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Rasmus Andreasen

Fractionation behavior of neodymium isotopes during thermal ionization mass spectrometry

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  • R. Andreasen
  • M. Sharma, Dartmouth College, Department of Earth Sci., 6105 Fairchild Hall, Hanover, NH 03755, United States mukul.sharma@dartmouth.edu
With the introduction of new generation multi-collector thermal ionization mass spectrometer (TRITON) a number of studies have investigated the 142Nd isotope excesses/deficits in terrestrial and extra- terrestrial materials. A key issue is the extent to which the fractionation behavior of Nd in the ion-source affects the accuracy and the ultimate experimental precision to which the Nd isotopes can be measured using thermal ionization mass spectrometry. We investigated if there is a systematic fine structure to the observed mass dependent isotope fractionation of Nd isotopes during thermal ionization, which violates the exponential law and requires its further refinement for high precision Nd isotope data. We find that: 1. The exponential fractionation law is fully adequate in correcting for mass dependent fractionation introduced in the TIMS source at the level of precision presently obtainable. This is not the case for other fractionation laws, including the Rayleigh law. 2. Seemingly mass dependent correlations between fractionation corrected isotope ratios are correlated errors caused by counting statistics uncertainty. These correlations are not related to the fractionation behavior of the sample. 3. In the case of Nd where variations in the light isotopes 142Nd and 143Nd are of primary interest, more precise data are obtained when the isotope pairs with greater mass range (i.e., 150Nd/144Nd or 150Nd/142Nd) are used to assess fractionation factor. However, this only increases the precision with ~20%. The downside of using this normalization is an increase in the demand of the chemical purification of Nd to reduce/eliminate Sm (and Ce) interference. 4. Rreservoir mixing can introduce variations in isotope ratios as large as 50% of the external reproducibility. However, the fractionation behavior of a sample is not necessarily a good measure of the amount of mixing between reservoirs on the filament. For samples showing reverse fractionation, there appear to be mixing at the same extent before, during, and after the period of reverse fractionation. There is likely significant mixing taking place in samples showing only normal fractionation. 5. It is imperative to measure all the isotopes of Nd with the best possible precision to assess whether measured anomalies (positive or negative) in 142Nd are real or could be the product of reservoir mixing on the filament.
OriginalsprogEngelsk
TidsskriftAmerican Geophysical Union, Fall Meeting 2008
Vol/bind13
StatusUdgivet - 1 dec. 2008
Eksternt udgivetJa

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