Precise elemental and isotope ratio determination by simultaneous solutionnebulization and laser ablation-ICP-MS: application to U-Pb geochronology

We have developed a procedure for precise in situ elemental and isotope rat io measurements by simultaneous solution nebulization and laser ablation in ductively coupled plasma mass spectrometry, which can be applied to isotope and element ratio determinations (e.g., Li-6/Li-7, B-10/B-11, Ca/Sr and ot hers) covering the entire mass range. Using a quadrupole mass spectrometer, our procedure yields precision of less than or equal to 2.0% (all errors a re 2 sigma of the standard error) for Pb-206/U-238 and Pb-207/Pb-206 and le ss than or equal to 3% for Pb-207/U-235 in neo-Proterozoic or older zircons and baddeleyite with U contents greater than or equal to 65-270 ppm. Impor tantly, this is accomplished without the use of an external calibration sta ndard. We nebulize a solution containing known amounts of natural Tl and a U-235 Spike simultaneously with ablation of an unknown accessory phase. Thi s allows precise mass discrimination correction of Pb/Pb as well as Pb/U in the ablated signal. Laser-induced elemental fractionation of Pb from U is observed to be a near function of the number of laser pulses (crater depth) and is inversely exponentially correlated with spot size. These systematic s allow us to correct for elemental fractionation. Spots with diameters gre ater than or equal to 150 mu m show no appreciable Pb/U fractionation, wher eas for 35 mu m spots U becomes progressively depleted relative to Ph, with a factor of four increase in Pb/U over a 2-min ablation period. For the Ha rvard standard zircon, 91 500, we obtain a Pb-206/U-238 age of 1061 +/- 4 M a and a Pb-207/Pb-206 age of 1074 +/- 8 Ma (TIMS age: 1065 Ma for Pb-206/U- 238, [Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Ober, F., von Quant, A., Roddick, J.C., Spiegel, J., 1995. Three natural zircon s tandards for U-Th-Pb, Lu-Hf, trace element and REE analyses. Geostand. News l. 19, 1-23]); for the SHRIMP zircon standard, SL13, we obtain a Pb-206/U-2 38 age of 578 +/- 10 Ma and a Pb-207/Pb-206 age of 595 +/- 13 Ma (TIMS age: 572 Ma, [Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., 1995 . Two carboniferous ages: A comparison of SHRIMP zircon dating with convent ional zircon ages and Ar-40/Ar-39 analysis. In: Geochronology Time Scales a nd Global Stratigraphic Correlation. SEPM Special Publication, pp. 3-21], P b-206/U-238 age from SHRIMP: 580-565 Ma, [Compston, W., 1999. Geological ag e by instrumental analysis: The 29th Halmond Lecture. Mineralogical Magazin e 63, 297-311]). The Phalaborwa baddeleyite is strongly reverse discordant yielding an upper intercept age of 2057 +/- 8 Ma (TIMS age: 1 Ma, [Reischma nn, T., 1995. Precise U/Pb age determination with baddeleyite (ZrO2), a cas e study from the Phalaborwa igneous complex, South Africa. S. Afr. J. Geol. 98, 98]; 2059.8 Ma, [Heaman, L.M., LeCheminant, A.N., Paragenesis and U-Pb systematics of baddeleyite (ZrO2). Chemical Geology 110, 95-126]) and a lo wer intercept at similar to 0 Ma. These results demonstrate that LA-ICP-MS is capable of dating accessory phases with precision and accuracy comparabl e to SHRIMP. (C) 2000 Elsevier Science B.V. All rights reserved.