Sample preparation for LAM work:

Transcription

1 Sample preparation for LAM work: Grain mounts Grain separation via: Rock crushing, sieving, Heavy liquids, Magnetic separation and picking. Then Mounting Polishing Imaging by BSE or CL Thin sections Locate minerals in situ using SEM or optical Microscope. Imaging of individual grains by BSE or CL. Analyses by LAM

2 Magnetic separator Sample preparation Picking microscope Mounting in epoxy rings Polishing

3 Daily/routine maintenance To remove contamination, especially common Pb, from the surfaces of samples and standards, ICP sample cone and skimmer cone, they are ultrasonically cleaned in nanopure water. Periodically standards are re-polished to remove ablation pits and ablation residue from sample surfaces.

4 Daily/routine maintenance Ablation cell parts, ICP glass ware, Tubing and samples and standards are all acid-washed (2M HNO 3 ) and air-dried in a HEPA filtered clean air hood.

5 Mineral standards. Need mineral standards of similar matrix as unknowns. Must be homogeneous and concordant. Must contain reasonable amounts of radiogenic Pb and U. Large enough for long term LAM use.

6 Mineral standards. We presently have and use zircons that are: 295 Ma, 720 Ma, 1065 Ma and 1330 Ma. Monazites of 555 Ma and 2580 Ma. Titanite of 520 Ma Allanite 353 Ma Rutile 934 Ma Baddeleyite 2060 Ma

7 Other considerations: Argon gas supply. Gas or liquid? Clean acid and water for solution mixing.

8 Effect of laser wavelength on the precision and accuracy of LA ICPMS U-Th-Pb data Energy of photons (ev) F2 (157 nm) ArF (193 nm) NdYAG (213 nm) NdYAG (266 nm) NdYAG (532 nm) NdYAG Excimer NdYAG fundamental (1064 nm) Laser wavelength (nm)

9 Some minerals, e.g. calcite, better ablate with shorter UV wavelength NdYAG 266 nm NdYAG 213 nm Jackson S.E. (2001): The Application of NdYAG lasers in LA-ICP-MS. In: Sylvester P.J. ed.: Laser ablation ICPMS in the Earth Sciences, MAC Short course 29,

10 266 vs 213 nm NdYAG comparison / / je14b30 Time (seconds) 206/ / je14b07 Time (seconds) Zircon nm NdYAG single laser pit 10 Hz, 0.35 mj/pulse 206 Pb/ 238 U: 11.72% 1σ m 207 Pb/ 206 Pb: 1.53% 1σ m Zircon nm NdYAG single laser pit 10 Hz, 0.35 mj/pulse 206 Pb/ 238 U: 10.54% 1σ m 207 Pb/ 206 Pb: 2.31% 1σ m

11 266 vs 213 nm NdYAG comparison / / / / / je14b21 Time (seconds) Zircon nm NdYAG 100x100 µm raster 10 Hz, 0.35 mj/pulse 206 Pb/ 238 U: 0.52% 1σ m 207 Pb/ 206 Pb: 0.42% 1σ m / / / / /237 Zircon nm NdYAG 100x100 µm raster 10 Hz, 0.35 mj/pulse 206 Pb/ 238 U: 0.52% 1σ m je14b01 Time (seconds) 207 Pb/ 206 Pb: 0.56% 1σ m

12 266 vs 213 nm NdYAG comparison Zircon TIMS age 1065 Ma Zircon TIMS age 295 Ma

13 Femtosecond lasers NIST 610 NIST 610 NIST 610 NIST 610 ICP-MS time-dependent Pb and U intensities, and Pb/U ratios versus laser fluence. 800 nm Ti: Sapphire laser, 100 fs pulses at 10 Hz repetition rate, single spot. Russo et al., J. Anal. Atom. Spectrom. 2002, 17,

14 Single collector magnetic sector data Zircon n = 10 TIMS age 1065 Ma data-point error ellipses are 68.3% conf Pb/ 238 U Concordia Age = 1064 ± 4 Ma (2σ, decay-const. errs ignored) Pb/ 235 U Element 2, UP213, 10 Hz, 3 J/cm 2, 40 µm beam diameter, line raster 10 µm/s

15 Multi collector magnetic sector data Zircon n = 8 TIMS age 1065 Ma data-point error ellipses are 68.3% conf Pb/ 238 U Concordia Age = 1063 ± 6 Ma (2σ, decay-const. errs ignored) Pb/ 235 U Neptune, UP213, 10 Hz, 6 J/cm 2, 60 µm beam diameter, line raster 10 µm/s

16 Multi collection strategies Static all faradays no com. Pb cor. L4 L3 L2 L1 Ax FAR H1 H2 H3 H4 205 Tl 206 Pb 207 Pb 209 Bi Np 238 U Dynamic axial SEM com. Pb cor. 1) 2) 3) 4) L4 L3 L2 L1 Ax SEM H1 H2 200 Hg 201 Hg 202 Hg 201 Hg 202 Hg 204 Pb 203 Tl 205 Tl 206 Pb H3 H4 204 Pb 205 Tl 206 Pb 207 Pb Np 238 U 17 % mass dispersion (e.g. Neptune)

17 Pros and cons of using quadrupole and magnetic sector/multicollector ICPMS for laser ablation U-Th-Pb dating Parameter Ion energy spread Peak shape Scanning speed (incl. settling time) Detector array Detector type Mass range Quadrupole High Gaussian High Single SEM Large Magnetic sector Low Flat-top Low (magnet) High (electrostatic) Single/Multiple Faraday/SEM/Daly Limited in MC mode

18 Pros and cons of using quadrupole and magnetic sector/multicollector ICPMS for laser ablation U-Th-Pb dating Feature Length of analysis to achieve useful precision Laser-induced fractionation Common Pb correction using 204 method (where applicable) Analysis of small samples (spatial resolution) Quadrupole Long Often has to be corrected for Low 204 Pb intensity High 204 Hg background Requires detection by SEM Magnetic sector Short (static acquisition) Often is not apparent during the short acquisition Can measure 204 Pb precisely High 204 Hg background Not possible on Faraday detectors, requires detection by channeltrons, SEM/Daly

19 Bruguier O., Télouk P., Cocherie A., Fouillac A.M., Albarède F., Evaluation of Pb-Pb and U-Pb laser ablation ICP-MS zircon dating using matrix-matched calibration samples with a frequency quadrupled (266 nm) Nd-YAG laser. Geost Newslet 25: Cox R., Wilton D., Košler J., Laser ablation U-Th-Pb dating of zircon and allanite: an example of insitu dating from the October Harbour granite, Central Coastal Labrador. Can. Mineral., 41, Fryer B.J., Jackson S.E., Longerich H.P., The application of laser ablation microprobe-inductively coupled plasma mass spectrometry (LAM-ICPMS) to in situ (U)-Pb geochronology. Chem. Geol. 109, 1-8. Hirata T., Nesbitt R.W., U-Pb isotope geochronology of zircon: Evaluation of the laser probeinductively coupled plasma mass spectrometry technique. Geochim. Cosmochim. Acta 59, Horn I., Rudnick R.L., McDonough W.F., Precise elemental and isotope ratio measurement by simultaneous solution nebulisation and laser ablation-icp-ms: application to U-Pb geochronology. Chem. Geol. 164, Horstwood M.S.A., Foster G.L., Parrish R.R., Noble S.R., Nowell G.M., Common-Pb corrected in situ U Pb accessory mineral geochronology by LA-MC-ICP-MS. J. Anal. Atom. Spectr. 18, Jackson S.E., Longerich H.P., Horn I., Dunning G.R., The application of laser ablation microprobe (LAM)-ICP-MS to in situ U-Pb zircon geochronology. J. Conf. Abstr. 1, 283. Jeffries T.E., Fernandez-Suarez J., Corfu F., Gutierrez G., Advances in U-Pb geochronology using a frequency quintupled Nd:YAG based laser ablation system (λ = 213 nm) and quadrupole based ICP-MS. J. Anal. Atom. Spectr. 18, Ketchum J.W..F, Jackson S..E, Culshaw N.G., Barr S.M., Depositional and tectonic setting of the Paleoproterozoic Lower Aillik Geoup, Makkovik Province, Canada: evolution of a passive margin foredeep sequence based on petrochemistry and U-Pb (TIMS and LAM-ICP-MS) geochronology. Precam Res 105: Košler J., Tubrett M., Sylvester P., Application of laser ablation ICPMS to U-Th-Pb dating of monazite. Geost. Newslet. 25,

20 Košler J., Fonneland H., Sylvester P., Tubrett M., Pedersen R.B., U-Pb dating of detrital zircons for sediment provenance studies a comparison of laser ablation ICPMS and SIMS techniques. Chemical Geology, 182, Košler J., Sylvester P.J., Present trends and the future of zircon in geochronology: laser ablation ICPMS. In: Hanchar J.M. and Hoskin P.W.O. (eds), Zircon. Reviews in Mineralogy & Geochemistry, vol 53, Li X., Liang X., Sun M., Guan H., Malpas J.G., Precise 206Pb/238U age determination on zircons by laser ablation microprobe-inductively coupled plasma-mass spectrometry using continuous linear ablation. Chem. Geol. 175, Longerich H.P., Fryer B.J., Strong D.F., Determination of lead isotope ratios by inductively coupled plasma-mass spectrometry (ICP-MS). Spectrochim. Acta 42B, Ludwig K.R., IsoplotEx v Berkeley Geochronological Center Special Publication no. 1a. Machado N., Gauthier G., Determination of 207Pb/206Pb ages on zircon and monazite by laser ablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil. Geochim. Cosmochim. Acta 60, Machado N., Simonetti A., U-Pb dating and Hf isotopic composition of zircon by laser ablation-mc- ICP-MS. In: Sylvester P. (ed.): Laser ablation ICPMS in the Earth Sciences - Principles and Applications, MAC short course 29, Parrish R.R., Nowell G., Noble S.R., Horstwood M., Timmerman H., Shaw P., Bowen I.J., LA- PIMMS: A New Method of U-Th-Pb Geochronology Using Micro-Sampling Techniques. J. Conf. Abst. 4, 799. Tiepolo M., In situ Pb geochronology of zircon with laser ablation inductively coupled plasma sector field mass spectrometry. Chem. Geol., 199, Tiepolo M., Bottazzi P., Palenzona M., Vannucci R., 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. Can. Mineral. 41, Willigers B.J.A., Baker J.A., Krogstad E.J., Peate D.W., Precise and accurate in situ Pb-Pb dating of apatite, monazite, and sphene by laser ablation multiple-collector ICP-MS. Geochim Cosmochim Acta 66: