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- Slama, Jiri, et al.
(författare)
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Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis
- 2008
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 249:02-jan, s. 1-35
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Tidskriftsartikel (refereegranskat)abstract
- Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in inicrobeam U-Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plesovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U-Pb age with a weighted mean Pb-206/U-238 date of 337.13 +/- 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U-Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to similar to 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plesovice zircon (>0.9 wt.% Hf) is homogenous within and between the grains with a mean Hf-176/Hf-177 value of 0.282492 +/- 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plesovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.
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| 3. |
- Westhues, Anne, et al.
(författare)
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New constraints on the timing of host-rock emplacement, hydrothermal alteration, and iron oxide-apatite mineralization in the Kiruna District, Norrbotten, Sweden
- 2016
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 111:7, s. 1595-1618
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Tidskriftsartikel (refereegranskat)abstract
- High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30-70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05-5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores
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