Transcript IgPetIso

Radiogenic Isotopes
In Igneous Petrology
Francis, 2013
N
P
P
Proton No.
N
Tin P = 50
Neutron No.
Radioactive Decay
Conversion of protons to neutrons and vice versa – Weak Nuclear Force
Beta decay:
87Rb

87Sr
Beta Capture:
26Al

26Mg
+ e- +  + 
t1/2 = 5.2 1010 years
 = 1.42  10-11/ yr
e+ +  + 
t1/2 = 0.72 106 years
 = 9.8  10-7/ yr
+
(positron emission):
Loss of alpha particles – residual Strong Nuclear Force
Alpha decay:
147Sm

143Nd
+
4He
+ 
t1/2 = 1.06 1010 years
 = 6.54  10-12/ yr
Radioactive Decay
+  ray
Parent
Isotope
+  ray
+  ray
-δN / δt = λ × N
-δN / N = λ × δt
ln(N) = -λ × t + No
N = No × e-λt
D* = No - N
D* = N × (eλt – 1)
D = Do + D*
D = Do + N × (eλt – 1)
t1/2 = ln 2/λ
Law of Radioactivity
Rutherford and Soddy, 1902
Some Useful Radioactive Decay Schemes:
Beta decay:

129I

176Lu

187Re

87Rb

182Hf
182W
+
129Xe
+
176Hf +
187Os +
87Sr
+
eeeee-
+
+
+
+
+





+ 
+ 
+ 
+ 
+ 
t1/2 = 9 106 years
t1/2 = 16 106 years
t1/2 = 3.5 1010 years
t1/2 = 4.56 1010 years
t1/2 = 5.2 1010 years





=
=
=
=
=
7.7  10-8 /yr
4.3  10-8/ yr
1.94  10-11/ yr
1.52  10-11/ yr
1.42  10-11/ yr
t1/2 = 0.72 106 years
t1/2 = 3.7 106 years




=
=
=
=
9.8  10-7/ yr
1.9  10-7/ yr
0.581  10-10/ yr
4.962  10-10/ yr





=
=
=
=
=
6.54  10-12/ yr
9.8485  10-10/ yr
4.9475  10-11/ yr
1.55125  10-10/ yr
Beta Capture (positron emission):

53Mn 
40K

40K

26Al
26Mg
53Cr
40Ar
40Ca
+
+
+
+
e+ +  + 
e+ +  + 
e+ +  + 
e- +  + 
combined
t1/2 = 1.250 109 years
Alpha decay:

147Sm 
235U

232Th 
238U

146Sm
+ 4He + 
143Nd +
4He + 
207Pb
+ 7 4He + 4e- + 
208Pb
+ 6 4He + 4e- + 
206Pb
+ 8 4He + 6e- + 
142Nd
t1/2 =
t1/2 =
t1/2 =
t1/2 =
t1/2 =
103 106 years
1.06 1010 years
0.7038 109 years
14.010 109 years
4.468 1010 years
Summary Radioactive Decay Schemes:
Beta decay:
87Rb

87Sr
+
e- +
+
4He
 + 
t1/2 = 5.2 1010 yrs
 = 1.42  10-11/ yr
t1/2 = 1.06 1010 yrs
 = 6.54  10-12/ yr
Alpha decay:
147Sm

143Nd
235U

207Pb
+ 
+ 7 4He
+ 4e- + 
t1/2 = 0.7038 109 yrs
 = 9.8485  10-10/ yr
 = 4.9475  10-11/ yr
232Th

208Pb
+ 6 4He
+ 4e- + 
t1/2 = 14.010 109 yrs
238U

206Pb
+ 8 4He
+ 6e- + 
t1/2 = 4.468 1010 yrs
 = 1.55125  10-10/ yr
Rb – Sr System
87Rb

87Sr
+
e- +
 + 
t1/2 = 5.2 1010 years  = 1.42  10-11/ yr
Rb+ substitutes for K+ in the large W site of phases such as feldspar, mica, and
amphibole, whereas Sr2+ substitutes for Ca2+ in feldspars. In-grown 86Sr thus sits
in a site that is not only too large for it, but which may have been damaged by the
decay process. As a result, Rb-Sr isochrons are relatively easily disturbed. This
situation is aggravated by the fact that both Rb and Sr are relatively soluble in
aqueous solutions leading to the mobility of Rb and Sr during secondary
processes such as weathering and metamorphism.
Not useful in old metamorphosed rocks.
87Sr
=
87Sr/86Sr
87Sr
i
+
87Rb
× (eλt-1)
D = Do + N × (eλt – 1)
= (87Sr/86Sr)i + (87Rb/86Sr) × (eλt-1)
2 unknowns:
t = Time
(87Sr/86Sr)i
Y = Yi + a × X
a = (eλt-1)
i
The Effect of Metamorphism
Rb – Sr System
87Rb

87Sr
+ e- +  + 
During partial melting,
Liquid (Rb / Sr) > Residue (Rb / Sr)
KRb < KSr
Small degrees partial melting fractionates the
Parent / Daughter ratio Rb/Sr, such that
liquids have higher parent / daughter ratios
and residues have lower parent / daughter
ratios.
Sm - Nd System
147Sm

143Nd
+
4He
+ 
t1/2 = 1.06 1010 years  = 6.54  10-12/ yr
143Nd
=
143Nd
143Nd /144Nd
=
i
+
147Sm
× (eλt-1)
143Nd/144Nd
i
D = Do + N × (eλt – 1)
+ (147Sm/144Nd) × (eλt-1)
Both Sm3+ and Nd3+ substitute for Al3+ in clinopyroxene, amphibole, and are also
preferentially up taken by apatite. The Sm-Nd isotopic system is significantly
more robust than the Rb/Sr system because both the parent and daughter are
happy in similar crystallographic sites, and both are relatively insoluble and thus
immobile.
The similar chemical properties of Sm and Nd, however, means that it is more
difficult to find enough spread in the parent/daugther ratio to yield a good
isochron.
Oldest Age on the Moon
Sm - Nd System
147Sm
143Nd

=
143Nd
143Nd
+
i
+
4He
+ 
147Sm
× (eλt-1)
KSm > KNd
Liquid (Sm / Nd) < Residue (Sm / Nd)
In contrast to the Rb/Sr system, in the
Sm-Nd system partial melts have
lower parent /daughter ratios and the
solid residue of partial melting has
higher parent daughter ratios.
Note: this is the reverse of the
situation in the Rb/Sr
isotopic
system.
Sm - Nd:

147Sm
143Nd
=
143Nd
143Nd
i
+
+
147Sm
4He
+ 
× (eλt-1)
During partial melting,
Liquid (Sm / Nd) < Residue (Sm / Nd)
KSm > KNd
Nd isotopic evolution
143Nd/144Nd
= (143Nd/144Nd)i + (147Sm/144Nd) × (eλt-1)
εNd
and Model Ages
MORB
εNd(t)
= ((143Nd/144NdSample(t)) / (143Nd/144NdChur(t)) - 1) × 104
Mantle Extraction Ages:
It is important to distinguish
between crystallisation age and
mantle extraction age of the
continental crust. This problem
has been addressed by DePaolo
using a combination of zircon
crystallization ages and Nd
model mantle extraction ages.
His results indicate that 80% of
the Earth's continental crust was
formed by 1.6 Ga.
Many
younger crustal rocks must thus
represent reworked older crust.
εNd(t)
MORB
= ((143Nd/144NdSample(t)) / (143Nd/144NdChur(t)) - 1) × 104
Mantle
Array
Long-term depleted source that
has been recently enriched.
2 types
of
Enrichment
The incompatible trace
element enrichment of
E-MORB is associated
with elevated 87Sr/86Sr
and 143Nd/144Nd isotopic
ratios compared to NMORB, opposite to the
correlation observed at
many hot spots, such as
Hawaii.
Continental
flood Basalts
Continental flood Basalts
Calc-Alkaline
Arcs
U, Th, and Pb Isotopic Systems
235U

207Pb
+
74He
+ 4e- + 
t1/2 = 0.7038 109 yrs  = 9.8485  10-10/ yr
232Th

208Pb
+
64He
+ 4e- + 
t1/2 = 14.010 109 yrs  = 4.9475  10-11/ yr
+ 6e- + 
t1/2 = 4.468 1010 yrs  = 1.55125  10-10/ yr
238U

206Pb
+
84He
U3-6+, Th4+, and Pb2-4+ are highly incompatible in most
rock forming minerals (K << 1), with Th and U being
more incompatible than Pb, leading to a crust with high
U/Pb ratios. Furthermore, Th and U are lithophile and
preferentially partition into large sites in accessory phases
such as zircon, apatite, perovskite, and baddelyeite. Pb,
on the other hand, is largely excluded from zircon and is
significantly chalcophile, partitioning preferentially into
sulfides. Both U and Pb are relatively easily mobilized
and the use of these radiogenic isotopes as tracers is
largely restricted to modern, unaltered igneous rocks. Th,
on the other hand, is relatively immobile, and has been
successfully used as a tracer in older metamorphosed
rocks.
U/Pb
U – Pb Concordia Diagrams
235U

207Pb
+
74He
+ 4e- + 
t1/2 = 0.7038 109 years
 = 9.8485  10-10/ yr
238U

206Pb
+
84He
+ 6e- + 
t1/2 = 4.468 1010 years
 = 1.55125  10-10/ yr
The difference in geochemical behaviour of Pb versus Th and U works to our advantage in
using zircons for dating. The low levels of common Pb in zircon, combined with zircons high
resistance to alteration make U-Pb isotopes in zircon an excellent geochronometer of the past.
U – Pb Concordia Diagrams
235U

207Pb
+
74He
+ 4e- + 
t1/2 = 0.7038 109 years
 = 9.8485  10-10/ yr
238U

206Pb
+
84He
+ 6e- + 
t1/2 = 4.468 1010 years
 = 1.55125  10-10/ yr
Mackenzie River Zircons
Zircons in the Sands of Major Rivers
Mississippi River Zircons
Amazon River Zircons
Pb Isochrons and the Age of the Solar System
Pb – Pb Isochron Diagrams
235U

207Pb
+
74He
+ 4e- + 
t1/2 = 0.7038 109 years 
= 9.8485  10-10/ yr
238U

206Pb
+
84He
+ 6e- + 
t1/2 = 4.468 1010 years
 = 1.55125  10-10/ yr
Future Ages, Mixing Lines, & Pseudo-Isochrons?
MORB
The apparent future ages of MORB and OIB can be explained by multi-stage
fractionations, but the Pb paradox remains.
The first Pb Paradox: virtually all mantle reservoirs plot to the right of
the Geochron, where are the complimentary reservoirs required for mass
balance?
The lavas within many OIB suites
define approximately linear
arrays between two chemical and
isotopic
components,
one
relatively depleted and the other
relatively enriched. Originally
these were thought to correlate
with the MORB source and
primitive mantle respectively.
However, it rapidly became
apparent that these linear arrays
were different in different OIB
suites.
There are thus many "flavours"
of OIB suites, and at least five
different
components
are
required to explain them.
Furthermore,
there
are
geographic correlations in the
isotopic characteristics of OIB
suites. For example, the DUPAL
anomaly in the south Pacific is
defined by the abundance of EM
II OIB suites that appears to
correlate with a lower mantle
seismic tomography anomaly.
Mantle Components / Reservoirs
Bulk Silicate Earth (BSE) or Primitive Mantle (PM)
87Sr/86Sr
= .7045, 143Nd/144Nd = .5126, chondrite-defined
Depleted MORB Mantle (DMM) lava characteristics:
low 87Sr/86Sr <0.7025), high 143Nd/144Nd (>0.5130), low 206Pb/204Pb (~18)
source time integrated: low Rb/Sr, high Sm/Nd, low U/Pb
nature:
Primitive mantle minus continental crust or small degree melt.
Enriched Mantle I (EM 1):
Pitcairn Is., Tristan de Cunha
Hawaii
moderate 87Sr/86Sr (~0.7050), lowest 143Nd/144Nd (~0.5124),
low 206Pb/204Pb (< 17)
source time integrated: moderate Rb/Sr, low Sm/Nd, low U/Pb
nature:
subducted lower continental crust and/or lithospheric mantle, pelagic
sediments?
lava characteristics:
Enriched Mantle II (EM 2): lava characteristics:
Samoa, Society Islands
Dupal anomaly
HIMU (high U/Pb):
St. Helena Is., Austral Is., Azores
highest 87Sr/86Sr (>0.7080), low 143Nd/144Nd (~.5125), 206Pb/204Pb (~19)
source time integrated: high Rb/Sr, low Sm/Nd, low U/Pb
nature:
subducted upper continental crust and/or sediments, also
similar to Group II kimberlites and some olivine lamproites
low 87Sr/86Sr, (~ 0.7030), high 143Nd/144Nd (~0.5129),
highest 206Pb/204Pb (>20), high Ca
source time integrated: low Rb/Sr, high Sm/Nd, high U/Pb
nature:
subducted oceanic crust that has lost Pb because of seawater alteration.
lava characteristics:
Focal Zone (FOZO:
The apparent point of convergence of the linear
isotopic arrays of many OIB suites, thus
possibly representing a mantle component that
is common to all. It has a relatively depleted
composition compared to primitive mantle in
terms of Sr and Nd isotopes, but moderately
radiogenic Pb isotopes. It is thus not the
asthenospheric
mantle
(DMM),
and
not primitive mantle. It may be the figment of
a fertile imagination.
It is important to remember that
not only is the identity of these
different mantle components a
matter of debate, there is little
constraint on their physical
location,
They
are
typically
hidden in the deep mantle
Open Systems: Bulk Contamination:
General Mixing Equation:
Two data points 1 and 2 may be related by a mixing curve between 2 end-members M
and N provided the following relationship holds:
AX + BXY + CY + D = 0
A = a2b1Y2 – a1b2Y1
B = a1b2 – a2b1
C = a2b1X1 – a1b2X2
D = a1b2X2Y1 – a2b1X1Y2
ai = denominator of Yi
bi = denominator of Xi
r = a1b2 / a2b1
Mixing lines are hyperbolic curves whose curvature is proportional to r. The asymptotes of
mixing curves with large or small r’s can be used to define some of the ratios of the unseen endmembers.
Mixing in Ratio – Ratio Plots:
Two data points may be related by a mixing curve provided the following relationship holds:
AX + BXY + CY + D = 0
A = a2b1Y2 – a1b2Y1
B = a1b2 – a2b1
C = a2b1X1 – a1b2X2
D = a1b2X2Y1 – a2b1X1Y2
ai = denominator of Yi
Mixing lines are hyperbolic curves
bi = denominator of Xi
R = a1b2 / a2b1
Mixing lines are hyperbolic curves whose curvature is proportional to r
Contamination
and
Assimilation
Open Systems:
Assimilation Fractional Crystallization (AFC)
Parent Liquid + Contaminant
Daughter Liquid + Crystal Cumulates
Analytical solution for constant Di
Ciliq = Cio × F-z + (r × Cia × (1-F-z)) / ((r-1) × z × Cio)
z = (r + Di - 1) / (r-1)
DePaolo, 1981
r = assimilation rate / crystallization rate, ≤ 1 for closed system heat budget
Computer Models:
Daughter Liquid
(1-X) × Parent Liquid - X × Crystal Cumulates + r ×X × Contaminant
Open System: Compatible Elements
Computer Models:
Daughter Liquid
(1-X) × Parent Liquid - X × Crystal Cumulates + r ×X × Contaminant
Open System: Incompatible Elements
87Sr/86Sr
in
AOB & Hy-Norm Basalts
Within the northern Canadian Cordillera,
there is a clear correlation between
isotopic composition and the tectonic
belt within which recent alkaline
magmas erupt that mirrors that observed
in Mesozoic granitoids and late
Cretaceous shoshonites. All the alkaline
basalts of the Omenica Belt are have
more radiogenic Sr and Pb isotopes and
less radiogenic Nd isotopes than their
equivalents in the Intermontane Belt.
This clearly indicates the involvement of
lithospheric mantle and/or crust in their
origin.
YT
FS
Armstrong’s
CPB
Granitoid Lines
> 0.707
CC OMB
Ra
Hf
Ed
IMB
< 0.705
Mixing in Ratio - Element Plots:
AX + BXY + CY + D = 0
maybe
A = a2Y2 – a1Y1
B = a1 – a2
b=1
C = a2X1 – a1X2
D = a1X2Y1 – a2X1Y2
ai = denominator of Yi
bi = denominator of Xi = 1
R = a1 / a2
No
Mixing lines are still hyperbolic curves
Mixing in Element -Element Plots:
AX + BXY + CY + D = 0
A = Y 2 – Y1
B=0
C = X1 – X2
D = X 2 Y1 – X1Y2
ai = denominator of Yi = 1
bi = denominator of Xi = 1
r=1
Mixing lines are straight lines
The many "flavours" of OIB suites suggest
the lower mantle is “blobby” and contains
at least 5 different components.
PM
EM2
EM1
EM1
EM2
PM
EM2
EM2
EM1
Fe-Ni Core
PM
There is typically a systematic increase in 143Nd/144Nd and decrease in
87Sr/86Sr and Pb isotopes with decreasing Si, from Hy norm basalts through to
nephelinites. The values of nephelinites approach those of MORB.
Hy-Norm
OlNeph
The many "flavours" of OIB suites suggest
the lower mantle is “blobby” and contains
at least 5 different components.
PM
EM2
EM1
EM1
EM2
PM
EM2
EM2
EM1
Fe-Ni Core
PM