Title: An Introduction to U-Pb Geochronology
1EURISPET 2008
- An Introduction to U-Pb Geochronology
- (from a SHRIMPers point of view)
- Ian S. Williams
- Research School of Earth Sciences
2Geochronology The concept
Radioactive parent
P
D/P elt - 1
l
Stable daughter
D
3Geochronology The concept
Radioactive parent
P
l
Stable daughter
D
4Geochronology The concept
Radioactive parent
After one half life D P
P
l
Stable daughter
D
5Geochronology 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
6Geochronology in practice
Some initial assumptions
- No daughter isotope was present in the system to
start with, or if some was present, the amount
can be measured. - The system has remained closed. No parent or
daughter isotopes have been added from external
sources, no parent or daughter isotopes have been
lost.
7Geochronology in practice
K-Ar and Rb-Sr mineral ages
- High abundance host minerals (e.g micas)
- High radioactive parent content
- Low initial daughter content
- Reasonably simple sample preparation
- Reasonably simple analysis
- Good analytical precision (0.5)
8Closure temperatures
Minerals only retain radiogenic daughter products
below their closure temperatures
9Closure 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
10Rb-Sr whole-rock analysis
Concept Although individual minerals might
leak, the whole rock remains closed
11Rb-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
12The 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.
13The 40Ar-39Ar technique
40Ar 39Ar
Age
Ar is released from progressively more retentive
domains by step heating
0
100
Percent gas release
14The 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
15The U-Pb technique
The decays take place via many intermediate
radioactive daughter products
16The 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
17The 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
18The 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
19The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
207Pb/235U
Paces Miller, 1993
20The U-Pb technique
ID-TIMS
Duluth Anorthosite zircon
206Pb/238U
Isotope Dilution Thermal Ionisation Mass
Spectrometry is extremely precise
207Pb/235U
Paces Miller, 1993
21The 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
22The 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
23The 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
24The U-Pb technique
ID-TIMS
Mundil et al., 2003, Triassic platform
carbonates, northern Italy
Age differences can nevertheless be measured with
high precision
25The U-Pb technique
Micro-analysis
Zircon
Accurate age measurements on complex crystals
requires micro-sampling
50 µm
Transmitted light, crossed polarisers
26Laser ablation ICP-MS
27Laser ablation ICP-MS
ArF Excimer (193 nm) laser
Lasers allow high precision micro-sampling, 100
ng per analysis
100 µm
28Secondary Ion Mass Spectrometry
SIMS
Ion microprobes sample on an even smaller
scale, 2 ng per analysis
29Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Chemical and isotopic analyses
Chemical analyses
30Secondary Ion Mass Spectrometry
How does an ion microprobe work?
High-energy primary ions bombard the target
surface
31Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Secondary ions and neutrals of atoms and
molecules are ejected
32Secondary Ion Mass Spectrometry
How does an ion microprobe work?
Secondary ions and neutrals of atoms and
molecules are ejected
33How 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
34How does an ion microprobe work?
35Why is the SHRIMP so big?
The secondary ion spectrum is complex, even from
relatively simple minerals
36Why 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
37Why is the SHRIMP so big?
The double-focusing mass spectrometer separates
the ions by energy and momentum
Sample
38SHRIMP 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
39SHRIMP 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
40SHRIMP 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
41SHRIMP U-Pb applications
Mineral ages of igneous rocks Zircon, monazite,
baddeleyite, titanite, perovskite, allanite
42SHRIMP U-Pb applications
Zircon ages of high grade orthogneiss protoliths
100 µm
43SHRIMP U-Pb applications
The Acasta Gneiss, Canada, the oldest-known rock
in the world
Bowring Williams, 1999
44SHRIMP U-Pb applications
Cooma biotite schist
Provenance of sedimentary rocks Zircon,
monazite, titanite, rutile
45Illustrating large data sets
The relative probability histogram
Allocate each measurement a Gaussian curve of
unit area reflecting its uncertainties
Relative probability
1
46Illustrating large data sets
The relative probability histogram
Sum the curves to show the relative probability
of the various ages
Relative probability
47Illustrating large data sets
The relative probability histogram
The histogram displays the probable age
distribution, considering the uncertainties
48SHRIMP U-Pb applications
Neoproterozoic metasediments in Scotland
Tectonic history and correlation of sedimentary
rocks
MOINE DALRADIAN
49SHRIMP U-Pb applications
Deposition ages from diagenetic
overgrowths Xenotime, monazite
50SHRIMP U-Pb applications
Jillamatong S-type granodiorite
Ages of the components in magmas from inherited
zircon cores
Cathodoluminescence
51SHRIMP U-Pb applications
Ages of the components in magmas from inherited
zircon cores
Relative probability
52SHRIMP U-Pb applications
Identification of likely magma source rocks
53SHRIMP U-Pb applications
Ages of metamorphic minerals Monazite, titanite
54SHRIMP U-Pb applications
Granulite-grade metapelite, Mallee Bore
Ages of high grade metamorphic zircon overgrowths
200 µm
Cathodoluminescence
55SHRIMP U-Pb applications
Ages of high grade metamorphic zircon overgrowths
50 µm
Cathodoluminescence
56SHRIMP U-Pb applications
Granulite-grade metapelite, Reynolds Range
Provenance and metamorphic ages of granulites
57SHRIMP U-Pb applications
Provenance signature
ln207Pb/206Pb
Provenance and metamorphic ages of granulites
Metamorphism
ln238U/206Pb
58Granulite 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
59Granulite stratigraphy in central Australia
ln207Pb/206Pb
Most of the granulites are definitely of mid
Proterozoic age
Metamorphism
ln238U/206Pb
60Granulite 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
61Granulite stratigraphy in central Australia
Pressure (kbar)
Stable crustal geotherm
P-T estimates are consistent with metamorphism at
a depth of 30 km
Temperature (C)
62Granulite stratigraphy in central Australia
Is it possible that the granulites and low-grade
superbasin sediments are related?
63Granulite stratigraphy in central Australia
The provenance of the superbasin sediments, as
recorded by detrital zircon, changes
systematically up section
64Granulite 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)
65Granulite stratigraphy in central Australia
Basin sediments
Metamorphic Complex
The age patterns in the metamorphic complex and
sedimentary basin can be correlated
66Granulite 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
67Granulite stratigraphy in central Australia
The detrital zircon age signature in the younger
rift sediments is a Gondwana margin detrital
signature
68Granulite stratigraphy in central Australia
The most likely source of the sediments is East
Aftrica
69Main 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.