John C. Rucklidge

AMS of heavy ions with small accelerators

Abstract Recent advances in the detection and the routine measurements of heavy elements by accelerator mass spectrometry (AMS) are reviewed. Particular emphasis will be given to the measurement of low energy (⩽ 15 MeV) and high-Z ions using small (⩽ 3 MV) accelerators.

The role of chlorine in serpentinization

Chlorine present in altered ultramafic rocks resides in a compound which is readily soluble in neutral and acidic aqueous solution. Iron is also dissolved by the same solutions giving Fe∶Cl = 2∶1. Statistical treatment of multiple microprobe analyses shows a positive correlation between Cl and Fe, and negative correlations between Cl and Mg, and Cl and Si. These, and other data, suggest that Cl is present in the form of submicroscopic grains of a compound for which a likely formula is Fe2(OH)3Cl, and which is dispersed through the regions of the rock suffering active alteration. Elsewhere, where alteration is complete the chloride phase is absent.A mechanism is proposed whereby initially acid chlorine bearing solutions become alkaline in bringing about the conversion of olivine to serpentine and at the same time transfer Fe into Fe2(OH)3Cl which precipitates in the reaction zone. Fresher, acid, solutions, following as the reaction front progresses, dissolve the Fe2(OH)3Cl phase, providing a possibility for long distance transport of Fe and recycling of Cl. pH gradients near the reaction zone may be matched by fO2 gradients which may bring about the formation of magnetite in the centres of veins between the olivine crystals. All evidence of Cl will be removed when the process is complete, and the serpentine which forms will usually have Fe/Mg greater than the original olivine.

Accelerator mass-spectrometric measurements of heavy long-lived isotopes

Abstract The potential for a systematic analysis of 129 I within the hydrosphere is explored using accelerator mass spectrometry. Examples are taken from measurements on pre- and post-bomb samples of marine organisms such as algae, sponges and some forms of coral.

The dispersal of 129I from the Columbia River estuary

129I/I isotopic ratios have been measured in algae samples, collected along the north-west portion of the Pacific coastline of North America, as part of a preliminary study, to determine the extent and concentration of 129I leaving the Columbia River. These isotopic ratios were found to be in excess of the nuclear weapons testing level, however all were significantly lower than those previously measured from European nuclear fuel reprocessing facilities.

Accelerator mass spectrometry of 129I at isotrace

Abstract 129I has been measured by an accelerator mass spectrometry (AMS) method which is compatible with the further development of small tandem accelerators for the study of heavy elements. The addition of an electrostatic analyser to the injector system, is a primary factor facilitating the detection of 129I at levels as low as 5 × 10−13 of 127I. The removal of a significant portion of the unwanted ions by this analyser results in a simplified high energy ion analysis system, because the possibility of mass, energy and charge ambiguities are reduced. The relevance of this and other improvements to heavy ion analysis at IsoTrace is discussed using the detection of 129I as a prime example.

A tertiary lamprophyre dike with high pressure xenoliths and megacrysts from Wiedemanns Fjord, East Greenland

Lamprophyric dikes, mainly of camptonite/monchiquite affinities occur in the Wiedemanns Fjord area. One example contains a complex assemblage of olivine-orthopyroxene-chrome spinel nodules, megacrysts of kaersutite, diopside, strongly reverse-zoned green salite and various spinel phases. Microprobe analyses are presented for all these phases and for the lilac-coloured titansalites and strongly-coloured kaersutites of the groundmass. It is concluded that these minerals record evolution under various P, T and oxidation regimes during the formation of a lamprophyric parent magma. The nodules provide evidence for deep fractures in this area supposedly associated with early rifting in the North Atlantic.

The occurrence of chlorine in serpentine minerals

Partially serpentinized dunites containing small amounts of Chlorine (< 0.5%) from Dumont, Quebec, and Horoman, Hokkaido, Japan, and one containing less than 0.05% Chlorine from Higashi-Akaishi-Yama, Ehime, Japan have been examined using the electron probe microanalyzer and scanning transmission electron microscope with X-ray analytical capabilities. Chlorine was found together with Si, Mg, Ca and Fe in the serpentine minerals of the Dumont and Hokkaido dunites but not in the Ehime dunite. Chlorine is found associated only with the most finely crystalline facies of the serpentine (grain size less than 10 nm). The Ehime dunite contained no such fine grained serpentine, and was thus effectively chlorine-free, as are the coarser grained serpentines of the other samples. The finegrained chlorine-bearing serpentine also has a much higher concentration of Fe, and can contain smaller amounts of Ca, Ni and Mn than the coarse-grained variety as well as minute awaruite (FeNi3) grains. This fine-grained serpentine probably represents an early stage in the transformation of olivine to serpentine, with chlorine from hydrothermal solutions assisting the necessary chemical changes. The Cl increases the reaction rate by lowering the activation barrier to reaction by the introduction of reaction steps.

Charge ratio mass spectrometry of heavy elements

Abstract A simple method of stable element assay at the parts per billion level using a sequence of charge changes at MeV ion energies has been demonstrated. Isotopic ratios and abundances of platinum have been studied using this technique in nickel sulphide and copper samples. Silver isotopic distributions have also been investigated in a number of terrestrial and meteoritic materials. A similar charge changing sequence between electrostatic analyzers at keV ion energies was shown to reduce backgrounds for a mass independent search of fractionally charged particles. The methods described form the bases of a broad band mass spectroscopic system under development at IsoTrace.

129I in moss down-wind from the Sellafield nuclear fuel reprocessing plant

Abstract We have measured by AMS at IsoTrace 129 I/ 127 I ratios in mosses collected along a 60 km transect of the English Lake District down-wind from the nuclear fuel reprocessing plant at Sellafield, Cumbria. Values measured range from 6693(1188) × 10 −8 to 14.7(2.1) × 10 −8 .

In situ trace element determination by AMS

Abstract Because of its negligible background, AMS can be used for the direct measurement of trace element concentrations in materials at the lowest possible levels. The primary beam can be made small enough to be positioned within the boundaries of individual mineral grains in a polished rock section. Standards can be prepared which relate well to the complex natural matrices found in minerals, so that concentrations down to the ppb level can be measured. Enrichments and depletions of platinum group elements (PGE) and Au have been found in minerals such as millerite, arsenopyrite, pyrite, etc. Sensitivities vary widely because of the different ion yields for different elements. Sample surfaces should be electrically conducting to avoid charging and beam instability.