Simon Chenery

Contrasted monazite hydrothermal alteration mechanisms and their geochemical implications

In spite of the major importance of monazite as a repository for the rare earths and Th in the continental crust, for U-Th-Pb geochronology, and as a possible form for high-level nuclear waste, very little work has been carried out so far on the behaviour of this mineral during fluid-rock events. This contribution describes two contrasting examples of the hydrothermal alteration of monazite. The first case comes from a sample of the Carnmenellis granite (Cornwall, Southwest England), chloritized at 284 ± 16°C, whereas the other occurs in the Skiddaw granite (Lake District, Northwest England), which underwent greisenization at 200 ± 30°C. An integrated study involving backscattered scanning electron microscopy, electron microprobe analyses, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) reveals that the chloritization event was characterized by the coupled substitution 2REE3+ ⇌ Th4+ + Ca2+ in the altered parts of the monazite, thus leaving the P-O framework of the crystal untouched. In contrast, greisenization led to the coupled substitution REE3+ + P5+ ⇌ Th4+ + Si4+, and therefore involved a partial destruction of the phosphate framework. The resulting rare earth element patterns are quite different for these two examples, with a maximum depletion for Dy and Er in the altered parts of the Carnmenellis monazite, whereas the Skiddaw monazite shows a light rare earth depletion but an Yb and Er enrichment during alteration. This latter enrichment, accompanied by an increase in U but roughly unchanged Pb concentrations, probably resulted from a decrease in the size of the 9-coordinated site in monazite, thereby favouring the smaller rare earths. These contrasted styles of monazite alteration show that the conditions of fluid-rock interaction will not only affect the aqueous geochemistry of the lanthanides, actinides and lead, and the relative stability of the different minerals holding these elements. Variations in these conditions will also lead to various possible chemical exchanges between the crystalline phases and the hydrothermal fluids. The occurrence of common lead along penetrative cracks in the Carnmenellis monazite shows that only a leaching, prior to the U-Pb analyses of the whole-grain, will permit an accurate determination of the magmatic crystallization age. In contrast, for the Skiddaw case it may be possible to date the fluid-rock event by in situ 207Pb/206Pb geochronology. The observation that the altered parts of both monazite examples display Nd leaching and no significant Sm/Nd fractionation indicates that they should not affect the host whole-rock Nd isotopic signatures. Finally, it appears that monazite-like ceramics designed for the containment of high-level nuclear wastes will retain Th and the geochemically equivalent transuranic elements during fluid-rock events similar to those documented in this study but may release Nd, U and the corresponding radionuclides to the environment.

Sources and uptake of trace metals in otoliths of juvenile barramundi (Lates calcarifer)

Abstract We conducted an experiment to determine if the concentrations of three trace metals (Cu, Sr, and Pb) in the otoliths of juvenile barramundi ( Lates calcarifer ) were related to their concentrations in the water or diet. Fish were kept in freshwater aquaria for 22 days and either fed diet enhanced with 10 μg g −1 Cu, Sr or Pb, or had 10 mg l −1 (Sr and Pb) or 0.5 mg l −1 Cu added to the water. A control group were then transferred to seawater and maintained for a further 83 days. The otoliths were initially ablated for five trace metals (Li, Cu, Sr, Ba and Pb) with LA-ICPMS. Ablation intervals corresponded to between 7 and 11 days of growth. There was no detectable increase in the otolith concentration of any metal for the fish in the enhanced-diet treatments. There was significant uptake of the three metals added to the water (Cu, Sr and Pb). Three (Sr and Pb) or six fish (Cu) were reanalysed with the ICPMS in high-sensitivity mode enabling 5 μm wide ablations to be made at intervals equal to every day or second day during the experiment. These analyses showed that there was considerable variation in uptake rate among individual fish and that the concentration fluctuated widely before an equilibrium concentration was reached after about two weeks exposure. Analysis of the otoliths of the control group of fish that were transferred to seawater showed that the concentration of Li, Sr and Ba in the otoliths varied between the periods of fresh and seawater residency. These variations were related to changes in the concentration of these metals in the water. The fluctuations in Sr concentration were similar to those seen in wild diadromous fish and not as great as those measured in the fish kept in Sr-enhanced water. Sr concentration in the otoliths was strongly negatively correlated with Ba concentration in the experimental fish, but not in wild fish that migrate from the sea into freshwater. Sr/Ca, Ba/Ca and Cu/Ca ratios in the otoliths were positively correlated with water ratios. The partition coefficients of Sr and Ba were similar to published values for marine fish. The concentrations of Li and Pb in the water were below the detection limit in some treatments so no comparisons could be made for these metals. When taken in conjunction with the results of other recent studies of trace metal uptake in otoliths, our data indicate that under experimental conditions, otolith concentrations of several trace metals are related to water concentrations. However, these patterns do not simply extrapolate to the natural environment where the mechanisms involved appear to be more complex.

Electron microprobe and LA-ICP-MS study of monazite hydrothermal alteration:: Implications for U-Th-Pb geochronology and nuclear ceramics

This paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration, namely sericitization, chloritization, and greisenization. The samples were taken from three Paleozoic granites: the St. Nectaire granite (Massif Central, France), the Carnmenellis granite (Cornwall, England), and the Skiddaw granite (Lake District, England). Fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340°C and salinities from 3 to 18 wt% NaCl equivalent. Possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy (BSE-SEM), electron microprobe, laser Raman spectroscopy and laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) include cationic substitutions, monoclinic to hexagonal structure transition accompanied by chemical exchanges, selective Th removal, dissolution-reprecipitation, and dissolution with replacement by a different mineral. These results show that, despite the compositional and crystallographic simplicity of monazite, it exhibits varied chemical responses to different mineral-fluid interactions. Elemental and isotopic measurements by LA-ICP-MS for the unaltered monazites yield statistically significant 232Th-208Pb and 238U-206Pb magmatic crystallization ages. In some cases it was possible to give a reasonable estimate of the age of mineral-fluid interaction using the altered parts of the monazites or newly precipitated crystals. For other minerals, 232Th-208Pb and 238U-206Pb systematics were strongly disturbed by variable inputs and/or depletions of U, Th, and Pb. The data suggest that monazite-like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies, especially if the fluids are oxidizing or tend to dissolve monazite.

Determination of rare earth elements in single mineral grains by laser ablation microprobe–inductively coupled plasma mass spectrometry—preliminary study

The application of a laser ablation microprobe with high spatial resolution coupled with an inductively coupled plasma mass spectrometer to determine rare earth elements (REE) in single mineral grains is described. The REE were determined down to sub-ppm concentrations using a 40 µm diameter ablation crater; quantification at the 100 ppm level using a 4 µm diameter crater was demonstrated. A dual gas flow sample introduction system was used to permit calibration of the ICP mass spectrometer with aqueous standards. Repeatability of the order of ± 10% relative was achieved with the use of an internal standard when ablating a grain of clinopyroxene. Laser ablation craters with diameters of less than 5 µm were used to probe REE distributions within a grain of monazite. Comparison with data obtained by electron probe microanalysis (EPMA) showed that the detection limits using high resolution laser ablation ICP-MS were superior to those of EPMA for this application.

Laser ablation ICP-MS elemental analysis of individual fluid inclusions: An evaluation study

Details are given of the elemental analysis of single fluid inclusions using a UV laser ablation microprobe interfaced to an inductively coupled plasma mass spectrometer. The UV laser, a frequency quadrupled Nd:YAG operating at 266 run, allows higher spatial resolution (<2 μm) than can be achieved using near-IR or visible wavelengths. Tests have been carried out on 10–100 μm diameter aqueous (liquid + vapour) inclusions in fluorite, quartz, and halite up to 60 μm beneath the surface. A key feature of the system is a novel high temperature ablation cell which substantially improves the efficiency and reproducibility of fluid release. Calibration was carried out using a dual gas flow system that allowed use of standard solutions and NIST glasses for tuning the instrument and for obtaining relative sensitivity factors. As an alternative to synthetic fluid inclusions, a new calibration approach is described involving the encapsulation of microdroplets of standard solutions in hydrophobic epoxy resins fluid inclusion analogues. To illustrate the scope and performance of the instrument, data are reported for Ba, Ca, Cs, Cu, K, Mg, Mn, Na, Pb, Rb, Sr, and Zn in saline aqueous inclusions associated with evaporite and low temperature base metal deposits. Element detection limits vary according to the mass of material released for analysis and are thus related to the volume and composition of each inclusion. Precision is estimated to be better than 30%.

Calibration studies in laser ablation microprobe, inductively coupled plasma atomic emission spectrometry

The use of tandem techniques to separate the processes of mobilisation and excitation in atomic spectrometry is increasingly popular, both to aid the introduction of difficult test materials and to minimise the matrix effects associated with mobilisation. The use of the laser ablation microprobe for the introduction of material into the plasma for analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES) has been demonstrated by a number of groups using solid standard calibration. However, the problems associated with the use of solid standards, including matrix effects, test material homogeneity and cost of preparation, are well known. This paper describes a study to determine whether matrix effects could be found by comparing element-relative sensitivities of test materials mobilised by laser ablation and aqueous nebulisation. On finding matrix effects, an attempt was made to characterise them as either associated with the laser ablation process or as spectrochemical effects in the ICP atomic emission spectrometer. A majority of the matrix effects are ascribed to the ablation process and it has been shown that a set of empirical matrix coefficients could be produced and used in conjunction with aqueous calibration.

Nature of particulate matter produced by laser ablation—implications for tandem analytical systems

Laser ablation processes and the products thereby formed have been investigated by scanning electron microscopy and energy dispersive X-ray analysis (EDX). The structures examined consisted of ablation craters and the material mobilised into a gas stream by ablation and collected by filtration. Ablation events have also been studied by high-speed cine photography. Three types of particulate material have been identified in the ejecta. The major component from metals was spherical droplets of solidified melt-splash. With brittle materials, sharp-edged spall-breccia was also found. In both instances a minor amount of amorphous condensate from the vapour phase was present. The results of chemical analysis by EDX suggest that the ablated product might not necessarily be representative of the test material bulk. These observations account for many of the difficulties of laser ablation in remote excitation techniques and suggest methods of alleviating them.

Calibration strategies for the elemental analysis of individual aqueous fluid inclusions by laser ablation inductively coupled plasma mass spectrometry

Using a combination of synthetic fluid inclusions in halite, microvolume aqueous solutions and National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 611 Glass, calibration graphs were established for the determination of elemental ratios in natural fluid inclusions by laser ablation–inductively coupled plasma mass spectrometry (ICP-MS). For simultaneous multi-element analysis, optimization studies demonstrate the necessity to adopt a compromise set of operating conditions since ICP-MS sensitivity (signal:background) may differ from element to element as a function of argon flow, radiofrequency power and spray chamber temperature. Synthetic fluid inclusions were prepared by crystallization from saturated sodium chloride solutions containing up to 13 major and minor cations. The microvolume calibration standards, ‘microwells’, consisted of small holes (3 × 3 × 2 mm3) drilled into plastic sheet, filled with a standard solution and hermetically sealed. In order to allow direct comparison between the different test materials, all the elements (Li, Na, Mg, K, Ca, Mn, Cu, Zn, Rb, Cs, Ba, Pb, B, Cl, Br) were ratioed to strontium. The relative standard deviations for the element ratios were generally better than 25%, indicating that the nature of the sample (salt, glass and aqueous solution) does not markedly affect the consistency of ablation or the efficiency of transfer between the ablation chamber and ICP torch. Element ratios for the synthetic fluid inclusions were linear over several orders of magnitude and in close agreement with those for the NIST SRM 611 Glass and microwell solutions, irrespective of inclusion size (20–100 µm) and depth in the sample (up to 80 µm). Statistical t-tests on the mean element ratios confirm that microwells and glasses constitute suitable alternatives to synthetic fluid inclusions for the calibration and routine analysis of natural fluid inclusions.

How cold were the Early Permian glacial tropics? Testing sea-surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Abstract: Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater palaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting δ18O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The δ18O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 °C (for δ18Oseawater = 0‰ V-SMOW) or from +30.3 to +35.4 °C (for δ18Oseawater = +1.0‰ V-SMOW) for the early Asselian. Results for δ18O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 °C (for δ18Oseawater = 0‰) or +29.2 to +32.8 °C (for δ18Oseawater = +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 °C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.

Storage and behavior of As, Sb, Pb, and Cu in ombrotrophic peat bogs under contrasting water table conditions.

Concentration depth profiles and inventories of solid-phase As, Sb, Pb, and Cu were determined in ²¹⁰Pb-dated cores from an ombrotrophic peat bog in northwest England. Cores were collected from the peat dome and adjacent to an eroding gully. Down-core distributions of As, Sb, Pb, and Cu in the dome core are almost identical. The water table is close to the dome surface with only short-term draw-down. Under these conditions, As, Sb, Pb, and Cu are immobile, allowing the reconstruction of trends in historical contaminant deposition. The peak in atmospheric deposition of As, Sb, Pb, and Cu (4.59, 2.78, 147, and 26.7 mg m⁻² y⁻¹, respectively) occurred during the late 19th century. Stable Pb isotope ratios reveal that Pb deposition during this period was from indigenous and foreign sources. The mean water table is much lower at the gully edge, and there are pronounced interannual fluctuations. These conditions have not affected the integrity of the Pb and Cu records but have caused postdepositional mobilization and redistribution of As and Sb. Cumulative inventories show significant loss of As and Sb at the gully edge site. Long-term water table draw-down in ombrotrophic peat bogs has the potential to alter the geochemistry and fate of previously deposited As and Sb.