HR: 1340h
AN: V23B-1437    [Abstracts]
TI: Advances in Hardware and Data Reduction Protocol in 40Ar/39Ar Dating; On Single and Multi- Collector Mass Spectrometer Systems
AU: * Turrin, B D
EM: bturrin@rci.rutgers.edu
AU: Swisher, C C
EM: cswish@rci.rutgers.edu
AB: The two limiting factors in obtaining accurate high precision 40Ar/39Ar ages are the accurate and precise determination of the mass spectrometer mass discrimination (usually expressed as 1 a.m.u.) and of the 36Ar background corrections. These two corrections have a direct affect on the accuracy of the neutron flux monitor (J which is applied to the unknowns) and the sample unknowns. The mass discrimination correction has the most significant influence on samples with low concentrations of radiogenic Ar because measured 36Ar/39Ar ratio is multiplied by 1 a.m.u. to the third power resulting in a correction of up to 20 per mil to 36Ar/39Ar ratio. This ratio if further increased by the atmospheric 40Ar/36Ar (295) to correct for atmospheric Ar. However, the mass discrimination is not trivial for samples with high radiogenic 40Ar concentrations either because the measured 40Ar/39Ar ratio is directly multiplied by a 1 a.m.u correction, resulting in a 2 to 10 per mil correction to this ratio. In some cases this variation in mass discrimination is hidden in J, however, this is not the case for sample measured at different time frames or methods. Similarly, the 36Ar background corrections has the most significant influence on samples with low concentrations of radiogenic Ar because measured 36Ar/39Ar ratio is multiplied by 1 a.m.u. to the third power and then increased by the atmospheric 40Ar/36Ar (295) to correct for atmospheric Ar. Improvements in the determination of these two corrections, is required to improve the accuracy and precision 40Ar/39Ar dating. We have developed a protocol that closely tracks any changes in the mass spectrometer mass discrimination during the time frame of the experiments thereby improving the accuracy of our age determinations. To further improve the accuracy of the 36Ar measurements, we have also implemented digital ion-counting and multi-collector data acquisition, significantly improving our low-level signal measurements resulting in a major improvement in the accuracy and precision of 40Ar/39Ar age determinations.
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting