements by both techniques was 1.5%, providing elemental concentrations extended over a significant range. MMDLs varied from element to element but for XRF/powder pellet data were found to be approximately equivalent to instrumental detection limits (IDLs) calculated from background count rates. However, for trace elements determined by ICP-AES/acid attack, MMDL were found to be on average three times larger than IDLs measured from repeated analysis of an aqueous blank. As a result of an evaluation of these data, it is proposed that appropriate figures of merit to describe the analytical performance of a technique are: (1) median precision in the determination of major elements; and (2) the number of trace elements that can be determined to MDLs of less than one-tenth the crustal abundance of the element. These factors should then be evaluated in conjunction with logistical factors including the rate at which samples can be analysed and the cost per determination. The influence of these factors on applications of the techniques studied in pure and applied geochemistry are discussed.

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.

X-ray fluorescence spectrometry

Abstract X-ray fluorescence analysis is a technique that is widely used for the determination of both major and trace elements in a wide variety of geological materials. In this review, the analytical characteristics of both wavelength and energy dispersive instrumentation are considered as well as operating and calibration procedures applicable to the analysis of both silicate rocks and exploration-type samples.

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.

Heat flow, heat production and thermo-tectonic setting in mainland UK

New heat flow data for the United Kingdom, together with additional heat flow and heat production determinations for Caledonian-age granites, have led to a revision of the UK heat flow map and a re-examination of the relationship between heat flow (q0) and heat production (A0) for granites and basement rocks. Previously recognized broad belts of above-average heat flow are now resolved into separate zones which reflect, to a greater extent, the geological structure and tectonic history of the UK. The zones of highest heat flow are spatially associated with voluminous, high heat production granitoid batholiths in SW England, northern England and the Eastern Highlands of Scotland. A single linear correlation between q0 and A0 is no longer tenable and an analysis in terms of broad heat flow provinces, each with a characteristic upper-crustal heat production distribution and deep heat flow contribution, is also considered to be an oversimplification. On the q0–A0 plot, the data form four separate clusters; three corresponding to the granite batholiths in SW England, northern England and the Eastern Highlands of Scotland, and the fourth to the basement rocks of central England and Wales. An explanation of the q0–A0 data is proposed in terms of the crustal structure and thermo-tectonic setting of each area. In the case of the granite batholiths the data reflect the contrasting depth extent and radioelement-depth functions of the intrusions. These parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced.

Energy-dispersive X-ray fluorescence analysis of silicate rocks: comparisons with wavelength-dispersive performance

The performance of routine energy-dispersive X-ray fluorescence (ED-XRF) analysis using direct tube excitation is compared with that of current wavelength-dispersive practice for the determination of both major and trace elements in silicate rocks. The influence of detector resolution is considered and a comparison is made between limits of determination, analysis times, instrumental precision and routine performance using data for typical wavelength-dispersive X-ray fluorescence (WD-XRF) techniques taken from the literature. It is shown that although ED-XRF limits of determination of the lighter major elements are inferior to those of WD-XRF data, analytical precisions at the levels normally encountered in silicate rocks are comparable. For the ED-XRF analysis of the heavier trace elements that are effectively excited by the silver X-ray tube used in this work (especially Rb, Sr, Y, Zr, Nb, Pb and Th) limits of determination, instrumental precision and routine analytical performance are equivalent to results obtained by WD-XRF. It is suggested that the over-all accuracy of either technique can be influenced significantly by uncertainties in the recommended values for international rock reference materials that are generally used to set up and/or test calibrations.

Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis

Abstract A fusion procedure commonly used for major element analysis of silicate rocks (1:5 sample to flux ratio, using 0.7 g of sample) was used to determine a small group of trace elements (Cu, Zn, Rb, Sr, Y, Zr, Nb) by X-ray fluorescence. Compton scatter peak ratioing was successfully used for matrix corrections, despite the thickness of the glass discs being only 1.7 mm. Precision and detection limits were compared with pressed pellet values. For most elements, routine analytical precision is better with powder pellets and detection limits are worse in glass discs, by an average factor of 2, considering all elements analyzed. Accuracy was evaluated with international reference materials and for most of the determinations, good or excellent agreement with recommended values was obtained. The main advantages of using glass discs for the trace element determination are the possibility of analyzing small amounts of sample and the suppression of any mineralogical effects in the glass discs, which can also be used to determine major elements.

Ritual in some Early Bronze Age gravegoods

This systematic study of selected ‘Wessex’ Early Bronze Age burial assemblages was undertaken to reassess the original purpose of the artefacts deposited and their significance in the burial ritual. Use-wear analysis and fragmentation assessment has been undertaken on daggers, whetstones, bone objects and beads. Lithological and physical analysis was also undertaken on the stone objects. The results show a remarkable disparity in use and fragmentation between certain artefact types. Some objects and groups might be seen as symbolic depositions placed by mourners, or as parts of ceremonial costume, rather than as possessions of the deceased.