The ARGUS multicollector noble gas mass spectrometer: Performance for 40Ar/39Ar geochronology

We describe a new high-sensitivity multicollector noble gas mass spectrometer (ARGUS) that has been specifically designed for simultaneous collection of all Ar isotopes and hence is ideally suited for 40Ar/39Ar geochronology. The instrument uses a detector housing that holds five Faraday collectors. One collector is equipped with a 10(11) ohm resistor (for 40Ar), and the remaining four are fitted with 10(12) ohm resistors (for Ar-36 to 39Ar). ARGUS has a mass resolution of 225-250 and a measured sensitivity of 7 x 10(-14) mol/V at 200 mu A trap current. During the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40Ar/Ar-36 of 300.67 +/- 0.07 (2 sigma, eight data points rejected, and mean square weighted deviation = 1.18). The ages of three mineral standards (Taylor Creek Rhyolite sanidine, Heidelberg biotite, and Alder Creek sanidine) are within error of the accepted ages, and uncertainties for two are improvements on previously published data. ARGUS decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak-jumping single-collector mass spectrometers.