The UW-Madison 5-collector mass spectrometer for high-precision 40Ar/39Ar geochronology

Efforts to improve the Geologic Time Scale to the EARTHTIME goal of per mil precision have prompted the implementation of multicollector mass spectrometers for Ar/Ar geochronology in several laboratories. The advantages of multicollection vs. traditional single collector (MAP or VG) instruments that use analog electron multipliers are mainly twofold: (1) significant improvement in the accuracy and precision of the Ar measurement, and (2) simultaneous measurement of the Ar/Ar and Ar/Ar ratios required for age determination. We have commissioned a 5-collector Nu Instruments Noblesse mass spectrometer in the UW-Madison Rare Gas Geochronology Laboratory and are developing protocols for its use. The instrument comprises two low mass ion counters (IC), an axial Faraday detector, and two high mass ICs; it is attached to a fully automated 200 cm extraction system featuring a 50 W CO laser. We have developed a two step procedure for multicollector analysis, which requires a peak jump of one amu, but provides accurate determination of the essential isotope ratios. Sample analyses are interspersed with measurements of a standard gas to assess mass discrimination, intercalibrate between multiple detectors, and gauge instrumental drift over time. The in-house produced standard is a mixture of atmospheric Ar and Ar and has a Ar/Ar ratio of 2:1. Single aliquots from a 0.1 cm pipette give Ar signals of ~200,000 cps, commensurate with the sizes of sample unknowns. One drawback of the ion counters is that the dynamic range is reduced compared to analog multipliers. However, significant improvement in the signal to noise ratio for Ar and separation of hydrocarbon peaks from Ar at m/e = 36 allows for significantly smaller signals to be analyzed with improved precision, resulting in age uncertainties from a single analysis that approach those obtained for individual zircon crystals via the CA-TIMS U/Pb method, i.e., per mil level. We present laser total fusion data from sanidine in a variety of geologic materials between 1 and 91 Ma. Samples were analyzed using both the Noblesse and our existing single-collector MAP 215-50, thus we provide a comparison of the sample size requirements, analysis time, and age uncertainties characteristic of each instrument.