Valerie Olive

A Re—Os isotope study of ultramafic xenoliths from the Matsoku kimberlite

Ultramafic xenoliths erupted within the Matsoku kimberlite pipe display a range of petrographic features that have been interpreted as the products of melt injection or infiltration and metasomatic processes. We have analysed Re and Os concentrations and the Os isotope composition of these samples with the aim of investigating how such processes affect the Re—Os system. In particular, we have focused on the relationships between: (1) veins, dykes and sheets, that are thought to record melt injection or infiltration processes; (2) the coarse peridotite wallrocks to the veins, dykes and sheets, which usually do not show modal metasomatism but have been affected by low-volume melt infiltration; and (3) common coarse peridoties which show little, if any, evidence of modal metasomatism or melt infiltration. Peridotite samples (i.e., groups 2 and 3 above) all display low 187Os/188Os and thus mimic the signature that is now well established for the sub-continental mantle lithosphere. We demonstrate a relationship between Os isotope composition and modal mineralogy that suggests a link between Re depletion and extraction of a basaltic component to establish the refractory compositions of lithospheric mantle samples. Model age calculations suggest a mean age of melt extraction older than 2200 Ma. The development of ilmenite-rutile-phlogopite-sulphide metasomatic assemblages does not appear to affect strongly Re/Os. By contrast, pyroxenite dykes have substantially higher 187Os/188Os which is broadly supported by elevated Re/Os. High Re/Os is consistent with the pyroxenites having formed by precipitation from melts intruded into the lithospheric mantle. Such melts, as is observed for the dykes, would also be expected to carry a low Sm/Nd signature which with time would develop negative eNd. Wallrock peridotites and other rocks affected only by low-volume melt migration also show low Sm/Nd but with accompanying low Re/Os, thereby demonstrating the potential for the generation of the low 143Nd/144Nd, low 187Os/188Os signature which appears to be characteristic of sub-continental lithospheric mantle.

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

Abstract 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.

Discrete Site Surface Complexation Constants for Lanthanide Adsorption to Bacteria As Determined by Experiments and Linear Free Energy Relationships

Bacteria are abundant in many natural and engineered environments where they are thought to exert important controls on the cycling, mobility, bioavailability, and toxicity of metal contaminants. In order to probe their role in moderating the behavior of lanthanides, pH-dependent adsorption edges of 13 individual lanthanides and yttrium to the Gram-negative bacterium Pantoea agglomerans were used to generate discrete site surface complexation constants. The calculated surface complexation constants were compared with stability constants estimated using linear free energy relationships based on a number of hydroxyl-containing ligands. The experimental data suggests that lanthanide adsorption edges below pH 6.5 are consistent with adsorption to phosphate groups for the light and some of the middle lanthanides (La to Gd), whereas some of the middle and heavy lanthanides appear to favor carboxyl co-ordination (Tb to Yb), although exceptions occur in each grouping. The experimentally derived surface complexation constants for carboxyl coordination were of similar magnitude to stability constants estimated from linear free energy correlations using fulvic acid stability constants. The implication is that the adsorption of lanthanides to bacterial surfaces could be modeled reasonably well using lanthanide stability constants for natural organic matter, except perhaps at low pH where phosphate binding dominates.