Jan Jan KoKošleršler11 and Mike Tubrettand Mike Tubrett22

www.natur.cuni.cz/ugmnz/icplabwww.natur.cuni.cz/ugmnz/icplabhttp://sparky2.esd.mun.cahttp://sparky2.esd.mun.ca

UU--ThTh--PbPb datingdatingof geological samplesof geological samples

by laser ablation ICPMSby laser ablation ICPMSWinter Conference on Plasma Winter Conference on Plasma SpectrochemistrySpectrochemistry

Fort Lauderdale, Florida, January 3, 2004Fort Lauderdale, Florida, January 3, 2004Short course STShort course ST--1919

1. Department of Geochemistry, Charles University Prague, Czech 1. Department of Geochemistry, Charles University Prague, Czech RepublicRepublicand Department of Earth Science, University of Bergen, Norwaand Department of Earth Science, University of Bergen, Norwayy

2. Department of Earth Sciences, Memorial University of Newfound2. Department of Earth Sciences, Memorial University of Newfoundland, Canadaland, Canada

Principles of U-Pb-Th dating and why geologists needan in-situ dating techniqueBrief intro to the laser ablation ICPMS techniqueElemental fractionation during laser ablationInstrument (ICPMS) mass biasData reduction, error and age calculation in LAMDATEDating minerals with and without correction for initial common PbApplications (igneous, metamorphic, sedimentary provenance,fission track dating)Some practical aspects: sample preparation, daily maintenanceQuality control and available standardsWhere are we heading – laser wavelength and pulse width,quadrupole and magnetic sector instruments, multiple collection

Outline of the short course

238U → 206Pb half-life 4.468 Ga235U → 207Pb half-life 0.704 Ga232Th → 208Pb half-life 14.010 Ga

U-Th-Pb geochronology

( )( )

207

2061

137 881

1

235

238

PbPb

e

e

t

t

−

−

t0

=.

*λ

λ

( )1e=UPb t

235

207235 −

λ

11=235

238

235

207

238

206

−

+

λλ

UPb

UPb

( )1e=UPb t

238

206238 −

λ

Concordia

( )1e=ThPb t

232

208232 −

λ

λ = decay constantt = time (age of the sample)® = radiogenic Pb

Age equations

1.0

1.5

2.0

2.5

3.0 Ga

5 10 15

0.2

0.4

0.6

207Pb/235U

206Pb/238U

Upper interceptage

206Pb/238U age

207Pb/235U age

207Pb/206Pb age

Lower interceptage

Concordia

DiscordiaPb-loss trajectory

Concordia diagram

Concordia = locus of points with identical 207Pb/235U and 206Pb/238U ages

Mineral U content Common Pb(ppm) (% of total Pb)

Zircon 1 to >10 000 <2%Monazite 282 to >50 000 <2%Baddeleyite 58 to 3410 <2%Rutile <1 to 390 <2 to 95%Xenotime 5000 to 29 000 <5%Titanite 4 to 500 5 to 40%Allanite 130 to 600 5 to 30%

Data from Heaman and Parrish (1991), Parrish and Tirrul (1989) and Noble and Searle (1995)

Geochronologically important minerals

Some geochronologically important minerals have complex internalstructures 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 mineralsto the crystallisation of other mineral phases.

Other applications in geochronology include:Check for homogeneity and presence of parent isotope- ordaughter isotope-rich inclusions in dated mineral phases.Constraints on elemental mass balance and mineral reactions thatproduce geochronologically important mineral phases.

Why there is a need for in-situ dating ?

200 µm

BSE image of allanite

Backscattered electron image of garnetwith inclusions of REE, U and Th rich minerals

EBC(WR)

Pure garnet

Measured garnet

Inclusion

143Nd144Nd

147Sm/144Nd

Measuredslope

Trueslope

Garnet

ApatiteMonazite

Effect of REE-rich inclusions on age determination

25 µm

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, 255-274.

44Ca

90Zr145Nd147Sm

Gas blankGarnetablation Garnet ablation

Zircon inclusions

0 5020 60 80 100 140120

0 20 40 60 80

1000000

100000

10000

1000

100

10

Data acquisition time (seconds)

Laser ablation time (seconds)Co

unts

per

seco

nd100

Identification of zircon inclusions duringlaser ablation of a metamorphic garnet

Lower level

Upper level

Lower level

Upper level

Emission

Absorption

NON INVERTED POPULATION

INVERTED POPULATION

Light amplificationby stimulatedemission

Photons

Photons

Inverted population is formed by externalpumping of light to the active medium.

Electrons

Light attenuationby absorption

How lasers workLight Amplification by Stimulated Emission of Radiation

Modified from Silfvast WT (1991) Lasers. In: Encyclopedia of lasers and optical technology. Meyers RA (ed), Acad. Press, p 209-226

Carrier gas To ICP

CCDcamera

2nd HG

Ablation cell

NdYAG laser

WAVELENGTH CONVERSION

BEAM SIZECONTROL

VIEWINGSYSTEM

Shutter

Adjustablebeam expander

Prism

½ waveplate

Dump

Dump

OPT

ICA

L AT

TENU

ATO

R

Beamsplitter ObjectiveSlit

4th HG 5th HG Harmonic generators

Powermeter

Dielectricmirrors

Dielectricmirror

Objective

CCDcamera

To ICP

Laser beam

IMAGED LASER BEAM

FOCUSED LASERBEAM

Laser ablation system

Jackson SE (2001) In: Laser ablation-ICPMS in the Earth Sciences. Sylvester P (ed), MAC Short Course Volume 29, p 29-46

Laser parametersLaser 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 beamflat vs gaussian profile

Laser energy density, energy per pulsethe amount of laser radiation that interacts with the sample (J/cm2, mJ/pulse)

Laser repetition ratenumber of laser shots per second (1 – 20 Hz)

Focus of the laserfocused on the sample surface, above it or active focus on the bottom of the pit

Ambient gas – He or Ar

LASER ABLATION MICROPROBE

TV

Ar in

Ar out

PETROGRAPHIC MICROSCOPE

PRISM

YAG ROD

SHG

POWER METER

FHG

LASER HWP POL

APERTURE

Laser probe

Laser beam path Sample cell

Laser sampling

MUN MUN MUN

MUN VGEAustralian/MUN

and many other …

Different laser cell designs

Laser ablation of zircon

Plasma plume in He

Ablation pit

ICP MS interface

0 1 2 cm

To rotary pump10 -10 torr0 -1

To turbo pump10 - 10 torr-4 -5

Coil Electrostatic lenses

Samplecone

Skimmercone

Torch

Plasma

Ion beamSample

Modified from Houk RS (1986). Anal Chem 58:97A-105A

Torch

Sample

Turbo pumps

Quadrupole DetectorLenses

Interface

Coil

Mass filter

Quadrupole ICPMS

Modified from Agilent Technologies

Torch

Sample CoilInterface

Modulationand extraction

region Lenses Reflector

Detector

Energyfilter

Time-of-flight ICPMS

Based on Ray SJ, Hieftje GM (2001) J Anal Atom Spectrom 16:1206-1216

Single-collector magnetic sector ICPMS

Multi-collector magnetic sector ICPMS

ICP source

Magneticsector

MulticollectorZoom optics

Amplifierbox

Ion optics

Electricsector

AnalyzervalveZoom

optics

RPQSEM

Slit

Slit

Multiple collection of ions