| 1. |
- Luvizotto, G. L., et al.
(författare)
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Rutile crystals as potential trace element and isotope mineral standards for microanalysis
- 2009
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Ingår i: Chemical Geology. - 0009-2541. ; 261:3-4, s. 346-369
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Tidskriftsartikel (refereegranskat)abstract
- The present paper reports trace element concentrations of 15 elements (V, Cr, Fe, Zr, Nb, Mo, Sn, Sb, Hf, Ta, W, Lu, Pb, Th and U) as well as Ph and Hf isotope data for four relatively homogeneous and large (centimeter size) rutile grains. Methods employed are SIMS, EMP, LA-ICP-MS, ID-MC-ICP-MS and TIMS. For most elements homogeneity is usually within +/- 10% and occasionally variations are even narrower (+/- 5%), particularly in the core of two of the studied grains. The trace element concentrations of the grains span a broad compositional range (e.g., Zr concentrations are ca. 4, 100, 300 and 800 ppm). Provisional concentration values, calculated based on the homogeneity of the element and agreement between techniques, are presented for Zr, Nb, Sn, Sb, Hf, Ta, W and U. The present work represents a significant step forward in finding a suitable mineral standard for rutile microanalysis and encourages not only further search for mineral standards but also applications of rutile in the field of geochemistry and geochronology. In this sense, the rutiles presented here are useful as mineral standard in general in-situ rutile measurements, particularly for Zr-in-rutile thermometry, quantitative provenance studies (Nb and Cr concentrations as index of source rock type) and U-Pb dating. One of the studied grains has a relatively high U concentration (ca. 30 ppm) and rather constant U-Pb ages (1085.1 to 1096.2 Ma (207)Pb/(235)U ages and 1086.3 to 1096.6 Ma, (206)Pb/(238)U ages), favoring its application as an age standard for U-Pb rutile dating. Since Lu concentrations in rutile are very low and Hf concentrations can be relatively high (tens of ppm), detrital rutiles may be suitable for obtaining initial Hf isotope composition of source rocks, therefore the rutiles presented here can be used as calibration material for in-situ rutile Hf isotope studies. (C) 2008 Elsevier B.V. All rights reserved.
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| 2. |
- Luvizotto, G. L., et al.
(författare)
-
Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany
- 2009
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Ingår i: Mineralogy and Petrology. - 0930-0708. ; 97:3-4, s. 233-249
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Tidskriftsartikel (refereegranskat)abstract
- Metamorphic textures in medium-grade (similar to 500-550A degrees C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (similar to 480A degrees C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from similar to 500 to 630A degrees C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (similar to 530A degrees C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.
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| 3. |
- Triebold, S., et al.
(författare)
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Deducing source rock lithology from detrital rutile geochemistry: An example from the Erzgebirge, Germany
- 2007
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Ingår i: Chemical Geology. - 0009-2541. ; 244:3-4, s. 421-436
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Tidskriftsartikel (refereegranskat)abstract
- This study evaluates the applicability of rutile trace element geochemistry to provenance studies. The study area is the Erzgebirge in eastern Germany, where metamorphic rocks ranging from lower greenschist facies conditions up to granulite facies conditions are exposed. We collected sand and rock samples from small catchment areas for a comparative analysis of rutile geochemistry using wavelength-dispersive electron microprobe. Our results show that rutile geochemistry is a powerful tool in provenance studies, allowing for the identification of source lithologies and an evaluation of the host orogen's metamorphic history. The log (Cr/Nb) ratio has proven to be decisive in discriminating between mafic and metapelitic lithologies. It is also useful for identifying different source rocks when plotted versus a third element or proxy. Furthermore, our results suggest that rutile thermometry can be applied to a much wider range of lithologies than previously assumed. A quantification of temperature populations within single sand samples shows that at high-grade metamorphic conditions, such as those found in the Erzgebirge, more than 65% of rutiles do not re-equilibrate during retrograde metamorphism and thus retain their peak temperature chernistry. Such samples, which have equilibrated at recent metamorphic conditions, can be identified by their 2-sigma standard deviations of less than 120 degrees C. Below 550-600 degrees C, no complete equilibration is reached. Rutiles from greenschist facies and lower metamorphic conditions in the Erzgebirge still inherit relict temperatures from a former metamorphic cycle. They partly record very high temperatures >950 degrees C and supposedly derive from erosion of the west African craton in Ordovician time. (C) 2007 Elsevier B.V. All rights reserved.
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| 4. |
- Triebold, S., et al.
(författare)
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Discrimination of TiO2 polymorphs in sedimentary and metamorphic rocks
- 2011
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Ingår i: Contributions to Mineralogy and Petrology. - 0010-7999. ; 161:4, s. 581-596
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Tidskriftsartikel (refereegranskat)abstract
- Investigation by Raman spectroscopy of samples from different geological settings shows that the occurrence of TiO2 polymorphs other than rutile can hardly be predicted, and furthermore, the occurrence of anatase is more widespread than previously thought. Metamorphic pressure and temperature, together with whole rock chemistry, control the occurrence of anatase, whereas variation of mineral assemblage characteristics and/or fluid occurrence or composition takes influence on anatase trace element characteristics and re-equilibration of relict rutiles. Evaluation of trace element contents obtained by electron microprobe in anatase, brookite, and rutile shows that these vary significantly between the three TiO2 phases. Therefore, on the one hand, an appropriation to source rock type according to Nb and Cr contents, but as well application of thermometry on the basis of Zr contents, would lead to erroneous results if no phase specification is done beforehand. For the elements Cr, V, Fe, and Nb, variation between the polymorphs is systematic and can be used for discrimination on the basis of a linear discriminant analysis. Using phase group means and coefficients of linear discriminants obtained from a compilation of analyses from samples with well-defined phase information together with prior probabilities of groupings from a natural sample compilation, one is able to calculate phase grouping probabilities of any TiO2 analysis containing at least the critical elements Cr, V, Fe, and Nb. An application of this calculation shows that for the appropriation to the phase rutile, a correct-classification rate of 99.5% is obtained. Hence, phase specification by trace elements proves to be a valuable tool besides Raman spectroscopy.
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