R cks as Cl cks: Principles of Geochronology Nesse Ch' 5, p'9394

1
R cks as Cl cksPrinciples of
GeochronologyNesse Ch. 5, p.93-94
2
Principles of GeochronologyNesse Ch. 5, p.93-94
1. Atoms and Isotopes2. Decay Processes3.
U-Th-Pb Geochronology4. Thermochronology
3
Geochronology Radioactive decay Age
equation Useful decay systems What do we
actually date?
4
Geochronology
What is geochronology? determination of time
intervals on a geologic scale, through either
absolute or relative dating methods (Oxford
Concise Dictionary of Earth Sciences)
absolute dating involves measurements
of radioactive elements and their decay products
and knowledge of their rates of decay to
calculate an age in years for a particular rock,
mineral, or fossil
date time in years calculated from age
equation age date with an interpretation of its
significance
5
P
D
www.wasistzeit.de
6
Geochronology
Radioactive decay spontaneous change of one
atom into another atom of a
different element
Parent (radioactive)
subatomic particles energy
Daughter (radiogenic)
7
Geochronology
Radioactive decay spontaneous change of one
atom into another atom of a
different element
Parent (radioactive)
subatomic particles energy
Daughter (radiogenic)
beta particle antineutrino energy
e.g.
87Rb (Z37)
87Sr (Z38)
8
Geochronology
The age equation
Mussett Khan, Ch. 15 (geophysics text)

• t time (y)
• decay constant (y-1)
• (proportional to 1/T1/2)
• D/N daughter/parent isotope ratio
• (modified to suit system of
  interest)

9
Geochronology
The age equation
Mussett Khan, Ch. 15 (geophysics text)

• t time (y)
• decay constant (y-1)
• (proportional to 1/T1/2)
• D/N daughter/parent isotope ratio
• (modified to suit system of
  interest)

what you want to know
known from experiment
measured in lab (mass spectrometer)
10
Geochronology

• Geologically useful decay systems
• Parent daughter isotopes abundant enough to be
• measured accurately ( to ppm level)
• Decay constant matched to time frame of interest
• (billions of years? thousands of years?)
• Elements in question relatively immobile
• Minerals in question common and resistant to
  alteration
• 5. Daughter isotopes formed by radioactive
  decay only
• (not incorporated into minerals by other
  processes)

11
Geochronology
Geologically useful decay systems
Among the most widely used geochronometers
are 238U ? 206Pb 235U ? 207Pb 232Th ?
208Pb 147Sm ? 143Nd 87Rb ? 87Sr 40K ?
40Ar (40Ar/39Ar) also 187Re ? 187Os, 176Lu ?
176Hf, 14C ? 14N
12
Geochronology
Geologically useful decay systems
Among the most widely used geochronometers
are 238U ? 206Pb 235U ? 207Pb 232Th ?
208Pb 147Sm ? 143Nd 87Rb ? 87Sr 40K ?
40Ar (40Ar/39Ar) also 187Re ? 187Os, 176Lu ?
176Hf, 14C ? 14N
except for 14C, decay constants suitable for
dating materials millions ? billions of years old
13
Radioactive isotopic systems used for
geochronology
14
Geochronology
Geologically useful decay systems
Among the most widely used geochronometers
are 238U ? 206Pb 235U ? 207Pb 232Th ?
208Pb 147Sm ? 143Nd 87Rb ? 87Sr 40K ?
40Ar (40Ar/39Ar) also 187Re ? 187Os, 176Lu ?
176Hf, 14C ? 14N
generally regarded as the most useful and
reliable for dating crystallisation of
minerals in igneous and metamorphic rocks
15
U-Th-Pb Geochronology
What do we actually date??
Meaningful dates obtained from material that is
isotopically homogeneous isotopically closed
since time of formation known geological/petrologi
cal significance
16
U-Th-Pb Geochronology
What do we actually date??
Meaningful dates obtained from material that is
isotopically homogeneous isotopically closed
since time of formation known geological/petrologi
cal significance
Generally requires that we date MINERALS not
ROCKS!!! (better yet, parts of minerals)
17
Age zoning in monazite (EMP maps) must analyse
individual zones separately!
Goncalves et al., Am. Min., 2005
18
U-Th-Pb Geochronology
What do we actually date??
238U ? 206Pb 235U ? 207Pb 232Th ? 208Pb
generally regarded as the most useful and
reliable for dating crystallisation of
minerals in igneous and metamorphic rocks
U and Th occur as trace elements in many
accessory minerals (lt5 of rock)
including zircon ZrSiO4 titanite CaTiSiO5
rutile TiO2 apatite Ca5(PO4)3(OH,F,Cl) mona
zite (Ce,La,Th)PO4 xenotime YPO4
19
U-Th-Pb Geochronology

• Zircon - ZrSiO4
• round, irregular, euhedral
• all sorts of complex internal structure, cores
  sometimes visible
• UgtgtTh 0-20,000 ppm U
• little common Pb (inherited)
• magnetism NM at 1.8A, 15oFS 10oSS, poorer grains
  are more magnetic

igneous crystal with lengh-wise inclusion
indicates a lack of xenocrystic core
occurs in most felsic igneous rocks, some mafic
rocks with baddeleyite, common detrital mineral,
generated in high grade metamorphic rocks via
mineral reactions, esp. granulites.
20
U-Th-Pb Geochronology

• Monazite (Ce,La,Th)PO4
• monoclinic, yellow
• Th-rich, 3-10
• U 0.1-1
• little common Pb
• magnetism 0.3-0.5A with 15oFS 10oSS

common accessory in low-Ca igneous and
metamorphic rocks pelitic schist, slates as
nodular crystals, abundant in leucogranities with
mica, especially muscovite sometimes in
hydrothermally altered rocks
21
U-Th-Pb Geochronology

• Titanite - CaTiSiO5
• also called sphene
• rhombic shape is typical
• colourless to deep honey-brown, depending on U
• U up to 300 ppm
• some common Pb
• magnetism 1.0-1.6A, 15oFS 10oSS

common in I-type igneous plutonic rocks common
in some calc-schists and impure marbles very
common accessory mineral in mafic and pelitic
metamorphic rocks
22
U-Th-Pb Geochronology

• Rutile - TiO2
• round to euhedral
• brilliant lustre, very reflective
• orange-red to black
• U up to 100ppm
• very little common Pb
• magnetism 1.0-1.6A, 15oFS 10oSS

common in granulites and pelitic high grade
metamorphic rocks common detrital mineral due to
hardness
23
U-Th-Pb Geochronology

• Xenotime - YPO4
• tetragonal, yellow-green-brown
• UgtgtTh
• U 0.1-3
• little common Pb
• magnetism 0.3-0.5A with 15oFS 10oSS

low-Ca igneous and metamorphic rocks pelitic
schist, abundant in leucogranities with mica,
especially muscovite often much less abundant
than monazite, keen eye needed to distinguish
24
U-Th-Pb Geochronology

• Apatite - Ca5(PO4)3(OH,F,Cl)
• round to euhedral
• can be confused with zircon
• U up to 100ppm
• considerable common Pb
• magnetism NM at 1.8A, 15oFS 10oSS, poorer grains
  are more magnetic
• density about 3.0-3.2

ubiquitous in rocks rarely used for U-Th-Pb
dating but widely used in fission track and
U-Th-He analysis
25
U-Th-Pb Geochronology
What do we actually date??
Two different types of analytical methods 1.
Mineral separate analysis rock is crushed
minerals of interest are separated certain
grains are selected for dating by
hand-picking selected grains are abraded and
dissolved and/or mounted for analysis in mass
spectrometer TIMS thermal ionisation mass
spectrometry
26
U-Th-Pb Geochronology
What do we actually date??
Two different types of analytical methods 2. In
situ analysis minerals of interest are located
in thin section or rock chip selected grains
are bombarded by ion beam or laser isotopes
are analysed by mass spectrometer in some
cases EMP analysis is possible SHRIMP
sensitive high-resolution ion microprobe LA-ICP-M
S laser ablation inductively coupled plasma -
mass spectrometry
27
U-Th-Pb Geochronology
What do we actually date??
Two different types of analytical methods 1.
Mineral separate analysis (TIMS) rock is
crushed minerals of interest are
separated certain grains are selected for
dating by hand-picking selected grains are
abraded and dissolved and/or mounted for
analysis in mass spectrometer 2. In situ analysis
(SHRIMP, LA-ICP-MS) minerals of interest are
located in thin section or rock chip selected
grains are bombarded by ion beam or laser
isotopes are analysed by mass spectrometer in
some cases EMP analysis is possible
28
U-Th-Pb Geochronology
What do we actually date??
Two different types of analytical methods 1.
Mineral separate analysis (TIMS) advantage
high analytical precision disadvantage
lose petrological context 2. In situ analysis
(SHRIMP, LA-ICP-MS) advantage retain
petrological context disadvantage lower
analytical precision ideally, should do both
types of analysis
29
U-Th-Pb Geochronology

SHRIMP sensitive high-resolution ion microprobe
30
U-Th-Pb Geochronology

SHRIMP sensitive high-resolution ion microprobe

• spatial resolution down to 10 mm
• good vertical profiling
• application to damaged or complex grains
• application to metamorphic and provenance studies

31
U-Th-Pb Geochronology
vertical profiling with SHRIMP
32
Mina Gonzalito Gneiss overgrowths on cores
300-600 ppm U
33
U-Th-Pb Geochronology
What do we actually date??
Techniques available at Dal U-Th-Pb (monazite,
using EMP) Ar-Ar (hornblende, muscovite, biotite,
K-feldspar) fission tracks (zircon,
apatite) (U/Th)-He (apatite) cosmogenic nuclides
(quartz)
34
U-Th-Pb Geochronology
What do the data look like?
independent decay of 235U and 238U ? 2
independent dates from same sample if they agree
concordant if they disagree (more common)
discordant
U-Pb concordia plot
van der Pluijm Marshak, 2004 Fig. 13.9
35
U-Th-Pb Geochronology
What do the data look like?
independent decay of 235U and 238U ? 2
independent dates from same sample if they agree
concordant if they disagree (more common)
discordant
U-Pb concordia plot
concordia line along which two dates
agree discordia line joining discordant dates
has upper and lower intercepts with concordia
van der Pluijm Marshak, 2004 Fig. 13.9
36
U-Th-Pb Geochronology
What do the data look like?
independent decay of 235U and 238U ? 2
independent dates from same sample if they agree
concordant if they disagree (more common)
discordant
U-Pb concordia plot
concordant dates age of crystallisation discorda
nt dates may yield information about
post-crystallisation processes
van der Pluijm Marshak, 2004 Fig. 13.9
37
U-Th-Pb Geochronology
the Wetherill concordia diagram
38
U-Th-Pb Geochronology
What do the data look like?
Rb-Sr isochron plot
elt -1 (slope) age of sample
D/S D0/S P/S (elt -1) y b mx
S concentration of stable isotope (constant)
van der Pluijm Marshak, 2004 Fig. 13.8
39
U-Th-Pb Geochronology
Monazite petrochronology age domain maps by
EMP (Williams et al. 1999 Goncalves et al. 2005)
extract element concentrations from chemical
maps (Th, U, Pb, Y) solve age (Montel) equation
pixel- by-pixel for selected domains or entire
grain link to spot analyses and other
textural information
Goncalves et al., Am. Min., 2005
40
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting?
leucosome host
leucosome
CGB migmatites
TIMS Timmermann et al. (CJES 1997)
41
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting?
leucosome host
1457 Ma
cores age of protolith
rims age of melting
leucosome
CGB migmatites
1064 Ma
TIMS Timmermann et al. (CJES 1997)
42
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting? what about
SHRIMP?
CGB migmatites
43
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting? what about
SHRIMP?
CGB migmatites
SHRIMP Slagstad et al. (CJES 2004)
44
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting? what about
SHRIMP?
CGB migmatites
rims age of melting 1067 9 Ma
SHRIMP Slagstad et al. (CJES 2004)
45
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting? TIMS vs
SHRIMP
TIMS better analytical precision
rims (LI) age of melting 1064 18.5 Ma
CGB migmatites
rims age of melting 1067 9 Ma
SHRIMP better spatial resolution
46
Application 1 Grenville Orogen U-Pb (zircon)
time and duration of partial melting? TIMS vs
SHRIMP
TIMS better analytical precision
rims (LI) age of melting 1064 18.5 Ma
CGB migmatites
same age!! (ca. 1065 Ma)
rims age of melting 1067 9 Ma
SHRIMP better spatial resolution
47
P
D
www.wasistzeit.de