Abstract

High precision TIMS analyses of the La Jolla Nd standard show that although the internal precision (2σmean) of most runs is better than 4 ppm for the 142Nd/144Nd and 143Nd/144Nd values, the external reproducibility (2σ) is a factor of two worse. This is because in several runs, the mass-dependent fractionation of the isotope ratios does not follow the exponential law for the whole run. Divergence from exponential law behavior starts at a different point in time for each run and is accompanied by a switch from normal to ‘reverse’ fractionation, where isotope ratios get lighter with time. Ratios measured thereafter display increased scatter, upward shifts in 142Nd/144Nd, 143Nd/144Nd, 148Nd/144Nd, and 150Nd/144Nd, and a downward shift in 145Nd/144Nd. The deviations are to the concave side of the exponential law curves in XNd/144Ndvs. 146Nd/144Nd space, and hence can be explained by mixing of ions derived from isotopically heterogeneous sample reservoirs that fractionate independently along the exponential law curve. Such variably fractionated sample domains would be expected to develop on the filament in response to prevailing temperature gradients and heterogeneous sample distribution. Such an effect can also account for the apparent stalling or reversal of the mass fractionation. Isotope mixing during mass spectrometry is of great concern when measuring 142Nd anomalies because an inevitable consequence of the phenomenon is that 142Nd/144Nd deviations will always be positive when using 146Nd/144Nd for mass fractionation correction. This might preclude the detection of potential negative 142Nd anomalies, or produce spurious positive 142Nd anomalies. Upward shifts in 142Nd/144Nd as high as 70 ppm can be observed during an analysis. In practice, this could result in measurement errors and uncertainties of about the same magnitude as the natural anomalies geochemists try to resolve, i.e., <50 ppm. The ultimate attainable accuracy and reproducibility of isotope ratios will thus be constrained by ion-counting statistics as well as the extent of isotope mixing during the measurement on a mass spectrometer.

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