Download Lecture 7.3 - Heat production.key

Geodynamics
Heat conduction and production
Lecture 7.3 - Heat production
Lecturer: David Whipp
[email protected]
Geodynamics
www.helsinki.fi/yliopisto
1
Goals of this lecture
•
Discuss radiogenic heat production in the Earth, how it
varies and the implications for geodynamic processes
2
Radiogenic heat production
•
Radiogenic heat production, 𝐴 or 𝐻, results from the decay
of radioactive elements in the Earth, mainly 238U, 235U, 232Th
and 40K. 𝐴 is generally used for volumetric heat production and
𝐻 for heat production by mass.
•
These elements occur in the mantle, but are concentrated in
248
Heat Transfer
the crust, where radiogenic
heating can be significant
•
Table
Typicalheat
Concentrations
of the Heat-Producing
Elements
The4.3
surface
flow in continental
regions
is ~65inmW m-2
Several Rock Types and
Average Concentrations in Chondritic
and
~37 mW m-2 isthefrom
radiogenic heat production (57%)
Meteorites
Type
Rock
Reference
undepleted (fertile) mantle
“Depleted” peridotites
Tholeiitic basalt
Granite
Shale
Average continental crust
Chondritic meteorites
U (ppm)
Concentration
Th (ppm)
K (%)
0.031
0.001
0.07
4.7
3.7
1.42
0.008
0.124
0.004
0.19
20
12
5.6
0.029
0.031
0.003
0.088
4.2
2.7
1.43
0.056
Turcotte and Schubert, 2014
3
Radiogenic heat production
•
Radiogenic heat production, 𝐴 or 𝐻, results from the decay
of radioactive elements in the Earth, mainly 238U, 235U, 232Th
and 40K. 𝐴 is generally used for volumetric heat production and
𝐻 for heat production by mass.
•
These elements occur in the mantle, but are concentrated in
248
Heat Transfer
the crust, where radiogenic
heating can be significant
•
Table
Typicalheat
Concentrations
of the Heat-Producing
Elements
The4.3
surface
flow in continental
regions
is ~65inmW m-2
Several Rock Types and
Average Concentrations in Chondritic
and
~37 mW m-2 isthefrom
radiogenic heat production (57%)
Meteorites
Type
Rock
Reference
undepleted (fertile) mantle
“Depleted” peridotites
Tholeiitic basalt
Granite
Shale
Average continental crust
Chondritic meteorites
U (ppm)
Concentration
Th (ppm)
K (%)
0.031
0.001
0.07
4.7
3.7
1.42
0.008
0.124
0.004
0.19
20
12
5.6
0.029
0.031
0.003
0.088
4.2
2.7
1.43
0.056
Turcotte and Schubert, 2014
4
Radiogenic heat production
Heat production
Rock type
By mass, 𝐻 [W kg-1]
Reference undepleated (fer/le) mantle
7.39E-12
2.44E-08
0.024
"Depleated" perido/tes
3.08E-13
1.02E-09
0.001
Tholeii/c basalt
1.49E-11
4.41E-08
0.044
Granite
1.14E-09
3.01E-06
3.008
Shale
7.74E-10
1.86E-06
1.857
Average con/nental crust
3.37E-10
9.26E-07
0.927
Chondri/c meteorites
3.50E-12
1.15E-08
0.012
By volume, 𝐴
By volume, 𝐴
[W m-3]
[µW m-3]
Calculated from Turcotte and Schubert, 2014
5
Radiogenic heat production
Heat production
Rock type
By mass, 𝐻 [W kg-1]
Reference undepleated (fer/le) mantle
7.39E-12
2.44E-08
0.024
"Depleated" perido/tes
3.08E-13
1.02E-09
0.001
Tholeii/c basalt
1.49E-11
4.41E-08
0.044
Granite
1.14E-09
3.01E-06
3.008
Shale
7.74E-10
1.86E-06
1.857
Average con/nental crust
3.37E-10
9.26E-07
0.927
Chondri/c meteorites
3.50E-12
1.15E-08
0.012
By volume, 𝐴
By volume, 𝐴
[W m-3]
[µW m-3]
Calculated from Turcotte and Schubert, 2014
•
Typical heat production values for upper crustal rocks are
2-3 µW m-3, but the crustal average is <1 µW m-3
•
What does this suggest, and why might this occur?
6
Radiogenic heat production
Stüwe, 2007
•
𝐻
Because radiogenic heat production is the result
of radioactive decay, the parent isotope
concentrations have continually decreased
throughout Earth’s history
•
Today, the total amount of radiogenic heat is
about half of what was produced in the
ArcheanS at ~3 Ga
Crustal heat production over time
7
Radiogenic heat production
Stüwe, 2007
•
𝐻
Because radiogenic heat production is the result
of radioactive decay, the parent isotope
concentrations have continually decreased
throughout Earth’s history
•
Today, the total amount of radiogenic heat is
about half of what was produced in the
ArcheanS at ~3 Ga
Crustal heat production over time
8
LETTER
RESEARCH LETTER
doi:10.1038/nature13728
Spreading continents kick-started plate tectonics
Patrice F. Rey1, Nicolas Coltice2,3 & Nicolas Flament1
Temperature (K)
Continent
c
d
e
Depth (km)
b
Depth (km)
Depth (km)
Aa
that of present-day tectonic forces driving orogenesis1. To explore the
Stresses acting on cold, thick and negatively buoyant oceanic
litho- 2,000
0 1,000
0
sphere are thought to be crucial to the initiation of subduction
and tectonic impact of a thick and buoyant continent surrounded by a stag1,2
the operation of plate tectonics , which characterizes the present- nant lithospheric lid, we produced a series of two-dimensional thermo200 was hotter mechanical
1,820 K numerical models of the top 700 km of the Earth, using
day geodynamics of the Earth. Because the Earth’s interior
293 K
in the Archaean eon, the oceanic crust may have been thicker, thereby temperature-dependent densities and visco-plastic rheologies that depend
400
making the oceanic lithosphere more buoyant than at
present3, and on temperature, melt fraction and depletion, stress and strain rate (see
400 km whether subduction and plate tectonics occurred during this time is Methods). The initial temperature field is the horizontally averaged tem600
ambiguous, both in the geological record and in geodynamic models4. perature profile of a stagnant-lid convection calculation for a mantle
Here we show that because the oceanic crust was thick and buoyant5, ,200 K hotter than at present (Fig. 1A, a and Extended Data Fig. 2). The
lateral
temperature gradients
ensures that
no convective
stresses(g cm–3)
early continents may have produced intra-lithospheric gravitational absence ofB
a Deviatoric
Reference
density
stress (MPa)
stresses large enough to drive their gravitational spreading, to initi- act on the lid, allowing100
us to isolate
dynamic effects2.8
of the3.0
continent.
300 the
500
3.2 3.4
0 depleted mantle
0 continent 225 km thick (strongly
ate subduction at their margins and to trigger episodes of subduc- A buoyant and stiff
Basaltic crust
tion. Our model predicts the co-occurrence of deep to progressively root 170 km thick overlain by felsic crust 40 km thick; see Fig. 1B, a) is
shallower mafic volcanics and arc magmatism within continents in inserted within the lid, on the left side of the domain to exploit
the symCrust
40
40
a self-consistent geodynamic framework, explaining the enigmatic metry of the problem (Fig. 1A, a). A mafic crust 15 km thick covers the
multimodal volcanism and tectonic record of Archaean cratons6. More- whole system (Fig. 1A, a), consistent with the common occurrence of
Strongly
over, our model predicts a petrological stratification and tectonic struc- thick greenstone80
covers on continents, as well as80
thick basaltic
crust on
3
depleted
ture of the sub-continental lithospheric mantle, two predictions that the oceanic lid .
are consistent with xenolith5 and seismic studies, respectively, and conOur numerical solutions show that the presence oflithospheric
a buoyant consistent with the existence of a mid-lithospheric seismic discontinuity7. tinent imparts 120
Depletion
a horizontal force large enough120
to induce mantle
a long period
The slow gravitational collapse of early continents could have kick- (,50–150 Myr) of slow collapse of the whole continental lithosphere
Strain
rate
started transient episodes of plate tectonics until, as the Earth’s inte- (Fig. 1 and Extended
Fig.
3), in agreement with the dynamics of
160 Data
–15
s–1 a continent of larger volume leads
10 19. Hence,
rior cooled and oceanic lithosphere became heavier, plate tectonics spreading for gravity currents
225
became self-sustaining.
to larger gravitational power and faster collapse. BecauseAsthenosphere
of lateral spreadPresent-day plate tectonics is primarily driven by the negative buoy- ing of the continent,
the100
adjacent
3.2 3.4
2.8pushed
3.0 under
300lithospheric
500 lid is slowly
b
0 b and Extended Data Fig. 3A,0a). For gravitational
ancy of cold subducting plates. Petrological and geochemical proxies of its margin (Fig. 1A,
Basaltic crust
subduction preserved in early continents point to subduction-like pro- stress lower than the yield stress of the oceanic lid, thickening of the margin
cesses already operating before 3 billion years (Gyr) ago8,9 and perhaps of the lid is slow, and viscous drips (that is, Rayleigh–Taylor instabilities)
40 (Extended Data Fig. 3A, a and40
Lithospheric
as early as 4.1 Gyr ago10. However, they are not unequivocal, and geo- detach from its base
b). These
instabilities,
20
mantle of
dynamic modelling suggests that the thicker basaltic crust produced by typical of stagnant-lid convection , mitigate the thermal thickening
partial melting of a hotter Archaean or Hadean mantle would have had the lid.
80
80
increased lithospheric buoyancy and inhibited subduction3,4. Mantle conWhen gravitational stresses overcome the yield stress of the lithospheric
vection under a stagnant lid with extensive volcanism could therefore lid, subduction is initiated (Fig. 1A, b and c). Depending on the half-width
have preceded the onset of subduction11. In this scenario, it is classically of the continent120
and its density contrast with the120
adjacent oceanic lid (that
Strain
rate
assumed that the transition from stagnant-lid regime to mobile-lid regime is, its gravitational power)
three
situations
can
arise: first,
subduction
Asthenosphere
–15 s–1
10
and the onset of plate tectonics require that convective stresses overcame initiates and stalls
(Extended Data Fig. 3b); second,
160the slab detaches and
160
the strength of the stagnant lid12 at some stage in the Archaean.
the lid stabilizes (Fig. 1A, d and e and Extended Data Fig. 3c); or third,
On the modern Earth, gravitational stresses due to continental buoyancy recurrent detachment of the slab continues until recycling of the oceanic
can contribute to the initiation of subduction2,13. The role of continental lid is completed, followed by stabilization (Extended Data Fig. 3d). When
9
LETTER
doi:10.1038/nature13728
Spreading continents kick-started plate tectonics
Patrice F. Rey1, Nicolas Coltice2,3 & Nicolas Flament1
Temperature (K)
Continent
Depth (km)
Aa
that of present-day tectonic forces driving orogenesis1. To explore the
Stresses acting on cold, thick and negatively buoyant oceanic
litho- 2,000
0 1,000
0
sphere are thought to be crucial to the initiation of subduction
and tectonic impact of a thick and buoyant continent surrounded by a stag1,2
the operation of plate tectonics , which characterizes the present- nant lithospheric lid, we produced a series of two-dimensional thermo200 was hotter mechanical
1,820 K numerical models of the top 700 km of the Earth, using
day geodynamics of the Earth. Because the Earth’s interior
293 K
in the Archaean eon, the oceanic crust may have been thicker, thereby temperature-dependent densities and visco-plastic rheologies that depend
400
making the oceanic lithosphere more buoyant than at
present3, and on temperature, melt fraction and depletion, stress and strain rate (see
400 km whether subduction and plate tectonics occurred during this time is Methods). The initial temperature field is the horizontally averaged tem600
ambiguous, both in the geological record and in geodynamic models4. perature profile of a stagnant-lid convection calculation for a mantle
Here we show that because the oceanic crust was thick and buoyant5, ,200 K hotter than at present (Fig. 1A, a and Extended Data Fig. 2). The
lateral
temperature gradients
ensures that
no convective
stresses(g cm–3)
early continents may have produced intra-lithospheric gravitational absence ofB
a Deviatoric
Reference
density
stress (MPa)
stresses large enough to drive their gravitational spreading, to initi- act on the lid, allowing100
us to isolate
dynamic effects2.8
of the3.0
continent.
300 the
500
3.2 3.4
0 depleted mantle
0 continent 225 km thick (strongly
ate subduction at their margins and to trigger episodes of subduc- A buoyant and stiff
Basaltic crust
tion. Our model predicts the co-occurrence of deep to progressively root 170 km thick overlain by felsic crust 40 km thick; see Fig. 1B, a) is
shallower mafic volcanics and arc magmatism within continents in inserted within the lid, on the left side of the domain to exploit
the symCrust
40
40
a self-consistent geodynamic framework, explaining the enigmatic metry of the problem (Fig. 1A, a). A mafic crust 15 km thick covers the
multimodal volcanism and tectonic record of Archaean cratons6. More- whole system (Fig. 1A, a), consistent with the common occurrence of
Strongly
over, our model predicts a petrological stratification and tectonic struc- thick greenstone80
covers on continents, as well as80
thick basaltic
crust on
3
depleted
ture of the sub-continental lithospheric mantle, two predictions that the oceanic lid .
are consistent with xenolith5 and seismic studies, respectively, and conOur numerical solutions show that the presence oflithospheric
a buoyant consistent with the existence of a mid-lithospheric seismic discontinuity7. tinent imparts 120
Depletion
a horizontal force large enough120
to induce mantle
a long period
The slow gravitational collapse of early continents could have kick- (,50–150 Myr) of slow collapse of the whole continental lithosphere
Strain
rate
started transient episodes of plate tectonics until, as the Earth’s inte- (Fig. 1 and Extended
Fig.
3), in agreement with the dynamics of
160 Data
–15
s–1 a continent of larger volume leads
10 19. Hence,
rior cooled and oceanic lithosphere became heavier, plate tectonics spreading for gravity currents
225
became self-sustaining.
to larger gravitational power and faster collapse. BecauseAsthenosphere
of lateral spreadPresent-day plate tectonics is primarily driven by the negative buoy- ing of the continent,
the100
adjacent
3.2 3.4
2.8pushed
3.0 under
300lithospheric
500 lid is slowly
b
0 b and Extended Data Fig. 3A,0a). For gravitational
ancy of cold subducting plates. Petrological and geochemical proxies of its margin (Fig. 1A,
Basaltic crust
subduction preserved in early continents point to subduction-like pro- stress lower than the yield stress of the oceanic lid, thickening of the margin
cesses already operating before 3 billion years (Gyr) ago8,9 and perhaps of the lid is slow, and viscous drips (that is, Rayleigh–Taylor instabilities)
40 (Extended Data Fig. 3A, a and40
Lithospheric
as early as 4.1 Gyr ago10. However, they are not unequivocal, and geo- detach from its base
b). These
instabilities,
20
mantle of
dynamic modelling suggests that the thicker basaltic crust produced by typical of stagnant-lid convection , mitigate the thermal thickening
partial melting of a hotter Archaean or Hadean mantle would have had the lid.
80
80
increased lithospheric buoyancy and inhibited subduction3,4. Mantle conWhen gravitational stresses overcome the yield stress of the lithospheric
vection under a stagnant lid with extensive volcanism could therefore lid, subduction is initiated (Fig. 1A, b and c). Depending on the half-width
have preceded the onset of subduction11. In this scenario, it is classically of the continent120
and its density contrast with the120
adjacent oceanic lid (that
Strain
rate
assumed that the transition from stagnant-lid regime to mobile-lid regime is, its gravitational power)
three
situations
can
arise: first,
subduction
Asthenosphere
–15 s–1
10
and the onset of plate tectonics require that convective stresses overcame initiates and stalls
(Extended Data Fig. 3b); second,
160the slab detaches and
160
10
the strength of the stagnant lid12 at some stage in the Archaean.
the lid stabilizes (Fig. 1A, d and e and Extended Data Fig. 3c); or third,
On the modern Earth, gravitational stresses due to continental buoyancy recurrent detachment of the slab continues until recycling of the oceanic
can contribute to the initiation of subduction2,13. The role of continental lid is completed, followed by stabilization (Extended Data Fig. 3d). When
b
d
Larger horizontal stresses force subduction
of buoyant oceanic lithosphere
Slab detaches, margin stabilizes
e
Depth (km)
c
Depth (km)
Continent weakens, spreads laterally
Let’s see what you’ve learned…
•
If you’re watching this lecture in Moodle, you will now be
automatically directed to the quiz!
•
References:
Rey, P. F., Coltice, N., & Flament, N. (2015). Spreading continents kick-started plate tectonics. Nature,
513(7518), 405–408. doi:10.1038/nature13728
11