Abstract:
The combination of enriched isotopes and conventional elemental internal standards permits the precise determination of >40 trace elements by ICPMS in a broad spectrum of geological matrixes. Enriched isotopes expand the suite of available reference isotopes spaced through the mass spectrum, so that the complex mass-dependent variations in sensitivity encountered during ICPMS analysis can be monitored and deconvolved. The method we have developed is straightforward, entailing simple sample preparation, instrument calibration, and data reduction procedures, as well as providing extended element coverage, improved precision, and both time and cost benefits compared to alternative analytical strategies. Analytical precision near or better than 1% RSD (relative standard deviation) is achieved for most elements with mass >80 amu and between 1% and 4% (RSD) for elements with mass < 80 amu, while maintaining low detection limits (< to < 10 ng g-1 for elements with mass > 80 amu and < 10 ng g-1 to 1 μg g-1 for elements with mass < 80 amu). The subtle geochemical differences which can be resolved using this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in magmas from ocean islands and subduction zones. These data reveal significant departures from chondritic Zr/Hf and Nb/Ta values, and systematic trends which are consistent with greater incompatibility of Zr relative to Hf and also of Nb relative to Ta during melting of the upper mantle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta ratios in Nb-poor subduction zone magmas, supports the notion that the depletion of high-field strength elements in subduction magmas is due to their removal from the mantle wedge by prior melting events.