A LASER PROBE COUPLED WITH ICP DOUBLE-FOCUSING SECTOR-FIELD MASS SPECTROMETER FOR IN SITU ANALYSIS OF GEOLOGICAL SAMPLES AND U Pb DATING OF ZIRCON

Quadrupole ICP-MS is currently the method of choice for laser-ablation applications, since sector-field instruments are generally considered too slow for the acquisition of rapidly varying signals. However, the new generation of double-focusing sector-field mass spectrometers is capable of rapidly scanning the mass spectrum both magnetically and electrostatically, and the combination of these two scan modes approaches the performance of quadrupoles for most practical applications. We present the instrumental configuration and operating conditions of a LA-ICP-MS system, assembled by coupling a 266 nm laser source to a double-focusing magnetic-sector mass spectrometer ("Element", Finnigan MAT). The capability of the adopted Configuration to furnish hi.-h-quality ill situ trace-element analysis of minerals and U-Pb geochronological data is described. Concentrations of thirty geochemically relevant elements (LILE, HFSE, REE, actinides) were determined on geological samples with good precision and accuracy (<10%), evaluated using two USGS basaltic glass reference materials, BCR-2 and BIR-1. The high sensitivity and low background allowed extremely low limits of detection (down to the ppb level) for the heaviest elements, and between 10 and 100 ppb for the lighter masses. The first results on U:Pb ratio determinations are also very positive. By adopting an external matrix-matched standard for corrections of mass bias and laser-induced fractionation of elements, precision and accuracy in age determinations to better than similar to2% are attained on the zircon standard 91500.