Recrystallisation rims in zircon (Valle d'Arbedo, Switzerland): an integrated cathodoluminescence, LA-ICP-MS, SHRIMP, and TEM study

Recrystallization rims are a common feature of zircon crystals that underwent metamorphism. We present a microstructural and microchemical study of partially recrystallized zircon grains collected in polymetamorphic migmatites (Valle d'Arbedo, Ticino, Switzerland). The rims are bright in cathodo-luminescence (CL), with sharp and convex contacts characterized by inward-penetrating embayments transgressing igneous zircon cores. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data and transmission electron microscopy (TEM) imaging indicate that the rims are chemically and microstructurally different from the cores. The rims are strongly depleted in REE, with concentrations up to two orders of magnitude lower than in the cores, indicating a significant loss of REE during zircon recrystallization. Enrichment in non-formula elements, such as Ca, has not been observed in the rims. The microstructure of zircon cores shows a dappled intensity at and below the 100 nm scale, possibly due to radiation damage. Other defects such as pores and dislocations are absent in the core except at healed cracks. Zircon rims are mostly dapple-free, but contain nanoscale pores and strain centers, interpreted as fluid inclusions and chemical residues, respectively. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages show that the recrystallization of the rims took place >200 Ma ago when the parent igneous zircon was not metamict. The chemical composition and the low-Ti content of the rims indicate that they form at sub-solidus temperatures (550-650 degrees C). Recrystallization rims in Valle d'Arbedo zircon are interpreted as the result of the migration of chemical reaction fronts in which fluid triggered in situ and contemporaneous interface-coupled dissolution-reprecipitation mechanisms. This study indicates that strong lattice strain resulting from the incorporation of a large amount of impurities and structural defects is not a necessary condition for zircon to recrystallize. Our observations suggest that the early formation of recrystallization rims played a major role in preserving zircon from the more recent Alpine metamorphic overprint.

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): Geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 +/- A 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 +/- A 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism.

The data treatment dependent variability of U-Pb zircon ages obtained using mono-collector, sector field, laser ablation ICP-MS

U-Pb dating of zircons by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is a widely used analytical technique in Earth Sciences. For U-Pb ages below 1 billion years (1 Ga), Pb-206/U-238 dates are usually used, showing the least bias by external parameters such as the presence of initial lead and its isotopic composition in the analysed mineral. Precision and accuracy of the Pb/U ratio are thus of highest importance in LA-ICPMS geochronology. We consider the evaluation of the statistical distribution of the sweep intensities based on goodness-of-fit tests in order to find a model probability distribution fitting the data to apply an appropriate formulation for the standard deviation. We then discuss three main methods to calculate the Pb/U intensity ratio and its uncertainty in the LA-ICPMS: (1) ratio-of-the-mean intensities method, (2) mean-of-the-intensity-ratios method and (3) intercept method. These methods apply different functions to the same raw intensity vs. time data to calculate the mean Pb/U intensity ratio. Thus, the calculated intensity ratio and its uncertainty depend on the method applied. We demonstrate that the accuracy and, conditionally, the precision of the ratio-of-the-mean intensities method are invariant to the intensity fluctuations and averaging related to the dwell time selection and off-line data transformation (averaging of several sweeps); we present a statistical approach how to calculate the uncertainty of this method for transient signals. We also show that the accuracy of methods (2) and (3) is influenced by the intensity fluctuations and averaging, and the extent of this influence can amount to tens of percentage points; we show that the uncertainty of these methods also depends on how the signal is averaged. Each of the above methods imposes requirements to the instrumentation. The ratio-of-the-mean intensities method is sufficiently accurate provided the laser induced fractionation between the beginning and the end of the signal is kept low and linear. We show, based on a comprehensive series of analyses with different ablation pit sizes, energy densities and repetition rates for a 193 nm ns-ablation system that such a fractionation behaviour requires using a low ablation speed (low energy density and low repetition rate). Overall, we conclude that the ratio-of-the-mean intensities method combined with low sampling rates is the most mathematically accurate among the existing data treatment methods for U-Pb zircon dating by sensitive sector field ICPMS.

Determination of 241Pu in nuclear waste slurries: a comparative study using LSC and ICP-MS.

(241)Pu was determined in slurry samples from a nuclear reactor decommissioning project at the Paul Scherrer Institute (Switzerland). To validate the results, the (241)Pu activities of five samples were determined by LSC (TriCarb and Quantulus) and ICP-MS, with each instrument at a different laboratory. In lack of certified reference materials for (241)Pu, the methods were further validated using the (241)Pu information values of two reference sediments (IAEA-300 and IAEA-384). Excellent agreement with the results was found between LSC and ICP-MS in the nuclear waste slurries and the reference sediments.

Insights into shallow magmatic processes in large silicic magma bodies: the trace element record in the Fish Canyon magma body, Colorado

Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y (< 7 ppm) and HREE (&SIM; 1 ppm Yb), hence high LREE/HREE (Ce/Yb > 40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing four- to six-fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass &SIM; 30% of those in whole rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (&SIM; 4%) + titanite (&SIM; 0.5-1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. Sr/Y in Fish Canyon glass (13-18) is lower than the typical ``adakitic'' value (> 40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated.

Characteristics and origin of agates in sedimentary rocks from the Dryhead area, Montana, USA

Agates from the Bighorn district in Montana (USA), the so-called Dryhead area, and their adjacent host rocks have been examined in the present study. Analyses by XRD, polarizing microscopy, LA-ICP-MS, cathodoluminescence (CL), SEM and of oxygen isotopes were performed to obtain information surrounding the genesis of this agate type. Investigations of the agate microstructure by polarizing microscopy and CL showed that chalcedony layers and macrocrystalline quartz crystals may have formed by crystallization from the same silica source by a process of self-organization. High defect densities and internal structures (e. g. sector zoning) of quartz indicate that crystallization went rapidly under non-equilibrium conditions. Most trace-element contents in macrocrystalline quartz are less than in chalcedony due to a process of `self-purification', which also caused the formation of Fe oxide inclusions and spherules. Although the agates formed in sedimentary host rocks, analytical data indicate participation of hydrothermal fluids during agate formation. Trace elements (REE distribution patterns, U contents up to 70 ppm) and CL features of agate (transient blue CL), as well as associated minerals (fluorite, REE carbonates) point to the influence of hydrothermal processes on the genesis of the Dryhead agates. However, formation temperatures <120 degrees C were calculated from O-isotope compositions between 28.9 parts per thousand (quartz) and 32.2 parts per thousand (chalcedony).

Retention half times in the skeleton of plutonium and 90Sr from above-ground nuclear tests: a retrospective study of the Swiss population.

Plutonium and (90)Sr are considered to be among the most radiotoxic nuclides produced by the nuclear fission process. In spite of numerous studies on mammals and humans there is still no general agreement on the retention half time of both radionuclides in the skeleton in the general population. Here we determined plutonium and (90)Sr in human vertebrae in individuals deceased between 1960 and 2004 in Switzerland. Plutonium was measured by sensitive SF-ICP-MS techniques and (90)Sr by radiometric methods. We compared our results to the ones obtained for other environmental compartments to reveal the retention half time of NBT fallout (239)Pu and (90)Sr in trabecular bones of the Swiss population. Results show that plutonium has a retention half time of 40+/-14 years. In contrast (90)Sr has a shorter retention half time of 13.5+/-1.0 years. Moreover (90)Sr retention half time in vertebrae is shown to be linked to the retention half time in food and other environmental compartments. These findings demonstrate that the renewal of the vertebrae through calcium homeostatic control is faster for (90)Sr excretion than for plutonium excretion. The precise determination of the retention half time of plutonium in the skeleton will improve the biokinetic model of plutonium metabolism in humans.

Uranium-enrichment in soils and plants in the vicinity of a pitchblende vein at La Creusaz-Les Marecottes (W of Martigny, Valais, Switzerland)

In the NE part of the Aiguilles Rouges Massif near Martigny, at the eastern contact of the Variscan Vallorcine granite to adjacent gneisses, a series of pitchblende (UO2)-veins occur. This paper determines the level of enrichment and mobility of uranium in soils situated in the vicinity of such a UO2-vein 7 km west of Martigny. Within an area of 50 x 100 m, situated on a relatively steep slope and characterized by a strong gramma-ray anomaly, six soil profiles including their plant cover and a reference soil profile outside the influence of the UO2-vein have been examined. The soil shows pH-values between 4 and 5 and is colluvial. The applied analytical methods for the metal contents include extraction methods, common for soil studies, and bulk analysis performed with X-ray fluorescence and ICP-MS. Uranium contents found in the uppermost 20 cm of the soil profiles vary from 2,500 ppm close to the vein to 15 ppm at the lowermost point of the study area. The reference soil has around 3 ppm uranium. At greater depth (20 to 40 cm) the U-content decreases to about half of the surface values, indicating a vertical transport of uranium within the soil profile. No systematic dependance of uranium-contents to grain size (amount of clay) nor to the amount of organic matter has been found. However, the good correlation between uranium and free iron oxide concentration suggests adsorption of uranium on iron oxy-hydroxides. The ashes of grass and mosses contain up to 90 ppm U, the blueberry and redwood only up to 3 ppm. Our observations suggest that at the surface the uranium is transported by downhill creep (solifluxion) of uranium-rich rock fragments. Liberated by oxidation of the uppermost fragments in a given soil column, the uranium migrates vertically until the conditions are favourable to adsorption onto Fe-oxy-hydroxides. However, as high U-contents of local surface water show, this adsorption does not lead to a significant retention of the uranium.

Further characterisation of the 91500 zircon crystal

This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA-ICP-MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses.

Geochemistry and 40Ar/39Ar geochronology of pseudotachylyte associated with UHP whiteschists from the Dora Maira massif, Italy

In situ UV-Iaser ablation Ar-40/(39) Ar geochronological and geochemical data, together with rock and mineral compositional data, have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the UHP whiteschists of the Dora Maira Massif, Italy. Several generations of fresh pseudotachylyte occur as irregular veins up to a few cur thick both parallel and at high angles to the foliation. Whole rock XRF data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith. Minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss. The pseudotachylyte contains newly crystallized grains of biotite and K-feldspar in a matrix of glass with partially fused grains of quartz, zircon, apatite, and titanite. Electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer-scale changes in mineralogy. UV-Iaser ablation ICP-MS traverses across the mylonitic gneiss-pseudotachylyte contact are consistent with cataclastic communition of REE carriers such as epidote, monazite, allanite, zircon, and apatite before melting as an efficient mechanism of REE homogenization in the pseudotachylyte. The 40Ar/39Ar data from one band of pseudotachylyte indicate formation at 20.1 +/- 0.5 Ma, when the mylonitic gneisses were already in a near surface position. The variable effects of top-to-the-west shear deformation within outcrops of the coesite-bearing unit are reflected in localized zones of protomylonite, cataclasite, ultracataclasite, and pseudotachylyte. Preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the UHP rocks. It is speculated that deeper crustal seismic events potentially played a role in the unroofing of the UHP rocks at earlier stages in their exhumation history. (c) 2005 Elsevier B.V. All rights reserved.

Retention half times in the skeleton of plutonium and Sr-90 from above-ground nuclear tests: A retrospective study of the Swiss population

Plutonium and Sr-90 are considered to be among the most radiotoxic nuclides produced by the nuclear fission process. In spite of numerous studies on mammals and humans there is still no general agreement on the retention half time of both radionuclides in the skeleton in the general population. Here we determined plutonium and Sr-90 in human vertebrae in individuals deceased between 1960 and 2004 in Switzerland. Plutonium was measured by sensitive SF-ICP-MS techniques and Sr-90 by radiometric methods. We compared our results to the ones obtained for other environmental compartments to reveal the retention half time of NBT fallout Pu-239 and Sr-90 in trabecular bones of the Swiss population. Results show that plutonium has a retention half time of 40 +/- 14 years. In contrast Sr-90 has a shorter retention half time of 13.5 +/- 1.0 years. Moreover Sr-90 retention half time in vertebrae is shown to be linked to the retention half time in food and other environmental compartments. These findings demonstrate that the renewal of the vertebrae through calcium homeostatic control is faster for Sr-90 excretion than for plutonium excretion. The precise determination of the retention half time of plutonium in the skeleton will improve the biokinetic model of plutonium metabolism in humans. (C) 2010 Elsevier Ltd. All rights reserved.

Structure and kinematics of the Siviez-Mischabel nappe in the Mattertal (western Alps of Switzerland) = Structure et cinématique de la nappe de Siviez-Mischabel le long de la vallée de Zermatt (Alpes de Suisse occidentale)

Résumé Cette étude porte sur le flanc inverse de la nappe de Siviez-Mischabel et sur les unités tectoniques sous jacentes (zone de Stalden supérieur et zone Houillère) dans la vallée menant à Zermatt. L'étude structurale du granite permien de Randa (orthogneiss oeillé) permet de mieux comprendre les effets de la déformation alpine sur les roches de socle. La cartographie détaillée de l'orthogneiss et de son encaissant, ainsi que l'étude lithostratigraphique des terrains sédimentaires associés permettent de proposer un schéma structural et cinématique du flanc inverse de la nappe de Siviez-Mischabel et de mieux comprendre ses relations avec les unités tectoniques sous-jacentes. L'analyse structurale de l'orthogneiss de Randa et de son encaissant révèle la superposition de plusieurs phases de déformation ductile. Cet orthogneiss formé sous des conditions métamorphiques du faciès schiste vert possède une forte schistosité alpine avec au moins deux linéations d'extension. La première, L1, orientée NW-SE est associée à la mise en place de la nappe. La seconde, L2, orientée SW-NE, se corrèle au cisaillement ductile du Simplon. La quantification de la déformation au moyen de la méthode de Fry sur les faciès porphyriques donne des ellipses à rapports axiaux compris entre 1.9 et 5.3, en accord avec les valeurs obtenues par d'autres marqueurs {tourmalines étirées, fibres). Les valeurs mesurées parallèlement à L1 ou L2 sont très semblables. La méthode de Fry a nécessité une étude théorique préalable afin de vérifier son applicabilité aux orthogneiss oeillés. La méthode requiert une distribution spatiale homogène et isotrope des marqueurs utilisés. Les tests statistiques effectués ont révélé que les phénocristaux de feldspath alcalin satisfont à cette condition et qu'ils peuvent être utilisés comme marqueur de la déformation au moyen de la méthode de Fry. Les valeurs obtenues révèlent l'importance du cisaillement ductile du Simplon sur la géométrie de la nappe dans la région d'étude. Le levé cartographique a permis d'améliorer la lithostratigraphie de la base de la nappe de Siviez-Mischabel. Trois formations en position renversée peuvent être observées sous les gneiss formant le coeur de la nappe. Ces trois formations forment le coeur du synclinal de St-Niklaus qui connecte la nappe de Siviez-Mischabel à la zone de Stalden supérieur. La datation par U-Pb de zircons détritiques et magmatiques par LA-ICP-MS permet de contraindre l'âge des formations observées (probablement Carbonifère à Trias précoce). Ces données ont des répercussions importantes sur la structure de la nappe dans la région, prouvant l'existence de plusieurs plis avec des séries normales et renversées bien préservées. La définition et la datation de ces formations, ainsi que leur identification dans la-Zone- Houillère avoisinante permettent de mieux comprendre la géométrie initiale et les relations tectoniques des nappes du Pennique moyen dans la vallée de Zermatt. Summary This study investigates the overturned limb of the Siviez-Mischabel nappe and underlying tectonic units (Upper Stalden zone and Houillère zone) in the Mattertal area. Detailed structural analysis in the Permian Randa granite (augen orthogneiss) allows a better understanding of the Alpine deformation effects on basement rocks. Detailed mapping of this orthogneiss and surrounding rocks, and the study of the lithostratigraphy in the related sedimentary horizons allow the proposition of a structural and kinematic model for the overturned limb of the Siviez-Mischabel and to better understand the relations with the underlying tectonic units. The structural analysis of the Randa orthogneiss and surrounding rocks revealed the superposition of several phases of ductile deformation. This orthogneiss formed under greenschist facies metamorphic conditions displays a strong Alpine foliation with at least two stretching lineations. The first lineation, L1, is oriented NW-SE and is related to the nappe emplacement northward. The second one, L2, is related to the Simplon ductile shear zone. Strain estimation using the Fry method has been performed on porphyritic facies of the Randa orthogneiss. The obtained ellipses have axial ratios varying between 1.9 and 5.3, in agreement with strain estimation obtained from other markers (stretched turmalines, fringes). The strain values are very similar if measured parallel to L1 or to L2. A theoretical approach was necessary to verify the relevant application of the Fry method to augen orthogneiss. This method requires that the distribution of the used markers has to be homogeneous and isotropic. Statistical tests have been done and revealed that K-feldspar phenocrysts satisfy these conditions and can be used as strain markers with the Fry method. The obtained strain measurements revealed the importance of the Simplon ductile shear zone on the geometry of the nappe in the studied area. Mapping has improved the lithostratigraphy at the base of the Siviez-Mischabel nappe. Three overturned formations can be observed below the gneisses forming the core of the nappe. These three formations form the St-Niklaus syncline, which connects the Siviez-Mischabel nappe to the underlying Upper Stalden zone. U-Pb dating of detrital and magmatic zircons by LA-ICPMS allowed the age of the observed formations to be constrained (presumably Carboniferous to Early Triassic). This data has critical implications for nappe structure in the region, composed of few recumbent folds with well preserved normal and overturned limbs. The definition and dating of these formations, as well as their identification in the adjacent "Houillère Zone" improve the understanding of the geometry and tectonic relations of the Middle Penninic nappes in the Mattertal.