1. No category

Lecture 17: time and metamorphism
Temperature, pressure and stress are three variables that drive metamorphic
change, but equally important is time:
at what age did a metamorphic suite reach its max T, how fast did it come back
up to the surface, how long did a contact metamorphic event last, etc etc
We obtain this information from dating:
Absolute dating of minerals
Dating of duration from kinetics
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Radioactive dating
Absolute dating is based on the regular decay of a radioactive element over
time, and is characterized by a decay constant or isotope half-life
0
Nd
Np λt
Nd
=
+
(e - 1)
Ns
Ns
Ns
We know these half-lifes reasonably well (but not perfectly - we’re still counting)
parent isotope
daughter isotope
40K
40Ar, 40Ca
87Rb
87Sr
147Sm
143Nd
176Lu
176Hf
187Re
187Os
232Th
208Pb
235U
207Pb
238U
206Pb
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half life (yr -1)
5.543 ⋅
good for
10-10
micas, plag, hbl
1.42 ⋅
10-11
micas, plag
6.54 ⋅
10-12
garnet, staurolite
1.94 ⋅
10-11
garnet, apatite
1.67 ⋅
10-11
sulfides, oxides
4.9475 ⋅
10-11
zircon, monazite
9.8485 ⋅
10-10
zircon, monazite
10-10
zircon, monazite
1.55125 ⋅
2
0
Radioactive dating - Nd
The biggest uncertainty is commonly the initial content of the daughter isotope
0
Nd
Np λt
Nd
=
+
(e - 1)
Ns
Ns
Ns
Nd
Ns
Np
Ns
0
Nd
Ns
0
Nd
Ns
age
age
time
Can go for minerals that do not contain any of the daughter element
Ca content in micas is very low, especially compared to K:
40K
→ 40Ca
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3
0
Radioactive dating - Nd
The biggest uncertainty is commonly the initial content of the daughter isotope
0
Nd
Np λt
Nd
=
+
(e - 1)
Ns
Ns
Ns
Can go for minerals that do not contain any of the daughter element
Ca content in micas is very low, especially compared to K:
40K
→ 40Ca
Can combine different minerals: N0d = fixed, but Np = rate of ingrowth, varies
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4
The U-Pb concordia
crystallization
metamorphism
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5
Mineral closure temperatures
Minerals will re-equilibrate their isotopic composition over time (e.g. they will kick out
daughter isotopes that don’t fit). However, at a certain temperature, this becomes too
slow and the age is frozen in: the closure temperature.
Different minerals have different closure temperatures and for a given mineral,
the closure temperature will be different for different isotopes
1000
800
700
600
500oC
1000
700
600
500
Rb-Sr
Nd-Sm
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300oC
400
K-Ar
Rb-Sr
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Mineral closure temperatures
Minerals will re-equilibrate their isotopic composition over time (e.g. they will kick out
daughter isotopes that don’t fit). However, at a certain temperature, this becomes too
slow and the age is frozen in: the closure temperature.
Different minerals have different closure temperatures and for a given mineral,
the closure temperature will be different for different isotopes
K-Ar
Tc (oC)
Rb-Sr
Tc (oC)
U-Pb
Tc (oC)
hornblende
550
muscovite
400-450
rutile
400-450
muscovite
350
biotite
~300
apatite
450-500
biotite
280
titanite
600
tourmaline
>700
zircon
>1000
pyrite
>550
monazite
>1000
can combine different minerals and different isotope systems to build up a
temperature-time path for a metamorphic rock
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Temperature-time path
Ms: Rb-Sr Tc = 400-450oC
K-Ar
Tc = 350oC
Bt: Rb-Sr Tc = 300oC
K-Ar
Tc = 280oC
Zrc: U-Pb Tc = >1000oC
Temperature
peak
http://minerva.union.edu/hollochk/c_petrology/old_drawings.htm
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time
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K-Ar dating of “pyrite”
Closure temperatures are for the diffusion of an element in the mineral structure: they
make no claim on how and if the element resides in the mineral structure
we can make use of this when dating phases with a low closure temperature that are
enclosed in a mineral with a high closure temperature
Most extreme example: dating fluid inclusions in Isua pyrite (Smith et al. 2005):
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Metamorphic P-T-t paths
Ms: Rb-Sr Tc = 400-450oC
K-Ar
Tc = 350oC
Bt: Rb-Sr Tc = 300oC
K-Ar
Tc = 280oC
Zrc: U-Pb Tc = >1000oC
intrusion
Temperature
peak
http://minerva.union.edu/hollochk/c_petrology/old_drawings.htm
Wednesday, March 18, 15
time
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Uplift for the Tauern region of the Alps
Uplift is a very important part of the metamorphic path, because this determines to a
large extent if we find anything of a rock’s earlier history preserved
uplift rate
in mm/yr
Rb-Sr
on Ms
3.6
K-Ar on Hbl
2.9
K-Ar on Ms
K-Ar on Bt
1.8
0.5
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Uplift and orogen erosion rates
Uplift is a very important part of the metamorphic path, because this determines to a
large extent if we find anything of a rock’s earlier history preserved
Typical erosion rate for young
mountain belts: 0.5 mm/yr
0.5 mm in 1 year
0.5 m in 1000 years
0.5 km in 1 Myr
500 km in 1 Gyr
Passive uplift by erosion is generally insufficient
to explain the rapid uplift we find in rocks. The
higher the P-T conditions, the less passive uplift
works: active uplift
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Tauern Window in the Alps
Vienna
Hoschek et al. 2010
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An example of a P-T-t path: Tauern Window, Alps
The Tauern Window shows an initial subduction P-T path, followed by transfer to the
24
overriding plate and finally uplift to the surface.
Uplift was active and fast.
22
40-50 Ma
20
18
16
ECL
14
P (kbar)
EB
12
36 Ma
10
EAM
8
27 Ma
42 Ma
6
mica
0.4 to 1.4 mm/yr needed to preserve
blueschists
ages from Zimmermann et al. 1994 (and refs therein)
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LC
2
est.
0
AM
24 Ma
AP
> ~1
Zeo
mm/yr
PPr
4
amp
GR
LB
GS
3.5 to 5
mm/yr
PrA
0
200
400
600
800
T (oC)
14
An example of a P-T-t path: Tauern Window, Alps
The Tauern Window shows an initial subduction P-T path, followed by transfer to the
24
overriding plate and finally uplift to the surface.
22
40-50 Ma
20
18
16
ECL
14
P (kbar)
EB
12
36 Ma
10
EAM
27 Ma
42 Ma
6
mica
LC
2
est.
0
AM
24 Ma
AP
> ~1
Zeo
mm/yr
PPr
4
amp
GS
3.5 to 5
mm/yr
PrA
ages from Zimmermann et al. 1994, section from Spear 1993
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GR
LB
8
0
200
400
600
800
T (oC)
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