An Introduction to U-Pb Geochronology

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EURISPET 2008

• An Introduction to U-Pb Geochronology
• (from a SHRIMPers point of view)
• Ian S. Williams
• Research School of Earth Sciences

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Geochronology The concept
Radioactive parent
P
D/P elt - 1
l
Stable daughter
D
3
Geochronology The concept
Radioactive parent
P
l
Stable daughter
D
4
Geochronology The concept
Radioactive parent
After one half life D P
P
l
Stable daughter
D
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Geochronology in practice
Several natural radioisotopes have half lives
suitable for geochronology
Half life
40K 40Ar (and 40Ca) 1.25
Ga 87Rb 87Sr
48.8 Ga 238U 206Pb
4.47 Ga 235U 207Pb
704 Ma 147Sm 143Nd
106 Ga
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Geochronology in practice
Some initial assumptions

1. No daughter isotope was present in the system to
   start with, or if some was present, the amount
   can be measured.
2. The system has remained closed. No parent or
   daughter isotopes have been added from external
   sources, no parent or daughter isotopes have been
   lost.

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Geochronology in practice
K-Ar and Rb-Sr mineral ages

1. High abundance host minerals (e.g micas)
2. High radioactive parent content
3. Low initial daughter content
4. Reasonably simple sample preparation
5. Reasonably simple analysis
6. Good analytical precision (0.5)

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Closure temperatures
Minerals only retain radiogenic daughter products
below their closure temperatures
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Closure temperatures
Approximate closure temperatures of commonly
dated minerals
Hornblende K-Ar 500C Muscovite
Rb-Sr 500C Muscovite K-Ar
400C Biotite K-Ar 350C Biotite
Rb-Sr 350C
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Rb-Sr whole-rock analysis
Concept Although individual minerals might
leak, the whole rock remains closed
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Rb-Sr whole-rock analysis
The Isochron
Isochrons give the age, initial isotopic
composition and a test that the system has
remained closed
Mineral 3
Slope e?t - 1
Mineral 2
Whole rock 3
Whole rock 2
87Sr/86Sr
Whole rock 1
Mineral 1
Initial 87Sr/86Sr
t 0
87Rb/86Sr
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The 40Ar-39Ar technique
39K is converted to 39Ar by fast neutron
irradiation. A neutron is captured and a proton
is lost. 39Ar becomes a proxy for K.
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The 40Ar-39Ar technique
40Ar 39Ar
Age
Ar is released from progressively more retentive
domains by step heating
0
100
Percent gas release
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The U-Pb technique
Half life
U-Pb is a paired decay scheme Two isotopes of U
decay to two isotopes of Pb at different rates
238U 206Pb 4.47 Ga 235U
207Pb 704 Ma
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The U-Pb technique
The decays take place via many intermediate
radioactive daughter products
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The U-Pb technique
The Wetherill concordia
206Pb/238U
4000
To common Pb
3500
Loss of radiogenic daughter can be detected as
discordance
Concordant
3000
2500
Isotopic disturbance
2000
Recent Pb loss
207Pb/235U
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The U-Pb technique
The Tera-Wasserburg concordia
207Pb/206Pb
To common Pb
Loss of radiogenic daughter can be detected as
discordance
4000
Concordant
Recent Pb loss
3500
3000
Isotopic disturbance
2500
2000
1500
238U/206Pb
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The U-Pb technique
Mineral U-Pb geochronology, pros and cons

• High radioactive parent content
• Low initial daughter content
• High closure temperatures (e.g. gt900C)
• Isotope Dilution gives exceptional analytical
  precision (0.02)
• Loss of radiogenic daughter is detectable
• Low abundance host minerals (e.g. zircon,
  monazite)
• Difficult chemistry
• Difficult mass spectrometry

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The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
207Pb/235U
Paces Miller, 1993
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The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
207Pb/235U
Paces Miller, 1993
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The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
207Pb/206Pb age 1099.01 0.58 Ma
207Pb/235U
Paces Miller, 1993
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The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
Concordia age 1099.24 0.26 Ma
207Pb/235U
Paces Miller, 1993
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The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
The accuracy of isotope dilution analyses is now
limited mainly by uncertainty in the decay
constants
207Pb/206Pb age 1099.0 5.1 Ma Concordia
age 1099.8 0.7 Ma
207Pb/235U
Paces Miller, 1993
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The U-Pb technique
ID-TIMS
Mundil et al., 2003, Triassic platform
carbonates, northern Italy
Age differences can nevertheless be measured with
high precision
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The U-Pb technique
Micro-analysis
Zircon
Accurate age measurements on complex crystals
requires micro-sampling
50 µm
Transmitted light, crossed polarisers
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Laser ablation ICP-MS
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Laser ablation ICP-MS
ArF Excimer (193 nm) laser
Lasers allow high precision micro-sampling, 100
ng per analysis
100 µm
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Secondary Ion Mass Spectrometry
SIMS
Ion microprobes sample on an even smaller
scale, 2 ng per analysis
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Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Chemical and isotopic analyses
Chemical analyses
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Secondary Ion Mass Spectrometry
How does an ion microprobe work?
High-energy primary ions bombard the target
surface
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Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Secondary ions and neutrals of atoms and
molecules are ejected
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Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Secondary ions and neutrals of atoms and
molecules are ejected
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How does an ion microprobe work?
Energy analyser
Magnet
1 METRE
Primary ion source
The sputtered secondary ions are analysed by a
large, high-resolution mass spectrometer
Ion counter
Sample
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How does an ion microprobe work?
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Why is the SHRIMP so big?
The secondary ion spectrum is complex, even from
relatively simple minerals
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Why is the SHRIMP so big?
Pb in zircon
206Pb
The mass spectrometer needs high mass resolution
to resolve the small mass differences between
atoms and molecules
HfSi
HfSi
HfO2
207Pb
HfO2
208Pb
Zr2O
Zr2O
HfSi
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Why is the SHRIMP so big?
The double-focusing mass spectrometer separates
the ions by energy and momentum
Sample
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SHRIMP U-Pb dating in practice
Sample 1
ln Pb/U
SHRIMP Pb/U measurements must be calibrated
against natural mineral standards
Sample 2
Sample 206Pb/238U
Standard analyses
Standard for sample 2
Standard for sample 1
ln UO/U
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SHRIMP U-Pb dating in practice
Pb/U ages can be measured with better than 1
precision
ln Pb/Ustd
ln Pb/Uunk
ln UO/U
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SHRIMP U-Pb dating in practice
Paterson Volcanics
Individual analyses are less precise than ID-TIMS
analyses, but they are equally accurate
ID-TIMS 328 2 Ma
SHRIMP 329 4 Ma
Claoué-Long et al., 1995
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SHRIMP U-Pb applications
Mineral ages of igneous rocks Zircon, monazite,
baddeleyite, titanite, perovskite, allanite
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SHRIMP U-Pb applications
Zircon ages of high grade orthogneiss protoliths
100 µm
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SHRIMP U-Pb applications
The Acasta Gneiss, Canada, the oldest-known rock
in the world
Bowring Williams, 1999
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SHRIMP U-Pb applications
Cooma biotite schist
Provenance of sedimentary rocks Zircon,
monazite, titanite, rutile
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Illustrating large data sets
The relative probability histogram
Allocate each measurement a Gaussian curve of
unit area reflecting its uncertainties
Relative probability
1
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Illustrating large data sets
The relative probability histogram
Sum the curves to show the relative probability
of the various ages
Relative probability
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Illustrating large data sets
The relative probability histogram
The histogram displays the probable age
distribution, considering the uncertainties
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SHRIMP U-Pb applications
Neoproterozoic metasediments in Scotland
Tectonic history and correlation of sedimentary
rocks
MOINE DALRADIAN
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SHRIMP U-Pb applications
Deposition ages from diagenetic
overgrowths Xenotime, monazite
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SHRIMP U-Pb applications
Jillamatong S-type granodiorite
Ages of the components in magmas from inherited
zircon cores
Cathodoluminescence
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SHRIMP U-Pb applications
Ages of the components in magmas from inherited
zircon cores
Relative probability
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SHRIMP U-Pb applications
Identification of likely magma source rocks
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SHRIMP U-Pb applications
Ages of metamorphic minerals Monazite, titanite
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SHRIMP U-Pb applications
Granulite-grade metapelite, Mallee Bore
Ages of high grade metamorphic zircon overgrowths
200 µm
Cathodoluminescence
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SHRIMP U-Pb applications
Ages of high grade metamorphic zircon overgrowths
50 µm
Cathodoluminescence
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SHRIMP U-Pb applications
Granulite-grade metapelite, Reynolds Range
Provenance and metamorphic ages of granulites
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SHRIMP U-Pb applications
Provenance signature
ln207Pb/206Pb
Provenance and metamorphic ages of granulites
Metamorphism
ln238U/206Pb
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Granulite stratigraphy in central Australia
A case study by David Maidment
The granulites of the Arunta Inlier are
surrounded and partly covered by remnants of
the Centralian Superbasin
ARUNTA INLIER
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Granulite stratigraphy in central Australia
ln207Pb/206Pb
Most of the granulites are definitely of mid
Proterozoic age
Metamorphism
ln238U/206Pb
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Granulite stratigraphy in central Australia
Some of the granulites are late Palaeozoic, the
same age as some of the superbasin sediments
ln207Pb/206Pb
Metamorphism
ln238U/206Pb
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Granulite stratigraphy in central Australia
Pressure (kbar)
Stable crustal geotherm
P-T estimates are consistent with metamorphism at
a depth of 30 km
Temperature (C)
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Granulite stratigraphy in central Australia
Is it possible that the granulites and low-grade
superbasin sediments are related?
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Granulite stratigraphy in central Australia
The provenance of the superbasin sediments, as
recorded by detrital zircon, changes
systematically up section
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Granulite stratigraphy in central Australia
U. Brady Gneiss

The provenance of the granulite-grade sediments,
as recorded by detrital zircon cores, also
changes systematically up section
L. Brady Gneiss
Meta igneous
Irindina Gneiss
Meta igneous
Naringa Calc-Silicate
U. Stanovos Gneiss
L. Stanovos Gneiss
Age (Ga)
Age (Ga)
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Granulite stratigraphy in central Australia
Basin sediments
Metamorphic Complex
The age patterns in the metamorphic complex and
sedimentary basin can be correlated
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Granulite stratigraphy in central Australia
Rift axis
Metamorphic complex
The metamorphic complex probably lies in a failed
Ordovician trans-continental rift
Ordovician continental margin
Aeromagnetic map of Australia
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Granulite stratigraphy in central Australia
The detrital zircon age signature in the younger
rift sediments is a Gondwana margin detrital
signature
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Granulite stratigraphy in central Australia
The most likely source of the sediments is East
Aftrica
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Main take-home points

• Because U-Pb dating uses a paired decay scheme,
  open system behaviour is much less of a problem
  than with other isotopic dating techniques.
• Individual in situ micro-analyses have much
  larger analytical uncertainties than ID-TIMS
  analyses.
• ID-TIMS is the most precise way to date simple
  crystals.
• In situ micro-analysis is the most accurate way
  to date complex crystals.
• Choose the method that best suits the problem.