A SIMPLE METHOD FOR THE PRECISE DETERMINATION OF GREATER-THAN-OR-EQUAL-TO-40 TRACE-ELEMENTS IN GEOLOGICAL SAMPLES BY ICPMS USING ENRICHED ISOTOPE INTERNAL STANDARDIZATION

The combination of enriched isotopes and conventional elemental intern al 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 m ass spectrum, so that the complex mass-dependent variations in sensiti vity encountered during ICPMS analysis can be monitored and deconvolve d. The method we have developed is straightforward, entailing simple s ample preparation, instrument calibration, and data reduction procedur es, as well as providing extended element coverage, improved precision , and both time and cost benefits compared to alternative analytical s trategies. Analytical precision near or better than 1% RSD (relative s tandard deviation) is achieved for most elements with mass > 80 amu an d between 1% and 4% (RSD) for elements with mass < 80 amu, while maint aining low detection limits (< 1 to < 10 ng g(-1) for elements with ma ss > 80 amu and < 10 ng g(-1) to 1 mu g g(-1) for elements with mass < 80 amu). The subtle geochemical differences which can be resolved usi ng this method are demonstrated by analyses of Nb, Ta, Zr, and Hf in m agmas from ocean islands and subduction zones. These data reveal signi ficant departures from chondritic Zr/Hf and Nb/Ta values, and systemat ic trends which are consistent with greater incompatibility of Zr rela tive to Hf and also of Nb relative to Ta during melting of the upper m antle. The occurrence of significantly subchondritic Zr/Hf and Nb/Ta r atios 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.