Improving the precision of single grain mica 40Ar/39Ar-dating on smaller and younger muscovite grains: Application to provenance studies

L. Gemignani*, K. F. Kuiper, J. R. Wijbrans, Xilin Sun, A. Santato

*Corresponding author for this work

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Current 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) show that previous ideas/arguments about the process of dilution of the Namche Barwa syntaxis age signal for muscovite were biased due to the measurement of exclusively larger grain-sizes (> 250 μm) of the analyzed samples. This outcome potentially has important implications for future provenance studies.

Original languageEnglish
Pages (from-to)100-111
Number of pages12
JournalChemical Geology
Early online date27 Feb 2019
Publication statusPublished - 20 Apr 2019


The work leading to the study presented here was supported by the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/People/2012/ITN, Grant agreement number 316966 , and from funding from the University of Turin , Italy. I would like to thank Natalie Vögeli and Gwladys Govin for their help in the field. Janne Koornneef is acknowledged for an early review of this paper. Onno Potsma is acknowledged for providing the Helix MC schema of fig. 2. This work benefited of the careful review of Fred Jourdan and Jeff Benowitz and of the editor Balz Kamber.

FundersFunder number
Seventh Framework Programme
Marie Curie
Seventh Framework Programme316966
Università degli Studi di Torino


    • Ar/ Ar thermochronology
    • Detrital muscovite
    • Helix MC plus
    • Provenance studies


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