Erratum to D. Gagnevin, J. S. Daly, T. E. Waight, D. Morgan, and G. Poli (2005) “Pb isotopic zoning of K-feldspar megacrysts determined by Laser Ablation Multi-Collector ICP-MS: Insights into granite petrogenesis”, Geochimica et Cosmochimica Acta 69, 1899–1915
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Laser Ablation
Optically stimulated luminescence
Thermoluminescence dating
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Glacial settings are considered to be the most challenging context for the application of luminescence dating. The optically stimulated luminescence (OSL) signal of quartz is often preferred for luminescence dating in partially-bleached settings as it resets (or bleaches) more rapidly in response to sunlight than the post-IR IRSL (pIRIR) signal of K-feldspar, and can therefore better characterise the well-bleached part of the partially-bleached De distribution. However, the relative bleaching extents of single grains of quartz and K-feldspar have not yet been compared for sedimentary samples from the natural environment. Here we compare the De distributions and accuracy and precision of ages determined using single grains of quartz and K-feldspar from sedimentary samples deposited in a proglacial setting with independent age control. We found that the extent of bleaching of the OSL signal of quartz and pIRIR225 signal of K-feldspar was similar (with similar over-dispersion), and therefore the pIRIR225 signal bleached to similarly low levels as the OSL signal of quartz in this partially-bleached setting. We also observed a consistent offset in over-dispersion between quartz and K-feldspar of ∼10% that can be linked to scatter arising from internal dose-rates of K-feldspar and should be included when applying age models. The results here demonstrate that the accuracy and precision of ages determined using the pIRIR225 signal of single grains of K-feldspar were similar to the OSL signal of quartz. However, K-feldspars were 5–18 times more efficient than quartz at determining the population of interest for age calculation as a larger proportion of K-feldspar grains emitted a detectable luminescence signal in comparison to quartz. These findings contradict our current understanding of the bleaching of K-feldspar and quartz grains in the natural environment, and are likely applicable to other partially-bleached settings (e.g. fluvial, alluvial).
Optically stimulated luminescence
Thermoluminescence dating
SIGNAL (programming language)
Optical dating
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The mineral samples analysed in this paper are K, Na, Ca aluminosilicates known as feldspars and belong to the granitic pegmatites developed in the ConţuNegovanu area, in the Southern Carpathians (Romania). Some of the samples were subjected to X-ray powder diffraction analysis and the obtained data enabled the identification of the precise feldspar terms, as well as some of their structural characteristics. The determined refractive indices and the respective birefringence data, interferometrically determined for yellow Na radiation were plotted on a determinative chart, confirming the previous determined chemical composition of the feldspar terms. The IR spectra also display characteristic features of the determined feldspar terms.
Pegmatite
Tourmaline
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Comminution
Silt
Texture (cosmology)
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Thermoluminescence dating
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The mineralogical study on the sub-surface obtained from drill-well JW-3, in the central part of the Kathmandu Basin was first performed by means of X-ray diffraction (XRD). The minerals detected in the sediments are quartz, feldspar, mica, smectite, chlorite, kaolinite, gypsum and calcite. The former three minerals are main constituent of the sediments and their relative proportion is over 70%. In general, the relative proportion of quartz is congruous with that of feldspar but is reverse proportion to that of mica. The clayey phyllosilicate minerals, such as smectite, chlorite and kaolinite are next dominant minerals in the sediments, and the relative proportion of these minerals shows a similar variation pattern to each other. Gypsum and calcite occur sporadically and their ratio is less than a few percentages, except in some horizons where they exceed 5%.
Variation curves of relative amounts of the minerals are mainly divided into two zones, based on the variation patterns of the minerals. In Zone I below115 m depth, the variation curves of minerals show gradual cyclic patterns with low amplitude except gypsum and calcite. On the other hand, the variations of mineral contents in Zone II above 115 m depth are larger than those in the Zone I. Particularly, the variation curves of quartz, feldspar and mica show repetition of shorter cycles at 4-7 m intervals which are overlapping a longer cycle at 30 m intervals. The change in variation pattern across 115 m depth of the drill-well is similar to that of frequency of each pollen in the same sediments, which depicts the climatic variations in the Kathmandu Basin (Fujii and Sakai 2002). Hence, the mineralogical variation must reflect not only the changes of depositional environments in the Kathmandu Basin but also the climatic variations there. Similar difference of variation pattern of mineral composition in the Zone I and Zone II are also reported from the pollen analysis of the same slimes. This difference seems to be related to global climatic changes.
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