Analysis Of Platinum Group Elements Research Articles

Contamination-free preparation of geological samples for ultra-trace gold and platinum-group element analysis

In order to determine the most contaminant-free sample processing procedures for ultra-trace platinum-group element (PGE) and gold analysis, we have crushed and milled pure quartz in a variety of media and then performed analysis by inductively coupled plasma mass spectrometry (ICP-MS) using isotope dilution (Pt, Pd, Ru and Ir) and external calibration (Au and Rh). Initial sample desegregation was best achieved in a Mn steel jaw crusher which contributed <0.1 ppb Pt and Au, <0.05 ppb Pd and Ru, <0.01 ppb Rh and <0.006 ppb Ir. Final milling must be performed in a ceramic mill (or presumably agate, although this mill composition was not investigated). Up to 0.008 ppb Ir and 0.08 ppb Au can be contributed from the ceramic mill (α-alumina in our case), but other elements were at or below the limits of detection. Tungsten carbide appears to be a problem, both in terms of Au contribution (up to 0.19 ppb from the tungsten carbide press) and an isobaric interference of tungsten oxide on 198Pt. The latter problem can be avoided by choosing alternate Pt isotopes for spike and reference (e.g., 194Pt and 195Pt, respectively). Based on these results, we have determined that the optimum low-contaminant processing for whole rock analysis is initial crushing in the Mn steel jaw crusher followed by milling in α-alumina ceramic. This results in below detection limit blanks for Pt, Pd and Ru and a total contribution of up to 0.008 ppb Ir, 0.01 ppb Rh and up to 0.18 ppb Au. For mineral separates such as magnetite, sulphide or chromite, a combination of steel crushers and mills can be used, provided suitable acid washing is performed prior to sample dissolution and analysis.

Isotope dilution ICP–MS analysis of platinum in road dusts from west central Scotland

In response to the increased use of catalytic convertors in motor vehicle exhausts systems, recent studies, employing GF–AAS or ICP–MS analysis of platinum group elements (PGEs), have identified a significant increase of Pt and Pd concentrations in road dust and roadside soils leading to concern over potential effects on human health. After deposition, the PGEs are subject to various physical and chemical transformations, potentially resulting in migration into other environmental compartments i.e. soils, the aquatic system and biota. However, the processes involved are poorly understood and the reactivity, bioavailability, and speciation of the PGEs are ill defined. In contrast to environmental studies, in geochemical research, Isotope Dilution ICP–MS (ID ICP–MS) has primarily been used for the quantification of Pt, providing a significant improvement in accuracy by allowing removal from the analyte solution of elements which could potentially give isobaric interference in the mass spectra. In the present work, an ID ICP–MS analytical technique with a detection limit of 0.1 μg kg −1 has been used for determination of concentrations of Pt in road dust samples from west central Scotland. The study highlights potential interference problems that may occur in analysis of Pt in environmental matrices using ICP–MS. Initial application of ID ICP–MS in a study of roadside environments in west central Scotland has revealed significantly enhanced concentrations of Pt, within the range 13–335 μg kg −1, in road dust samples from a trunk road and motorway, representing a significant increase relative to surface soils in this area that are remote from roads, which have Pt concentrations of less than 1.0 μg kg −1. In contrast, lower levels of contamination of 1.8–11.8 μg kg −1 were observed for dust samples from residential areas, in the town of East Kilbride.

Analysis of platinum group elements and gold in geological materials using NiS fire assay and Te coprecipitation; the NiS dissolution step revisited

The NiS fire assay–Te coprecipitation separation procedure for inductively coupled plasma mass spectrometry (ICP-MS) analyses of platinum-group elements (PGE) in silicate rocks has been revisited with the aim of reducing volatile PGE losses (Os, Pd). The NiS bead was dissolved in 20.2% HCl, in an open system allowing H 2S to escape without loss of HCl. Yield, accuracy and reproducibility were tested by replicate analyses of a CANMET reference material (UMT1), a mantle lherzolite (FON B 93), prepared as in-house standard, and abyssal harzburgites, analysed as unknowns. The yields as estimated from UMT1 range from 97% (Ir) to 93–94% (Rh, Pt, Pd) and 91% (Au). The Os value (7.13±1.4 ppb) is equal within 1 sigma level, to the CANMET provisional value. The mean Os content of FON B 93 (3.42 ppb) fits the Os concentration inferred for the terrestrial mantle very well, as does the Os/Ir ratio (1.053 vs. 1.063) while Pd concentrations are increased by 18% compared to previous analyses after open-beaker dissolution steps. The abyssal harzburgites also yield consistent Os contents (3.28±0.19 ppb) and a perfectly chondritic Os/Ir ratio (1.07). Thus modified, the NiS fire-assay method allows Os to be nearly completely recovered, while greatly reducing the volatility of Pd. Moreover, the PGE analyses of coarse-grained rocks are highly reproducible (1% for Rh, Pt, Pd; 4% for Ir, 5% for Ru, 6% for Au), if performed from large-sized powder aliquots.

Determination of platinum group elements in impact breccias using neutron activation analysis and ultrasonic nebulization inductively coupled plasma mass spectrometry after anion exchange preconcentration

An anion exchange procedure has been optimized for the analysis of platinum group elements (PGEs) in geological samples at low abundances by adding two stages: chlorination and replicate sorption onto a small anion exchange column. Combined with neutron activation analysis (NAA) and ultrasonic nebulization inductively coupled plasma mass spectrometry (USN-ICP-MS) determination, this procedure is evaluated by replicate analyses of PGEs (Ru, Pd, Os, Ir, Pt), Au, Ag and Re in two reference materials, WMG-1 and WITS-1. The suitability of the method for actual geological samples was demonstrated by analyzing target rocks and breccias from the Bosumtwi impact crater, Ghana.

SULFUR ENRICHMENT VIA VOLATILE-RICH METASOMATISM IN MONTFERRIER XENOLITHS (SOUTHERN FRANCE): CONSEQUENCES FOR HIGHLY SIDEROPHILE ELEMENTS IN THE SUB-CONTINENTAL MANTLE

Sulfides are the main hosts for highly siderophile elements (HSE); therefore the mantle S content is a crucial datum for geochemical studies relevant to platinum-group element abundances or Re-Os systematics. The estimated S content of the primitive mantle is about 250±50 ppm. Although S contents of fertile peridotite from orogenic massifs support this estimate, peridotites hosted in alkali basalt commonly contain only a few tens of ppm S, whatever their fertility (Lorand, 1990). Some xenoliths from southern France, i.e. Montferrier volcano (Herault) have very high sulfide abundances (up to 0.2 wt.%) with intergranular sulfides predominating over enclosed sulfides (Lorand and Conquere, 1983). The process responsible for such high sulfide modal abundances and its possible effect on HSE abundances are not clear. To shed some light on this process we have carried out an extensive study of 21 xenoliths from this locality, involving major elements and lithophile trace element data, chalcogenide (S, Se) geochemistry, platinum-group element analyses and Re-Os systematics. Montferrier xenoliths are either breccia-hosted or lavahosted. The former have a yellowish color indicating weathering, while the lava-hosted have a fresh green color and do not show obvious alteration features. Some xenoliths display disseminated Ti-pargasite (up to 6%) regardless of their host. Montferrier peridotites are characterized by a strongly deformed porphyroclastic microstructure giving them a mylonitic appearance. It has been shown that this deformation took place at low temperature (900-750°C), probably during an uplift on the nearby Cevennes lithospheric shear zone (e.g., Fabries et al., 1987). They also display very fertile compositions. Most of the studied samples contain between 3.0 and 4.5% Al2O3 and the least fertile still contains 2.0 wt.% Al2O3. The S concentration range (7 - 592 ppm) is the highest reported for basalt-hosted peridotite xenoliths. S contents are uncorrelated with fertility indices (e.g., Al2O3%). Breccia- hosted xenoliths have the lowest S contents ( 10). Ba and Rb enrichments are determined by amphibole. The second end-member is strongly enriched in LREE (La/SmN > 1). This LREE enrichment, is accompanied by large ion lithophile elements (Sr, U, Th...), but without concomitant enrichment of the high field strength elements (Nb, Ta, Zr, Hf). In contrast to the depleted type, the U/Th ratios are chondritic. Such enriched patterns are commonly ascribed to mantle metasomatism by carbonated and/or volatile-rich small volume melts. The positive correlation of figure 1B indicates that large amounts of S have been added by this metasomatism. In thin sections, the xenoliths containing more than c.a. 200 ppm S show numerous large intergranular sulfide blebs (up to 700 µm in maximum dimension) much richer in Fe and Cu than those from the depleted samples. Contrary to S, Se contents increase only moderately from 17-48 ppb in depleted samples to 50- 63 ppb in enriched xenoliths. The most S-rich samples may show markedly higher than chondritic S/Se ratios (104) which are inconsistent with a simple addition of immiscible sulphides during metasomatism. This observation suggests that S was a dissolved species in the volatile-rich metasomatic fluid whose effects are indicated by the trace element studies. Apart from the most refractory lherzolite which is unusually poor in PGE (total PGE = 5.2 ppb), the 8 xenoliths analyzed for the PGEs show a narrow PGE concentration range (20-28.2 ppb) independant of their fertility indices and S contents. Ru/Ir and Rh/Ir ratios are remarkably constant and slightly higher than chondritic. Pd/Ir ratios (1.37-2.26) are suprachondritic, as reported for Pyrenean orogenic lherzolites (Pattou et al., 1996) and some abyssal peridotites (Snow and Schmidt, 1998, Luguet et al., this volume). Pd/Ir ratios are correlated with S contents, mostly because the enriched samples have low Ir and Ru contents. Whole-rock fractionated Pd/Ir ratios are reflected in the pattern of the sulphides (LA-ICP-MS data). Os/Ir varies sympathetically with Pd/Ir, increasing from nearly chondritic values (1.1) in the depleted sample to suprachondritic (1.99) in the most S-rich xenolith. 187Os/188Os shows a similar increase from DMM-like values (0.128) in the depleted samples to highly radiogenic values (0.175) in the most S-rich sample. Our study provides further evidence that metasomatic process(es) involving volatile-rich small melt fractions may significantly alter the HSE relative abundances and Os isotopic composition of the lithospheric mantle (cf Gutierez Narbonna et al., this volume). Such parallel variations of Os, Pd and S are consistent with the experimentally-demonstrated affinity of these elements for S-bearing vapor phases at magmatic temperatures.

A new EPMA technique for determination of rare earth elements with the use of automated peak-overlap and modelled background corrections

Analysis of rare earth elements (REEs) plays an important part in geology and geochemistry for solving fundamental problems of ore formation, e.g., formation of gold and platinum deposits, as well as in ecology, in searching for materials suitable for burial of highly active wastes, where REEs are used in investigations as actinide imitators. However, electron microprobe analysis (EPMA) of REEs faces a series of problems related to the complex character of their X-ray spectra. For solving these problems, we suggest the use of an improved version of the program developed by us for the analysis of platinum group elements (PGEs), in which a computational method of background determination and an automatic correction for interfering lines are employed before applying the standard ZAF correction procedure. The program enables one to solve the problem of background determination, to simplify significantly REE analyses, to reduce random errors, to improve the correctness and speed of microprobe determinations, and to lower the limit of detection (LOD) of the elements analysed. When analysing thermally stable REE-bearing minerals, the LOD for most REEs lies in the concentration range 200-600 ppm. These values are obtained at an accelerating voltage of 20 kV, with a probe current of 100 nA and a counting time of 50 s at the peaks of analytical lines. For elements with multiple interference of analytical lines, such as Gd, Tm, Eu and Lu, the LOD estimated from the standard formula determines virtually the concentration level that might be achieved only in the case of correct measurements of overlap coefficients of all interfering lines. The use of a computational method for background determinations allows the required analysis time to be reduced by nearly a factor of two. We present in this paper the values of overlap coefficients and assess the reproducibility of results of the analysis of monazite, xenotime, apatite, and zirconolite. The basic principles of our software can be used successfully with various types of microprobe analysers including modern instruments.

Low-blank digestion of geological samples for platinum-group element analysis using a modified Carius Tube design

We have developed a modified Carius Tube design that permits the low-blank digestion of geological samples prior to platinum-group element analysis. The new Carius Tubes incorporate a liner of high-purity quartz glass that retains the sample and acids during the digestion procedure. This dramatically reduces the comparatively high Pt blank associated with dissolutions in conventional Carius Tubes. Using the new Carius Tube design we are able to achieve total procedural blanks for the determination of the PGE in geological samples that are at the 1–15 pg/g level for Ru, Pd, Ir and Pt. This constitutes a reduction of blank values by a factor of ∼10–100 compared to standard NiS fire assay sample preparation techniques.

Study of interfering nuclear reactions in determination of platinum group elements by neutron activation analysis

Eighteen interfering nuclear reactions that likely occur in neutron activation analysis of platinum group elements are studied. Their degrees are calculated by the basic principle of activation analysis. The calculation results indicate that most of the nuclear interferences become significant only if the interfering elements exist in major amount and the elements of interest in trace amount. However, the second-order interference to platinum analysis from gold and the fission interference to analysis of ruthenium and palladium from uranium cannot be neglected even if the gold and uranium exist as trace constituents in samples.

Non-chondritic platinum-group element ratios in the Earth's mantle

THE Earth's upper mantle has an overabundance of highly siderophile elements (platinum-group elements, gold and rhenium), relative to what would be expected from equilibrium with the metallic core1–3. This excess is now widely believed to have been introduced as a "late veneer" by meteorite bombardment during early Earth history, but after separation of the core4. Here we report high-precision analyses of platinum-group elements (PGEs) from fertile upper-mantle Iherzolites, which show variations in relative abundances that exceed those in the chondritic meteorites that are thought to have furnished the late veneer5. In particular, the Pd/Ir ratio, at about 1.76, is significantly greater than that of known chondritic meteorites6. Our results, combined with previous indications of both non-chondritic and nearly chondritic PGE ratios in the few other fertile mantle Iherzolites that have been analysed2,3,7–11, strongly suggest that the mantle is heterogeneous in its PGE content on scales of ∼100 kilometres. If the observed PGE ratios can be reconciled with a plausible meteoritic source, the observed heterogeneity may reflect an original spatial variation in the late veneer; otherwise, an additional fractionation mechanism seems to be required.

Interference by neutron induced second order nuclear reaction in activation analysis of platinum group elements after a nickel sulphide fire assay preconcentration

In determining the trace platinum group elements and gold in rocks and ores by the neutron activation analysis after a nickel sulphide fire assay preconcentration, there are interferences due to nuclides produced from second order nuclear reactions. This paper presents the degree of interference calculated over the ranges of long irradiation times and of reactor neutron flux from 1·1013 to 1·1015 n·cm−2·s−1. According to the results of these calculations, every one of the second order interfering reactions on the PGE+Au, except the197Au(n, γ)198Au(n, γ)199Au reaction, can be neglected under the long irradiation time or high reactor neutron flux. Special attention is given to the interference from gold in the determining platinum.

Neutron Activation Analysis of Platinum Group Elements as Indicators of Extraterrestrial Materials

This paper reviews some modern applications of the platinum group elements, mainly Ir and Os, in identifying and studying extraterrestrial materials, e.g. study of impact events resulting in biological mass extinctions in the earth's evolution, identification of cosmic dust, tracing of parent body of meteoritic craters on the earth's surface, searching for extrasolar components in the solar system, estimation of fluxes of extraterrestrial matters, etc. Also various representative neutron activation analysis methods developed for these purposes are summarized.

Platinum-group minerals in chromite seams of the Stillwater Complex, Montana

Platinum-group minerals are present as inclusions in both disseminated and massive chromite but are modally more abundant in massive chromite, in the ultramafic zone of the Stillwater Complex. They are small, generally less than 20 mu in diameter and characterized by euhedral and subhedral habits. Their distribution is random (not crystallographically controlled) within the chromite host. The platinum-group mineral inclusions may occur with silicate and base metal sulfide inclusions in an unfractured chromite host. All identified inclusions in unfractured chromite are laurite, (Ru, Os, Ir)S 2 . The Ru/(Ru + Os + Ir)(= Ruranges from 0.92 to 0.83 with Ru Os substitution more extensive than Ru Ir. Both Pd and Rh abundances in laurite are less than 2 wt percent. No Pt was detected. At present, the chemical data do not indicate a correlation between stratigraphic height and the Ru ratio.Several Pt-, Pd-, and Rh-bearing interstitial minerals and a single unusual inclusion phase have been identified in the A chromite seam at West Fork and the B seam at Mountain View. The interstitial platinum-group minerals are sperrylite and isoferroplatinum, whereas the unusual inclusion is a polyphase intergrowth of Pt-Pd-Ni arsenides with chalcopyrite and pent-landite.Whole-rock platinum-group element analyses of the chromite seams range from a few ppb to nearly 16 ppm with Pd > Pt > Rh > Ru > Ir (Os was not analyzed). This is in opposition to the data for the inclusions which show high Ru, Ir, and Os, low Rh and Pd, and no detectable Pt.Thus, in the Stillwater magma chamber, Ru-, Ir-, and Os-bearing minerals appear to have precipitated early and have been included in chromite. In contrast, Pd, Pt, and Rh apparently remained in solution in the magma and are in phases that precipitated late and are interstitial to chromite.

The terminal Cretaceous iridium anomaly in the Red Deer Valley, Alberta, Canada

Neutron activation analysis following fire-assay preconcentration in a nickel sulphide button allows processing of relatively large samples (25 g) for precision analysis of platinum group elements. Using this method, an iridium anomaly with maximum concentration of 3.36 ± 0.04 ppb has been located at the Cretaceous–Tertiary boundary at the base of the Nevis coal seam in the Red Deer Valley of Alberta. The iridium anomaly coincides with the disappearance of diagnostic late Cretaceous palynomorphs, including all but one species of Aquilapollenites.

Platinum-group elements in rocks from the voikar-syninsky ophiolite complex, Polar Urals, U.S.S.R.

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials.