HR: 11:20h
AN: V32B-05    [Abstracts]
TI: ⁴⁰Ar/³⁹Ar Geochronology by Multi-collector Mass Spectrometry
AU: * Cosca, M
EM: mcosca@unil.ch
AF: University of Lausanne, Institute of Mineralogy and Geochemistry, Anthropole, Lausanne, 1015, Switzerland
AB: The ushering in of a new generation of multi-collector mass spectrometers for ⁴⁰Ar/³⁹Ar geochronology represents a major technological advance, but what are the true benefits of making routine measurements with such machines versus the practical aspects of additional setup and calibration? Is argon multi-collection the way of the future? Recent experiences with a three collector Nu Instruments Noblesse mass spectrometer will be presented that address these questions. One immediate observation is that electronic stability improves average precision of the isotopic measurements. For example, on any given day 100 consecutive automated air pipette measurements yield ⁴⁰Ar/³⁶Ar ratios reproducible to 0.1%. One of the great advantages of multi-collection ⁴⁰Ar/³⁹Ar geochronology is the flexibility and ease of changing detector array configurations, which is useful when measuring irradiation packages with a range of K-content or grain size. For example, in situ UV laser ablation of micas or laser step-heating single grains of amphibole may be run by peak hopping on a single detector, while single grains of muscovite may be run by laser step-heating and analyzed by multi-collection. One of the drawbacks with multi-collection is the effort required with detector calibration and intercalibration. Source and detector discrimination and mass fractionation, however can be accomplished using air pipettes as an argon reference material. A series of experiments using sanidine standards have been performed that demonstrate the reproducibility of single collector and multi-collector measurements, with some as yet unexplained shifts using different detector arrays. Finally, several examples applying multi-collection ⁴⁰Ar/³⁹Ar data collection will be given that illustrate the utility of multi-collector measurements on a variety of mineral and rock materials.
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting