dating of geological samples by laser ablation ICPMS

Transcription

1 U-Th-Pb dating of geological samples by laser ablation ICPMS Winter Conference on Plasma Spectrochemistry Fort Lauderdale, Florida, January 3, 2004 Short course ST-19 Jan Košler 1 and Mike Tubrett 2 1. Department of Geochemistry, Charles University Prague, Czech Republic and Department of Earth Science, University of Bergen, Norway 2. Department of Earth Sciences, Memorial University of Newfoundland, Canada

2 Outline of the short course Principles of U-Pb-Th dating and why geologists need an in-situ dating technique Brief intro to the laser ablation ICPMS technique Elemental fractionation during laser ablation Instrument (ICPMS) mass bias Data reduction, error and age calculation in LAMDATE Dating minerals with and without correction for initial common Pb Applications (igneous, metamorphic, sedimentary provenance, fission track dating) Some practical aspects: sample preparation, daily maintenance Quality control and available standards Where are we heading laser wavelength and pulse width, quadrupole and magnetic sector instruments, multiple collection

3 U-Th-Pb geochronology 238 U 206 Pb half-life Ga 235 U 207 Pb half-life Ga 232 Th 208 Pb half-life Ga

4 Age equations Pb U Pb U Pb Th = = = ( e 1) λ t 235 ( e 1) λ t 238 ( e 1) λ t Pb Pb t 0 = 1 * ( λ t e 1) ( λ t e 1) λ = decay constant t = time (age of the sample) = radiogenic Pb 206 Pb U 238 Concordia = 207 Pb U λ λ

5 Concordia diagram Concordia = locus of points with identical 207 Pb/ 235 U and 206 Pb/ 238 U ages Pb/ 238 U 207 Pb/ 235 U age 207 Pb/ 206 Pb age 3.0 Ga Concordia Pb/ 238 U age Upper intercept Discordia age Pb-loss trajectory Lower intercept age Pb/ 235 U

6 Geochronologically important minerals Mineral U content Common Pb (ppm) (% of total Pb) Zircon 1 to > <2% Monazite 282 to > <2% Baddeleyite 58 to 3410 <2% Rutile <1 to 390 <2 to 95% Xenotime 5000 to <5% Titanite 4 to to 40% Allanite 130 to to 30% Data from Heaman and Parrish (1991), Parrish and Tirrul (1989) and Noble and Searle (1995)

7 Why there is a need for in-situ dating? Some geochronologically important minerals have complex internal structures on the scale of several tens to hundreds micrometers. Different parts of mineral grains may yield different isotopic ages. A single piece of rock may contain minerals of different ages. In-situ dating allows us to relate isotopic ages of dated minerals to the crystallisation of other mineral phases. Other applications in geochronology include: Check for homogeneity and presence of parent isotope- or daughter isotope-rich inclusions in dated mineral phases. Constraints on elemental mass balance and mineral reactions that produce geochronologically important mineral phases.

8 BSE image of allanite 200 µm

9 Backscattered electron image of garnet with inclusions of REE, U and Th rich minerals

10 Effect of REE-rich inclusions on age determination Apatite Monazite 143Nd 144Nd True slope Pure garnet EBC (WR) Measured garnet Measured slope Garnet 25 µm Inclusion 147Sm/ 144 Nd Prince C.I., Kosler J., Vance D., Günther D. (2000): Comparison of laser ablation ICP-MS and isotope dilution REE analyses - implications for Sm-Nd garnet geochronology.- Chemical Geology, 168,

11 Identification of zircon inclusions during laser ablation of a metamorphic garnet Counts per second Gas blank Garnet ablation Laser ablation time (seconds) Zircon inclusions Zr 145 Nd 147 Sm 44 Ca Garnet ablation Data acquisition time (seconds)

12 How lasers work Light Amplification by Stimulated Emission of Radiation NON INVERTED POPULATION Emission Upper level Photons Lower level Absorption Electrons Light attenuation by absorption Upper level Photons Lower level INVERTED POPULATION Inverted population is formed by external pumping of light to the active medium. Light amplification by stimulated emission Modified from Silfvast WT (1991) Lasers. In: Encyclopedia of lasers and optical technology. Meyers RA (ed), Acad. Press, p

13 Laser ablation system OPTICAL ATTENUATOR Dielectric mirrors ½ wave plate Dump WAVELENGTH CONVERSION 5th HG 4th HG 2nd HG Harmonic generators NdYAG laser Prism CCD camera CCD camera Dump Adjustable beam expander Power meter VIEWING SYSTEM Laser beam Objective Dielectric mirror To ICP Slit Beam splitter Shutter Objective IMAGED LASER BEAM BEAM SIZE CONTROL Carrier gas Ablation cell To ICP FOCUSED LASER BEAM Jackson SE (2001) In: Laser ablation-icpms in the Earth Sciences. Sylvester P (ed), MAC Short Course Volume 29, p 29-46

14 Laser parameters Laser wavelength shorter UV wavelength is better absorbed by most materials (266, 213, 193 nm) Laser pulse duration (pulse width) the shorter the pulse (ns vs fs), the less heat dissipation and sample melting Energy distribution across the laser beam flat vs gaussian profile Laser energy density, energy per pulse the amount of laser radiation that interacts with the sample (J/cm 2, mj/pulse) Laser repetition rate number of laser shots per second (1 20 Hz) Focus of the laser focused on the sample surface, above it or active focus on the bottom of the pit Ambient gas He or Ar

15 LASER ABLATION MICROPROBE PETROGRAPHIC MICROSCOPE TV PRISM POWER METER LASER HWP POL Ar in Ar out YAG ROD FHG SHG APERTURE

16 Laser probe Laser beam path Sample cell

17 Laser sampling

18 Different laser cell designs MUN MUN MUN Australian/MUN MUN VGE and many other

19 Laser ablation of zircon Plasma plume in He Ablation pit

20 ICP MS interface Skimmer cone Sample Torch Coil Plasma Sample cone Electrostatic lenses Ion beam cm To turbo pump torr To rotary pump torr Modified from Houk RS (1986). Anal Chem 58:97A-105A

21 Quadrupole ICPMS Sample Coil Lenses Quadrupole Detector Torch Interface Mass filter Turbo pumps Modified from Agilent Technologies

22 Time-of-flight ICPMS Sample Torch Coil Modulation and extraction Interface region Lenses Reflector Detector Energy filter Based on Ray SJ, Hieftje GM (2001) J Anal Atom Spectrom 16:

23 Single-collector magnetic sector ICPMS

24 Multi-collector magnetic sector ICPMS Magnetic sector Zoom optics Multicollector RPQ SEM Zoom optics Slit Analyzer valve Amplifier box Electric sector Slit Ion optics ICP source

25 Multiple collection of ions