1,721,221 research outputs found
Late Miocene to Early Pliocene depositional history of the intramontane Florina-Ptolemais-Servia Basin, NW Greece: Interplay between orbital forcing and tectonics
No full textUpper Miocene and Lower Pliocene shallow lacustrine deposits from the Florina-Ptolemais-Servia Basin in northwest Greece show a distinct m-scale sedimentary cyclicity of alternating marls and lignites or clays, which were shown to be primarily related to precession-induced climate variations ([van Vugt, N., Steenbrink, J., Langereis, C.G., Hilgen, F.J., Meulenkamp, J.E., 1998. Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record. Earth Planet. Sci. Lett. 164 (3-4) 535-551; Steenbrink, J., Van Vugt, N., Hilgen, F.J., Wijbrans, J.R., Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine succession of Ptolemais (NW Greece): Discrepancy betwee
Dynamics of magma generation and differentiation in the central-eastern Aegean arc: A geochemical and petrological study of Quaternary arc volcanism in Greece
Full text linkDavies, G.R. [Promotor]Wijbrans, J.R. [Promotor]Vroon, P.Z. [Copromotor
40Ar/39Ar (superschrift) geochronology of HP-UHP metamorphic rocks in North Qaidam, NW China
Full text linkWijbrans, J.R. [Promotor]Brouwer, F.M. [Copromotor]Qiu, H.N. [Copromotor
Provenance and Evolution of the Yangtze River constrained by Detrital Minerals
Full text linkWijbrans, J.R. [Promotor]Kuiper, K.F. [Copromotor
Intercalibration of 40AR/39AR geochronology and in situ terrestrial cosmogenic 3HE: Two scientific methods join at Earth's surface
Full text linkAndriessen, P.A.M. [Promotor]Wijbrans, J.R. [Copromotor
Extracting Erosion and Exhumation Patterns from Detrital Thermochronology: an example from the eastern Himalaya
Full text linkWijbrans, J.R. [Promotor]Beek, P. van der [Promotor]Najman, Y. [Copromotor
Improving the precision of single grain mica 40Ar/39Ar-dating on smaller and younger muscovite grains: Application to provenance studies
Full text linkCurrent generation multi-collector mass spectrometers allow for increasingly more precise measurements of small ion beams. The improvement of instrument sensitivity and resolution compared with older generation mass spectrometers has important implications for 40 Ar/ 39 Ar dating and allows the expanding of its range of applicability. Thermochronological analysis of detrital modern river sands is a powerful tool for unraveling provenance and exhumation histories of eroding hinterlands. Better instrument sensitivity allows refining the precision of dates for young and small grains, which in turn acknowledge an interpretation of the detrital signals from a wider range of micas. Previous studies have used the 40 Ar/ 39 Ar method to assess how the detrital mineral age signals can evolve downstream in the river trunk of an active mountain range. So far, however, there has not been a robust assessment of how grain-size variability can introduce biases in the analysis of age distributions. For example, the white mica signal from the Namche Barwa syntaxis in the eastern Himalaya is interpreted to be diluted downstream from its source due to the admixture of micas from downstream sources to the total population, but grain-size variability biases were not evaluated. Here we use the latest developments in multi-collector noble gas mass spectrometry to (1) test if the precision in the analysis of young and small muscovite samples can be improved by use of new faraday collector amplifier technology and (2) to apply this approach to test the variability of the age distribution as a function of the grain size from five modern rivers samples draining the Eastern Himalaya. The Helix MC plus at VUA is equipped with 10 13 Ohm amplifiers on the H2-H1 Faraday cups. We compare the functioning of the 10 13 Ohm amplifiers with the 10 12 amplifiers on the in-house Drachenfels (DRA) standard. The use of the new 10 13 Ohm amplifiers to measure the 40 Ar and 39 Ar ion beams improved the precision when measuring standards by a factor of two. We show that for larger catchment areas multi grain-size analyses lead to a more complete assessment of the full spectrum of ages obtained from different sources. The analyses of smaller grain sizes ( 250 μm) of the analyzed samples. This outcome potentially has important implications for future provenance studies
Impact of hydraulic sorting and weathering on mica provenance studies:An example from the Yangtze River
Full text linkDetrital muscovite and biotite 40Ar/39Ar analyses are useful tools for studying regional tectonic histories, sediment provenances and paleo-drainage reconstructions. During transport and recycling of detrital micas physical and chemical weathering occurs. This process effects the grain size and age populations ultimately found in river sediments, but is often ignored in provenance studies. Here, we present detrital muscovite and biotite 40Ar/39Ar results of 15 modern sediments from the Yangtze River to address the impact of grainsize on provenance age populations. The beam intensities of 39Ar, formed from 39K by neutron capture reaction during sample irradiation, have been used as an index for grain size. We found that relatively older detrital mica ages of the Yangtze River are often characterized by small 39Ar signals (i.e., grain sizes), and large grain sizes correspond to younger grains. This observation is also revealed by reanalysis of previously reported detrital mica studies in other major river systems (Red and Brahmaputra rivers) and sediments (Scotian Basin, Canada and Antarctic) and probably results from physical and chemical weathering during transport and recycling. Our Yangtze results indicate that detrital muscovite and biotite ages of grainsize ranging from 100 to 1000 μm cover all age components as identified in all dated grains (with a size of >100 μm), and thus indicate that detrital mica 40Ar/39Ar analyses should include also small grains from >100 μm to reduce the effects of hydraulic sorting and weathering. Grainsizes smaller than 100 μm have not been tested in this study, but will be more difficult to date due to both smaller beam intensities and possible recoil effects.</p
Reply to comment by M. A. Kendrick and D. Phillips (2009) on "The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/39Ar geochronology of eclogite garnet fluid inclusions" by Hua-Ning Qiu and J. R. Wijbrans (2008) [Earth Planet. Sci. Lett. 268 (2008) 501-514]
No full textThe 40Ar/39Ar apparent ages found by Qiu and Wijbrans [Qiu, H.N. and Wijbrans, J.R., 2008. The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/39Ar geochronology of eclogite garnet fluid inclusions. Earth Planet. Sci. Lett., 268, 501-514. doi: 10.1016/j.epsl.2008.01.042; Qiu, H.N. and Wijbrans, J.R., 2006. Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta, 70, 2354-2370. doi: 10.1016/j.gca.2005.11.030.] for crushing in vacuo experiments on Dabieshan eclogite garnets are best explained by a simple mixing model with radiogenic 40Ar* and air argon for the final plateau data, and excess 40Ar released in the initial crushing steps. More intricate models such as the hypothesized mixture of phengite, K-feldspar, amphibole and omphacite solid impurities by Kendrick [Kendrick, M.A., 2007. Comment on 'Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans'. Geochim. Cosmochim. Acta, 71, 6040-6045. doi: 10.1016/j.gca.2007.01.029.] and Kendrick and Phillips [Kendrick, M.A. and Phillips, D., 2009. Discussion of 'The Paleozoic metamorphic history of the Central Orogenic Belt of China from 40Ar/39Ar geochronology of eclogite garnet fluid inclusions by Qiu Hua-Ning and Wijbrans J.R.'. Earth Planet. Sci. Lett.] cannot explain the consistently observed 40Ar/39Ar age plateaux and isochrons because mixing of more than two K-containing end members each with different ages, different initial 40Ar/36Ar ratios and different proportional argon release patterns cannot define an isochron and age plateau, and the solid inclusion mixing model fails to address the consistently observed correlation between the radiogenic signal and chlorine derived 38Ar
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