Optimizing 40Ar/39Ar analyses using an Isotopx NGX-600 mass spectrometer

Abstract

40Ar/39Ar geochronology relies on magnetic sector mass spectrometers to determine relative isotopic abundances. Ongoing technological developments within noble gas mass spectrometers over the last decade have led to analysis of increasingly smaller samples and higher precision, but also result in more complex data correction and interpretation. We describe a new multi-collector noble gas spectrometer, the Isotopx NGX-600, that is configured to optimize 40Ar/39Ar measurements. The NGX-600 is equipped with 9 Faraday collectors and one ion counting electron multiplier. Each Faraday is equipped with Isotopx ATONA® amplifier technology, enabling measurements spanning a dynamic range of amplified beam current from below 10−16 A to above 10−9 A. The performance of the NGX-600 is evaluated using both a conventional Nier-type ion source, and a next generation low temperature ion source, which allows for trap current variation from 200 μA to 1000 μA. We have performed over 3000 analyses of atmospheric argon to: (1) assess optimal measurement and integration times for blanks, baselines, and air aliquots of various ion intensities, (2) quantify the sensitivity via measurements of first principles 40Ar/39Ar standards, (3) compare the sensitivity between the conventional and new low temperature Nier-type ion sources, and (4) evaluate corrections associated with inter-Faraday biases, instrumental mass bias, and Faraday-multiplier gain. In addition to optimization experiments, we report a comparative analysis of both single crystal fusion and incremental heating data from Quaternary volcanic rocks obtained using both the 5-collector Nu Instruments Noblesse and the NGX-600 spectrometers.