u-th-pb dating of geological samples by laser ablation icpms
TRANSCRIPT
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 ?
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
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
Multi-collector magnetic sector ICPMS
ICP source
Magneticsector
MulticollectorZoom optics
Amplifierbox
Ion optics
Electricsector
AnalyzervalveZoom
optics
RPQSEM
Slit
Slit