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

We have developed a procedure for precise in situ elemental and isotope ratio measurements by simultaneous solution nebulization and laser ablation inductively 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 others) covering the entire mass range. Using a quadrupole mass spectrometer, our procedure yields precision of less than or equal to 2.0% (all errors are 2 sigma of the standard error) for Pb-206/U-238 and Pb-207/Pb-206 and less 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. Importantly, this is accomplished without the use of an external calibration standard. We nebulize a solution containing known amounts of natural Tl and a U-235 Spike simultaneously with ablation of an unknown accessory phase. This 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 systematics allow us to correct for elemental fractionation. Spots with diameters greater than or equal to 150 mu m show no appreciable Pb/U fractionation, whereas 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 Harvard standard zircon, 91 500, we obtain a Pb-206/U-238 age of 1061 +/- 4 Ma 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 standards for U-Th-Pb, Lu-Hf, trace element and REE analyses. Geostand. Newsl. 19, 1-23]); for the SHRIMP zircon standard, SL13, we obtain a Pb-206/U-238 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 conventional zircon ages and 40Ar/39Ar analysis. In: Geochronology Time Scales and Global Stratigraphic Correlation. SEPM Special Publication, pp. 3-21], Pb-206/U-238 age from SHRIMP: 580-565 Ma, [Compston, W., 1999. Geological age by instrumental analysis: The 29th Halmond Lecture. Mineralogical Magazine 63, 297-311]). The Phalaborwa baddeleyite is strongly reverse discordant yielding an upper intercept age of 2057 +/- 8 Ma (TIMS age: 1 Ma, [Reischmann, T., 1995. Precise U/Pb age determination with baddeleyite (ZrO2), a case 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 lower intercept at similar to 0 Ma. These results demonstrate that LA-ICP-MS is capable of dating accessory phases with precision and accuracy comparable to SHRIMP. (C) 2000 Elsevier Science B.V. All rights reserved.