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

Abstract 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) of the analyzed samples. This outcome potentially has important implications for future provenance studies.