Chemical Geology, 81 ( 1990 ) 1- 16 1 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands [4] High-sensitivity Nb analysis by spark-source mass spectrometry (SSMS) and calibration of XRF Nb and Zr K.P. JOCHUM 1, H.M. SEUFERT 1 and M.F. THIRLWALL 2 1Max-Planck-Institut f~r Chemie, D-6500 Mainz (Federal Republic o[ Germany) 2Royal HoUoway and Bedford New College, University of London, Egham, Surrey TW20 OEX (Great Britain) (Received April 24, 1989; revised and accepted September 21, 1989) Abstract Jochum, K.P., Seufert, H.M. and Thirlwall, M.F., 1990. High-sensitivity Nb analysis by spark-source mass spectrom- etry (SSMS) and calibration ofXRF Nb and Zr. Chem. Geol., 81: 1-16. A new spark-source mass spectrometric (SSMS) technique for precise analysis of low Nb abundances in geological samples has been developed. The main improvements in precision, accuracy and detection power have stemmed from application of isotope dilution for accurate analysis of the internal standard element Zr, high mass resolution for sensitive measurement of low-abundance Nb peaks and precise determination of the element sensitivity of Nb for calibration. Nb concentrations of 5 ppb to 500 ppm (5 orders of magnitude) can be measured by this technique. Only small sample amounts ( ~ 50 mg) are needed for an analysis. Precision and accuracy are better than ~ _+ 4%, ~ _+ 8% and ~ _+12% for Nb concentrations higher than 1, 0.1 and 0.01 ppm, respectively. To demonstrate the capability of the technique, 16 international standard rocks have been analysed. Our investi- gations yield the first reliable Nb and Zr abundances of some standard samples. The agreement for most of our data and compiled literature data is within 10% for Nb concentrations > 5 ppm and Zr > 40 ppm. However, for samples with low Nb and Zr concentrations (e.g., DTS-I ), differences between our data and the literature can be as large as a factor of 70. These standard data have been used to calibrate X-ray fluorescence (XRF) analyses of Nb and Zr. Careful attention to spectral interferences and matrix corrections combined with long counting times (giving precision between _+ 0,4 and _+ 0.2 ppm 2a on Nb) leads to a calibration for 13 standards with MSWD of ~ 0.9 for Nb, and ~ 2.7 for Zr. This demonstrates that the SSMS data may be used to obtain consistent, high-precision calibrations for Nb analysis at many XRF laboratories worldwide. 1. Introduction Nb has become a key element to constrain the composition of the Earth's mantle and its evolution {e.g., Hofmann et al., 1986). This ele- ment has unusual geochemical properties: its abundance is anomalously high in oceanic ba- salts (mid-ocean ridge basalts, MORB; and oceanic island basalts, OIB ) and low in crustal rocks. This anomalous behaviour can be ex- plained if Nb is moderately incompatible dur- ing extraction of continental material from the mantle (i.e. it has a relative large partition coefficient) and highly incompatible during MORB and OIB production (i.e. small parti- tion coefficient; Hofmann et al., 1986). This is also the reason why Nb is used in geochemical discrimination diagrams for distinguishing MORB and OIB from island-arc and most in- tracontinental volcanics (Pearce and Norry, 1979). X-ray fluorescence spectrometry (XRF) is most commonly used for the analysis of Nb in rock samples. This method has the potential of 0009-2541/90/$03.50 © 1990 Elsevier Science Publishers B.V.