2006 lecture 12
TRANSCRIPT
-
8/8/2019 2006 Lecture 12
1/36
1
Lecture 12: Radioactivity
Questions
How and why do nuclei decay?
How do we use nuclear decay to tell time? What is the evidence for presence of now extinct
radionuclides in the early solar system?
How much do you really need to know about secularequilibrium and the U-series?
Tools
First-order ordinary differential equations
-
8/8/2019 2006 Lecture 12
2/36
2
Modes of decay
A nucleus will be radioactive if by decaying it can lower
the overall mass, leading to larger (negative) nuclearbinding energy Yet another manifestation of the 2nd Law of thermodynamics
Nuclei can spontaneously transform to lower mass nucleiby one of five processes
-decay
-decay positron emission
electron capture
spontaneous fission
Each process transforms a radioactive parent nucleus into
one or more daughter nuclei.
-
8/8/2019 2006 Lecture 12
3/36
3
-decayEmission of an -particle or 4He nucleus (2 neutrons, 2 protons)
The parent decreases its massnumber by 4, atomic number by 2.
Example: 238U -> 234Th + 4He
Mass-energy budget:238U 238.0508 amu234Th 234.04364He 4.00260
mass defect 0.0046 amu
= 6.86x10-13
J/decay= 1.74x1012 J/kg 238U= 7.3 kilotons/kg
This is the preferred decay mode of nuclei heavier than 209Biwith a proton/neutron ratio along the valley of stability
-
8/8/2019 2006 Lecture 12
4/36
4
-decayEmission of an electron (and an antineutrino) during
conversion of a neutron into a proton
The mass number does not change,the atomic number increases by 1.
Example: 87Rb -> 87Sr + e + Mass-energy budget:87Rb 86.909186 amu87Sr 86.908882
mass defect 0.0003 amu= 4.5x10-14 J/decay= 3.0x1011 J/kg 87Rb= 1.3 kilotons/kg
This is the preferred decay mode of nuclei with excessneutrons compared to the valley of stability
-
8/8/2019 2006 Lecture 12
5/36
5
-decay and electron captureEmission of a positron (and a neutrino) orcapture of an inner-
shell electron during conversion of a proton into a neutron
The mass number does not change,the atomic number decreases by 1.
Examples: 40K -> 40Ar + e+ + 50V+ e -> 50Ti +
+
In positron emission, most energy isliberated by remote matter-antimatterannihilation. In electron capture, a gamma
ray carries off the excess energy.
These are the preferred decay modes of nuclei with excess
protons compared to the valley of stability
-
8/8/2019 2006 Lecture 12
6/36
6
Spontaneous Fission
Certain very heavy nuclei, particular those with even mass
numbers (e.g., 238U and 244Pu) can spontaneously fission.Odd-mass heavy nuclei typically only fission in response to
neutron capture (e.g., 235U, 239Pu)
There is no fixed daughter product but rather astatistical distribution of fission products withtwo peaks (most fissions are asymmetric).
Because of the curvature of the valley of
stability, most fission daughters have excessneutrons and tend to be radioactive (-decays).
You can see why some of the isotopes peopleworry about in nuclear fallout are 91Sr and 137Cs.
Recoil of daughter products leavefission tracksof damage in crystals about 10 m long, whichonly heal above ~300C and are therefore
useful for low-temperature thermochronometry.
-
8/8/2019 2006 Lecture 12
7/36
7
Fundamental law of radioactive decay
Each nucleus has a fixed probability of decaying per unit
time. Nothing affects this probability (e.g., temperature,pressure, bonding environment, etc.)
[exception: very high pressure promotes electron capture slightly]
This is equivalent to saying that averaged over a largeenough number of atoms the number of decays per unit time
is proportional to the number of atoms present.
Therefore in a closed system:
dN
dt N (Equation 3.1)
N= number of parent nuclei at time t
= decay constant = probability of decay per unit time (units: s1)
To get time history of number of parent nuclei, integrate 3.1:
N t Noet
(3.2) No = initial number of parent nuclei at time t= 0.
-
8/8/2019 2006 Lecture 12
8/36
8
Definitions
The mean life
of a parent nuclide is given by the number
present divided by the removal rate (recall this later when we talkabout residence time):
N
N
1
(3.3)
The half life t1/2 of a nucleus is the time after which half theparent remains:
This is also the e-folding time of the decay:
N() Noe
Noe1
No
e
N(t1/2) No
2 Noe
t1/2 t1/2 ln2 t1/2 ln 2
.693
The activity is decays per unit time, denoted by parentheses:
N N (3.4)
-
8/8/2019 2006 Lecture 12
9/36
9
Decay of parent
0 2 3 4 5time
No
No
2
t1/2
No
e
0 2 3 4 5time
t1/2
0
-1
-2
-3
-4
-5
slope = -1
Activity
ln(N)ln(N
o)
Some dating schemes only consider measurement of parent nucleibecause initial abundance is somehow known.
14C-14N: cosmic rays create a roughly constant atmospheric 14C inventory,so that living matter has a roughly constant 14C/C ratio while it exchanges
CO2 with the environment through photosynthesis or diet. After deaththis 14C decays with half life 5730 years. Hence even through thedaughter 14N is not retained or measured, age is calculated using:
t
1
14 ln
(14
C) /C
(14C) /C o
-
8/8/2019 2006 Lecture 12
10/36
10
Radiocarbon dating in practice
-
8/8/2019 2006 Lecture 12
11/36
11
Radiocarbon dating in practice
-
8/8/2019 2006 Lecture 12
12/36
12
Evolution of daughter isotopes
Consider the daughter isotopeD resulting from decays of
parent isotopeN. There may be someD in the system at timezero, so we distinguish initialDo and radiogenicD*.
D t Do D* t
(3.5)
Under most circumstances,No is unknown, so substitute
Each decay of one parent yields one daughter (an extension
is needed for branching decays and spontaneous fission),
so in a closed system
D(t) Do No N t Do No 1 et
No Net
D(t) Do N t et1
-
8/8/2019 2006 Lecture 12
13/36
13
Evolution of daughter isotopes
Parent and daughter isotopes are frequently measured with
mass spectrometers, which only measure ratios accurately,
so we choose a third stable, nonradiogenic nuclide S such
that in a closed system S(t) = So:
(3.6)
D(t)
S(t) Do
S(t) N t
S(t)et1
t
D
S
o
D
S
t
N
S
e
t
1
0 2 3 4 5
No/S
o
t1/2
0
Daughter D/S
Parent N/S
time
C
oncentrationra
tios *
-
8/8/2019 2006 Lecture 12
14/36
14
Evolution of daughter isotopes When the initial concentration of daughter isotope can be taken as
zero, a date can be obtained using a single measurement of (D/S)tand (N/S)ton the same sample.
Example: 40K-40Ar dating Ar diffusivity is very high, so it is lost by minerals above some blocking
temperature (~350C for biotite). We assume 40Aro = 0 and measuretime since sample cooled through its blocking temperature.
If36Ar is used as the stable denominator isotope, an alternative toassuming 40Aro = 0 is to assume initial Ar of atmospheric composition.
40K/36Ar ratios are hard to measure well, so 40Ar-39Ar method is moreaccurate. The sample is irradiated with neutrons along with a neutronfluence standard of known age, converting 39K into 39Ar. 39K/40K isconstant in nature, so one gets the 40K content of the sample by step-
heating and measuring39
Ar/40
Ar ratios, which can be done very precisely. 40K has a branching decay; it can either electron capture to yield 40Ar or
-decay to 40Ca. The relevant decay constant is therefore (ec/40) Another example is U,Th-4He thermochronometry, which dates the passage of
apatite through the blocking temperature for4
He retention, ~80
C (!). This isuseful for dating the uplift of mountain ranges.
-
8/8/2019 2006 Lecture 12
15/36
15
K-Ar dating vs. Ar-Ar dating Here is an example of the relative precision of K-Ar and Ar-Ar
methods. The top point below is an Ar-Ar measurement, theothers are K-Ar.
-
8/8/2019 2006 Lecture 12
16/36
16
Isochron method Most often the initial concentration of neither parent nor daughter
is known, and more than one measurement is required to extract ameaningful date and also solve for the initial (D/S) ratio.
Ideally we need multiple samples ofequal age with equal initialratio (D/S)o but different ratios (N/S). In this case equation 3.6
defines a line on an isochron plot:
N/S
Do/So
slope = et-1
D/S
t
D
S
o
D
S
t
N
S
e
t1
y = intercept + x * slope
-
8/8/2019 2006 Lecture 12
17/36
17
Isochron method The best way to guarantee that all samples have the same initial
(D/S) ratio is to use different isotopes of the same element asDand S so that at high temperature diffusion will equalize this ratiothroughout a system.
The best way to guarantee that all samples have the same age is to
use different minerals from the same rock, which chemicallyfractionateNfromD when they crystallize. The whole rock canalso form a data point.
Example 1: 87Rb-87Sr The parent is 87Rb, half-life = 48.8 Ga
The daughter is 87Sr, which forms only 7% of natural Sr.
The stable, nonradiogenic reference isotope is 86Sr.
t
87Sr
86Sr
o
87Sr
86Sr
t
87Rb
86Sr
e
87t1
-
8/8/2019 2006 Lecture 12
18/36
18
Example 1: Rb-Sr systematics Rb is an alkali metal, very incompatible during melting,
with geochemical affinity similar to K.
Sr is an alkaline earth, moderately incompatible duringmelting, with geochemical affinity similar to Ca.
Age of the Chondritic meteoritesfrom Rb-Sr isochron: Acompilation of analyses of manymineral phases from manychondrites define a high precision
isochron with an age of 4.56 Gaand an initial 87Sr/86Sr of 0.698
implies solar nebula in chondriteformation region was well-mixedfor Sr isotope ratio and all
chondrites formed in a short time.
Rb+
Sr2+
Igneous processes like melting and crystallization thereforereadily separate Rb from Sr and generate a wide separation ofparent-daughter ratios ideal for quality isochron measurements.
-
8/8/2019 2006 Lecture 12
19/36
19
Example 2: Sm-Nd systematics Parent isotope is 147Sm, alpha decay half-life 106 Ga.
Daughter isotope is143
Nd, 12% of natural Nd. Stable nonradiogenic reference isotope is 144Nd.
t
143Nd
144
Nd
o
143Nd
144
Nd
t
147Sm
144
Nd
e
147t1
Nd isotopes are useful not only for dating but as tracers of large-scale geochemical differentiation. For these purposes, Nd isotope
ratios are given in the more convenient form Nd:
Nd t sample
143Nd
144Nd
CHUR
143Nd
144Nd
1
104
Nd 0 sample
143Nd
144Nd
sample
147Sm
144Nd e147t 1
CHUR
143Nd
144Nd
CHUR
147Sm
144Nd e147t 1
1
104
where CHUR is the chondritic uniform reservoir, the evolution of a reservoir withbulk earth or bulk solar system Sm/Nd ratio and initial 143Nd/144Nd.
(3.7)
-
8/8/2019 2006 Lecture 12
20/36
20
Example 2: Sm-Nd systematics Both Nd and Sm are Rare-Earth elements (REE or lanthanides), a coherent
geochemical sequence of ions of equal charge (+3), smoothly decreasing ionic
radius from La to Lu, and hence smooth variations in partition coefficients.
In most minerals, Nd is more incompatible than Sm (opposite of Rb-Sr system,where daughter Sr is more compatible than parent Rb). Hence after a partialmelting event, the rock crystallized from the extracted melt phase has a lower
Sm/Nd ratio than the source whereas the residual solids have a higher Sm/Ndratio than the source.
residue
crust
1
10
0.1
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb L u
primitive source
Sample
CI
chondrite
Normalizing concentration of
each element to CI chondriteserves two purposesit makesprimitive (aka chondritic)compositions a flat line and ittakes out the sawtooth patternfrom the odd-even effect in thesolar abundances.
-
8/8/2019 2006 Lecture 12
21/36
21
Example 2: Sm-Nd systematics
Since the rock crystallized from
the extracted melt phase has alower Sm/Nd ratio than thesource, it evolves with time to aless radiogenic isotope ratio.
Since the residual solids have ahigher Sm/Nd ratio than thesource they evolve with time to amore radiogenic isotope ratio.
143 Nd144 Nd
time
age
0 (today) 4.5 Ga
.511847
CHUR
meltingevent
residue
crust
Initial Nd isotope ratios are reported by extrapolating back to the measured orinferred age of the sample and comparing to CHUR at that time.
Thus, Nd(t)=0 in an igneous rock implies that the source was chondritic(or primitive) at the time of melting.Typical continental crust has Nd=-15 (requires remelting enriched source!) Typical oceanic crust has Nd=+10 (requires remelting depleted source!).
This is evidence that the upper mantle (from which oceanic crust recently
came) is depleted, and that the complementary enrichedreservoir is thecontinents. The mean age of depletion of the upper mantle is ~2.5 Ga.
One-stage Nd evolution
-
8/8/2019 2006 Lecture 12
22/36
22
Example 3: Extinct nuclides
Since the parent is extinct, we cannot use equation 3.6 to measure an isochron
We can show that certain nuclei with half-lives between ~1 and 100 Ma werepresent in the early solar system even though they are extinctnow.
Chronometry based on these short-lived systems gives superior time resolutionfor studies of early solar system processes.
Example: 26Al-26Mg
half-life of26Al is 0.7 Ma. It is present in supernova debris.
t
D
S
o
D
S
t
N
S
e
t1N 0,t
1
t
D
S
o
D
S
0 1 ?
Instead, to interpret measured (D/S) ratios we need another, stable isotope S2of the same element as short-lived parentN, so that we can expect (N/S2)owas constant. This gives a new equation for a line:
tDS
t o
DS o
NS o
DS o
NS2
S2S
-
8/8/2019 2006 Lecture 12
23/36
23
Example 3: Extinct nuclides
Wasserburg used stable 27Al as thesecond, stable isotope of Al to provethat 26Al was present when the Ca,Al-
rich inclusions in chondrites formed.
He demonstrated a correlation between26Mg/24Mg and Al/Mg amongcoexisting mineral phases.
The correlation proves the presence oflive 26Al when the inclusion formed,and the slope is the initial 26Al/Al ratio,~5 x 10-5 in the oldest objects.
Given estimates of26Al production insupernovae, this places a maximum ofa few million years betweennucleosynthesis and condensation of
solids in the solar system!
Example: 26Al-26Mg
half-life of26Al is 0.7 Ma. It is present in supernova debris.
-
8/8/2019 2006 Lecture 12
24/36
24
Joys of the U,Th-Pb system 238U decays to 206Pb through an elaborate chain of 8 -decays and 6 -decays,each with its own decay constant. To understand U-Pb (or Th-Pb)
geochronology, we need to understand decay chains.
238 U
234 Th
92
91
90
144 145 146
#
protons
# neutrons
#nucleo
ns
234
235
236
2
37
238
235 U
231 Th 232 Th
234 U
234 Pa231 Pa
230 Th228 Th227 Th
228 Ac227 Ac
223 Ra224 Ra 226 Ra 228 Ra
223 Fr
219 Rn220 Rn 222 Rn
219 At218 At215 At
218 Po216 Po215 Po214 Po210 Po211 Po 212 Po
215 Bi214 Bi210 Bi211 Bi 212 Bi
214 Pb212 Pb211 Pb210 Pb206 Pb207 Pb 208 Pb
210 Tl206 Tl207 Tl 208 Tl
206 Hg
89
88
87
86
85
84
83
82
81
80
141 142 143138 139 140135 136 137132 133 134129 130 131126 127 128124 125
229
230
231
232
233
224
225
226
227
228
219
220
221
222
223
214
215
216
217
218
212
213
210
211
209
207
208
4.5Ga0.7Ga
14Ga
247k a
7 h33 ka
24 d26 h80 ka2 a18 d
6 h22 a
6 a1.6ka4 d11 d
22 m
4 d55 s4 s
1 m2 s0.1m s
3 m0.15 s2 m s0.2m s0.3 s0.5 s138 d
7 m20 m61 m2 m5 d
27 m11 h36 m21 a
1 m3 m5 m4 m
7.5 m
Decay series of 238 U, 235 U, and 232 Th
-decay -decay
232 Th chain
s = 10 -6 seconds
ms = 10-3
secconds
s = seconds
m = minutes
h = hours
d = days
a = years
ka = 103
years
Ga = 109
years
half-life unit abbreviations:
235 U chain
238 U chain
(mas
sn
umb
ers
modul
o4
)
0
3
2
(length of
chain)
-deca
ys
-decay
s
6
7
8
4
4
6
-
8/8/2019 2006 Lecture 12
25/36
25
Decay chain systematics
Consider a model system of three isotopes:
N11 N2
2 N3
ParentN1 decays toN2. Intermediate daughterN2 decaystoN
3. Terminal daughterN
3is stable.
Evolution of this system is governed by coupled equations:
dN1dt
1N1 dN2dt
1N1 2N2 dN3dt
2N2
Solution forN1 is already known (eqn. 3.2), so we have:dN2dt
1N1oe
1t 2N2dN3dt
2N2N1 t N1oe
1t
-
8/8/2019 2006 Lecture 12
26/36
26
Decay chain systematics
The general solution for n isotopes in a chain was obtained
by Bateman (1910); for our 3 isotope case:
N2 (t) 1
2 1N1
oe
1t e2t N2oe2t (3.8a)N3(t) N1
o1 1
2 11e
2t 2e1t N2
o1 e2t N3
o (3.8b)
The behavior of this system depends on 1/2. Solutions fall into twoclasses. For 1/2>1, all concentrations and ratios are transient:
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25
N / N
1o
t/ 1
1/2=5N1
N2 N3
-
8/8/2019 2006 Lecture 12
27/36
27
Decay chain systematicsFor 1/2
-
8/8/2019 2006 Lecture 12
28/36
28
Decay chain systematics
Consider further the case 1/2 10-12 s-1)
In this case 21 ~ 2, so 3.8a simplifies to:
N2 (t) 1
2
N1o
e1t e2t N2oe2t (3.9)
Since 2 > 1, the e2t terms decay fastest, and after about5 mean-lives ofN2, we have
N2 (t)t5/2
1
2
N1oe
1t 1
2
N1(t)
2N2 1N1 N2 N1 This is the condition of secular equilibrium: the activities
of the parent and of every intermediate daughter are equal.
The concentration ratios are fixed to the ratios of decayconstants.
(3.10)
-
8/8/2019 2006 Lecture 12
29/36
29
Applications of U-series disequilibria Violations of secular equilibrium are extremely useful for studying
phenomena on timescales comparable to the intermediate half-lives,e.g.:
230Th, t1/2 = 75000 years
226Ra, t1/2 = 1600 years
210Pb, t1/2 = 21 years Some systems incorporate lots of daughter and essentially no parent
when they form. The daughter is unsupported and acts like the parent ofan ordinary short-lived radiodecay scheme. Example: measuring
accumulation rates in pelagic sediments, where Th adsorbs on particlesbut U remains in solution.
Some systems incorporate lots of parent and essentially no daughter.Surprisingly, the daughter grows in on the time scale of its own decay,not that of the parent. Example: corals readily incorporate U and
exclude Th during CaCO3 growth. In this caseN2o = 0, e1t~1, and
230Th
234U
230
Th 238
U 1 e230t
-
8/8/2019 2006 Lecture 12
30/36
30
Applications of U-series disequilibria During partial melting, the partition coefficients of parents and
daughters may differ, producing a secular disequilibrium in melt andresidue.
For the timescales of mantle melting and melt extraction to the crust,the relevant isotopes are 230Th (75 ka), 231Pa (33 ka), and 226Ra (1.6 ka)
During melting in the mantle at pressure
2.5 GPa, the mineral garnetpreferentially retains U over Th, leading to excess (230Th) in the melt.The melt would return to secular equilibrium within ~350 ka, so thepresence of excess (230Th) in erupted basalts proves both the role ofgarnet in the source region and fast transport of melt to the crust.
(238
U)/(232
Th)
( 232 Th)
( 230 Th)
melting
d e c a
y
equilin
e
-
8/8/2019 2006 Lecture 12
31/36
31
U,Th-Pb geochronology On timescales long enough that all intermediate nuclei reach secular
equilibrium, U and Th systems can be treated as simple one-step decays to Pb.
dNndt
n1Nn1 L 1N1
Nn(t) Nio
i2
n N1o e1t1 Nn
o N1o e1t1
t
206
Pb204Pb
o
206
Pb204Pb
t
238
U204Pb
e238t1
t
207Pb
204Pb
o
207Pb
204Pb
t
235U
204Pb
e235t
1t
208Pb
204Pb
o
208Pb
204Pb
t
232Th
204Pb
e
232t1
238U, t1/2=4.5 Ga
235
U, t1/2=0.7 Ga
232Th, t1/2=14 Ga
-
8/8/2019 2006 Lecture 12
32/36
32
U,Th-Pb geochronology
*207Pb
204Pb
*206Pb204
Pb
t
207Pb
204Pb
o
207Pb
204Pb
t
206
Pb204Pb
o
206
Pb204Pb
t
235U
238
U
e235t1
e
238t1 Conveniently, 235U/238U is globally constant (except for an ancient
natural fission reactor in Gabon, and perhaps near Oak Ridge, TN) at1/138. One does not have to measure U at all for this method.
Since 207Pb-206Pb age depends only on Pb isotope ratios, not Pb or Uconcentration, it is not affected by recent alteration whether Pb-loss or
U-loss. Only addition of contaminant Pb or aging after alteration willaffect the measured age (still need to correct for common Pb).
Each of these chronometers can be used independently. If they agree,the sample is said to be concordant. However, Pb is mobile in many
environments, and samples often yield discordantages from the 238U-206Pb, 235U-207Pb, and 232Th-208Pb chronometers.
Discordance due to recentPb loss, such as during weathering, isresolved by coupling the two U-Pb systems to obtain a 207Pb-206Pb date
U Th Pb geochronology
-
8/8/2019 2006 Lecture 12
33/36
33
U,Th-Pb geochronology Any concordant group of samples plots on an isochron line in
(207Pb/204Pb)*-(206Pb/204Pb)* space; the age is calculable from its slope.
Initial Pb isotope ratios can be neglected for many materials with veryhigh U/Pb ratios (e.g., old zircons), or measured on a coexisting mineralwith very low U/Pb ratio (e.g., feldspar, troilite).
10
11
12
13
14
15
16
1718
19
9 11 13 15 17 19 21 23
( 206 Pb/204 Pb)
( 207 P
b /
204
P b )
4.56 Ga ISOCHRON
evolution curves
=5
=8
=10=12
initial Pb (Canyon Diablo FeS, Patterson 1955)
In 1955 C.C. Patterson measured initial Pb in essentially U-free troilite (FeS)grains in the Canyon Diablo meteorite and thereby determined the initial Pb
isotope composition of the solar system. It follows from measurements ofterrestrial Pb samples that the Pb-Pb age of the earth is 4.56 Ga, and that theearth has evolved with a =(238U/204Pb) ratio of about 9 (chondrite value = 0.7)
U Th Pb geochronology
-
8/8/2019 2006 Lecture 12
34/36
34
U,Th-Pb geochronology If Pb was lost long enough in the past for continued decay of U to have
any significant effect on Pb isotopes, the 207Pb-206Pb may be impossible
to interpret correctly. In this case, we turn to the concordia diagram (G.Wetherill). Consider the family of all concordant compositions:
t
206Pb
204
Pb
o
206Pb
204
Pb
t
238U
204Pb
*206
Pb238U
e238t1
*207Pb
235U
e235t1
These equations parameterize a curve in (206Pb/238U)*(207Pb/235U)*space, the concordia.
0
0.2
0.4
0.6
0.8
0 5 10 15 20 25 30207 Pb*/235 U
0.51.0
1.5
2.0
2.5
3.0
3.5
concordant sample after 3.0 Ga
206 Pb*238 U
concordia
age in Ga
U Th Pb geochronology
-
8/8/2019 2006 Lecture 12
35/36
35
U,Th-Pb geochronology Imagine that a suite of samples underwent a single short-lived episode
of Pb-loss at some time. This event did not fractionate 206Pb from 207Pb,
so it moved the samples along a chord towards the origin in theconcordia plot:
If these now discordant samples age as closed systems, they remain on a line,whose intercepts with the concordia evolve along the concordia with time
0
0.2
0.4
0.6
0.8
0 5 10 15 20 25 30( 207 Pb*/235 U)
0.5
1.01.5
2.0
2.5
3.0
3.5
206
Pb*238 U
discordant samples at time of Pb-loss,
3.0 Ga after crystallization
0
0.2
0.4
0.6
0.8
0 5 10 15 20 25 30(207 Pb*/235 U)
0.5
1.01.5
2.0
2.5
3.03.5
discordant samples 0.5 Ga after Pb-loss,
3.5 Ga after crystallization
206 Pb*238 U
U Th Pb geochronology
-
8/8/2019 2006 Lecture 12
36/36
36
U,Th-Pb geochronology Example: the oldest zircons on Earth (actually, the oldest anything on
Earth), from the Jack Hills conglomerate in Australia
P k l GCA 65 4215 2001