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Genève, Switzerland

Gnos E.,Natural History Museum Geneva | Janots E.,ISTerre | Berger A.,Institute For Geologie | Whitehouse M.,Swedish Museum of Natural History | And 3 more authors.
Swiss Journal of Geosciences | Year: 2015

Monazite-bearing Alpine clefts located in the Sonnblick region of the eastern Tauern Window, Austria, are oriented perpendicular to the foliation and lineation. Ion probe (SIMS) Th–Pb and U–Pb dating of four cleft monazites yields crystallization ages of different growth domains and aggregate regions ranging from 18.99 ± 0.51 to 15.00 ± 0.51 Ma. The crystallization ages obtained are overlapping or slightly younger than zircon fission track ages but older than zircon (U–Th)/He cooling ages from the same area. This constrains cleft monazite crystallization in this area to ~300–200 °C. LA-ICP-MS data of dated hydrothermal monazites indicate that in graphite-bearing, reduced host lithologies, cleft monazite is poor in As and has higher La/Yb values and U concentrations, whereas in oxidised host rocks opposite trends are observed. Monazites show negative Eu anomalies and variable La/Yb values ranging from 520 to 6050. The positive correlation between Ca and Sr concentration indicates dissolution of plagioclase or carbonates as the source of these elements. The data show that early exhumation and cleft formation in the Tauern is related to metamorphic dome formation caused by the collision of the Adriatic with the European plate and that monazite crystallization in the clefts occurred later. Our data also demonstrate that hydrothermal monazite ages offer great potential in helping to constrain the chronology of exhumation in collisional orogens. © 2015, Swiss Geological Society. Source

Janots E.,Joseph Fourier University | Janots E.,University of Munster | Gnos E.,Natural History Museum Geneva | Hofmann B.A.,Natural History Museum Bern | And 4 more authors.
Meteoritics and Planetary Science | Year: 2012

A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5-10μm). Plagioclase (An 76-92) and clinopyroxene (En 48-62Wo 7-15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk-rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ 17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni=3940 and Co=159ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo 56-80), orthopyroxene (En 79Wo 2), and plagioclase (An 61-66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo 80 and En 79Wo 2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re-equilibration in a Si-undersaturated melt. JaH 556's unique composition enlarges the spectrum of howardite-analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group. © The Meteoritical Society, 2012. Source

Janots E.,Joseph Fourier University | Berger A.,Copenhagen University | Berger A.,University of Bern | Gnos E.,Natural History Museum Geneva | And 3 more authors.
Chemical Geology | Year: 2012

The timing and duration of hydrothermal activity during orogenesis are difficult to constrain, because such systems are open and multistage. Using mm-sized monazite crystals from two Alpine clefts from the Central Alps (Switzerland), we demonstrate that combined U-Th-Pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization. Our data highlight four major characteristics of the cleft monazites: (i) extreme Th/U ratios (ii) significant common Pb in the U system (up to 39% 206Pb), (iii) excess 206Pb (up to 54%), and (iv) precise and reliable Th-Pb ages. Comparison of our results with literature data indicates that Th/U in monazite is a remarkable discriminant of geological conditions, capable of distinguishing metasedimentary, magmatic and hydrothermal origin. Hydrothermal monazite crystals are characterized by Th/U ratios up to 629, among the highest values ever reported. Extreme Th/U ratios in monazite enhance incorporation of 230Th, a short-lived intermediate product in the 238U- 206Pb decay chain, generating 206Pb excess which, if not accounted for, results in 206Pb/ 238U ages that are too old, for example in the samples investigated here by as much as 300%.For the two zoned monazite crystals studied in detail, 208Pb/ 232Th ages are reliable for the different compositional domains visible in back-scattered electron (BSE) images. 232Th/ 208Pb ages for the older domains (15.2±0.3Ma and 14.1±0.3Ma, respectively) are consistent with the structural relationships of the hydrothermal veins, indicating early retrograde genesis. The youngest rim age at 13.5±0.4Ma for the Blauberg crystal marks termination of monazite precipitation. The resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity. In both crystals, the outer rims have the highest 206Pb excess, confirming a two-stage crystallization. Two scenarios are envisaged to account for the 206Pb excess evolution. In the first, increasing 206Pb excess is caused by a modification of the Th/U fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent U into the fluid. Alternatively, it is possible that 230Th increased with time in the fluid. In this second case, monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238U- 206Pb decay chain and progressively equilibrated (t<0.5Ma). © 2012 Elsevier B.V. Source

Berger A.,University of Bern | Janots E.,ISTerre | Gnos E.,Natural History Museum Geneva | Frei R.,Copenhagen University | And 2 more authors.
Applied Geochemistry | Year: 2014

In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299-363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing. © 2013 Elsevier Ltd. Source

Martins M.L.,Federal University of Santa Catarina | Pereira Jr. J.,Grande Rio University | De Chambrier A.,Natural History Museum Geneva | Mourino J.L.P.,Federal University of Santa Catarina
Journal of Helminthology | Year: 2011

We evaluated the relationship between infection by proteocephalid cestodes and the sex and weight classes of tucunaré (Cichla piquiti) captured between August 1999 and June 2001 in the Volta Grande Reservoir, Minas Gerais, Brazil. A total of 96 fish, 75.9±9.3% males and 88.9±6.4% females, were parasitized by Proteocephalus macrophallus and P. microscopicus, with total mean intensities of 76.6±23.9 and 145.2±36.7, respectively, during this period. In the majority of the months analysed, males showed 71.4-100% prevalence of parasitism and females 80-100%. Although there was no significant difference, females showed a higher mean intensity of infection (145.2±36.7) than males (76.6±23.9). Fish weighing 300-800g showed a higher mean abundance of parasites (P<0.05) compared with the biggest specimens weighing 801-2750g. Analysing both males and females together, the greatest mean intensities of infection were found in October and December (P<0.05) independent of the year, which coincides with the months of highest rainfall. These results show that fish living in reservoirs may be more susceptible to intermediate hosts than those that live in rivers. © 2011 Cambridge University Press. Source

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