Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v11e..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V11E-01
Other
8494 Instruments And Techniques
Scientific paper
A technique is presented for the determination of 232Th/230Th in volcanic rocks by plasma ionization multi-collector mass spectrometry (PIMMS) utilizing the ThermoFinnigan Neptune. These analyses were made statically, measuring 232Th on a Faraday cup and 230Th on the RPQ channel using the SEM. Because of the large dynamic range in the 232Th/230Th of volcanic rocks (> 105), accurate and precise measurement of 232Th/230Th using PIMMS requires: 1) high abundance sensitivity to minimize tailing of 232Th onto 230Th, and 2) explicit knowledge of the instrumental mass bias and the gain calibration of the two detectors used for the measurement. Using the RPQ on the Finnigan Neptune, the abundance sensitivity at 90% transmission was ~25ppb over 2 amu. Using an exponential fit the resulting tail correction of 232Th on 230Th is 0.7% for ratios of 3x105 and 0.3% for ratios of 1.5x105. To correct for both instrumental mass fractionation between masses 230 and 232 and the relative difference in the efficiency of the Faraday and SEM detectors we evaluated using both: 1) a linear interpolation of the 238U/^{236}U measured in the NBS U010 interspersed between each sample, and normalized to its certified value (14,432 ±149), and 2) a linear interpolation of the 232Th/230Th measured in the UCSC ThA interspersed between each sample, and normalized to its nominal value (170,760±2,049). Our results show that, due to both differences in instrumental mass bias and differences of the ion energies through the RPQ filter of U and Th, U does not work as an adequate proxy for Th. Replicate measurements of 232Th/230Th in synthetic and rock Th isotopic standards provide an overall reproducibility on the 232Th/230Th of <1% (2σ) and show excellent agreement with their `known` values established by other techniques within the reported errors, supporting the reliability and accuracy of this method. This PIMMS technique has considerable advantages over existing TIMS and SIMS techniques in terms of ionization efficiency and total sample consumption (and hence sample size requirement), as well as the rapidity of analysis.
Ball Lewis
Schwieters J.
Sims Kenneth W. W.
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