Ar-40/Ar-39 geochronology using a quadrupole mass spectrometer

摘要

Magnetic sector mass spectrometers dominate the field of Ar-40/Ar-39 geochronology. Recent advances in quadrupole mass spectrometer technology, especially improvements in resolution, have increased the performance of these instruments to the extent that they can be used for isotopic determinations. We describe a triple filter quadrupole mass spectrometer (Hiden HAL 3F Series Pulse Ion Counting Triple Filter QMS) linked to an automated furnace extraction and cleaning system dedicated to Ar-40/Ar-39 incremental heating experiments. The instrument produces peaks with broad flat tops and a width of 0.9 amu at 10 cps height and 0.84 amu at 500,000 cps height on a 1 million cps high peak (peak width at 0.01 parts per thousand and 50 parts per thousand peak height respectively). This allows measurement of ratios of the main At peaks in the 1 parts per thousand range. Measurements of 1.6 x 10(-12) mole of air reference gas over two years yields Ar-40/Ar-36 = 257.9 +/- 1.3 (1 sigma, n = 34). The ability of the instrument to produce Ar-40/Ar-39 ages from rocks/minerals of a wide age range, reaching into the late Quaternary, are demonstrated by a series of tests and comparison with geochronological data from other studies and an in-house MAP 215-50 magnetic sector mass spectrometer. We demonstrate that high-end quadrupole systems can be used for routine Ar-40/Ar-39 dating purposes.