| Last modified 06/2019 |
Heat production in granitic rocks
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
| |
 |
THE LITHOSPHERE
| The site is being updated irregularly |
Number of page visits
|
| Professor Irina M. Artemieva Geology Section, IGN University of Copenhagen Øster Voldgade 10 Copenhagen DK-1350 Denmark Email: iartemieva@gmail.com |
| Heat production in granitic rocks: Global analysis based on a new data compilation GRANITE2017 |
Granitic rocks play special role in the dynamics and evolution of the Earth and its thermal regime. First, their
compositional variability, reflected in the distribution of concentrations of radiogenic elements, provides constraints
on global differentiation processes and large scale planetary evolution, where emplacement of granites is
considered a particularly important process for the formation of continental crust. Second, heat production by
radioactive decay is among the main heat sources in the Earth. Therefore knowledge of heat production in
granitic rocks is pivotal for thermal modelling of the continental lithosphere, given that most radiogenic elements
are concentrated in granitic rocks of the upper continental crust whereas heat production in rocks of the
lower crust and lithospheric mantle is negligible.
We present and analyze a new global database GRANITE2017 (with about 500 entries) on the abundances of
heat producing elements (Th, U, K) and heat production in granitic rocks based on all available published data
Statistical analysis of the data shows a huge scatter in all parameters, but the following conclusions can be made.
(i) Bulk heat production in granitic rocks of all ages is ca. 2.0 μW/m3. It is very low in Archean-Early Proterozoic
granitic rocks (1.67 ± 1.49 and 1.25 ± 0.83 μW/m3, respectively) and there is a remarkable peak in heat
production in Middle Proterozoic granites (presently 4.36 ± 2.17 μW/m3) followed by a gradual decrease towards
Cenozoic granites (3.09 ± 1.62 μW/m3). Low heat production in the ancient continental crust may be
important for preservation of cratonic lithosphere. (ii) There is no systematic correlation between the tectonically
controlled granite-type and bulk heat production, although A-type (anorogenic) granites are the most
radioactive, and many of them were emplaced in Middle Proterozoic. (iii) There is no systematic correlation
between heat flow and concentrations of radiogenic elements. (iv) The present-day global average Th/U value is
4.75 ± 4.27 with a maximum in Archean-Early Proterozoic granites (5.75 ± 5.96) and a minimum in Middle-
Late Proterozoic granites (3.78 ± 2.69). The Th/U ratio at the time of granite emplacement has a minimum in
Archean (2.78). (v) The present-day K/U ratio is close to a global estimate for the continental crust only for the
entire dataset (1.46 ± 1.63) ×104, but differs from the global ratio for each geological time, and all anomalously
high values are observed only in Archean-Early Proterozoic granites. (vi) We do not observe a systematic
difference in radiogenic heat production between Archean and post-Archean granites, but rather recognize a
sharp change in radiogenic concentrations and ratios from the Early Proterozoic to Middle Proterozoic granites.
The Proterozoic anomaly may be caused by major plate reorganizations possibly related to the supercontinent
cycle when changes in the granite forming processes may be expected, or it may even indicate a change in global
thermal regime, mantle dynamics and plate tectonics styles. (vii) Our results provide strong evidence that secular
change in the Urey ratio was not monotonous, and that plate motions may have been the fastest in Middle
Proterozoic and have been decreasing since then. (viii) We estimate the total present-day heat production in the
granitic crust as 5.8–6.8 TW and in the continental crust as 7.8–8.8 TW.
compositional variability, reflected in the distribution of concentrations of radiogenic elements, provides constraints
on global differentiation processes and large scale planetary evolution, where emplacement of granites is
considered a particularly important process for the formation of continental crust. Second, heat production by
radioactive decay is among the main heat sources in the Earth. Therefore knowledge of heat production in
granitic rocks is pivotal for thermal modelling of the continental lithosphere, given that most radiogenic elements
are concentrated in granitic rocks of the upper continental crust whereas heat production in rocks of the
lower crust and lithospheric mantle is negligible.
We present and analyze a new global database GRANITE2017 (with about 500 entries) on the abundances of
heat producing elements (Th, U, K) and heat production in granitic rocks based on all available published data
Statistical analysis of the data shows a huge scatter in all parameters, but the following conclusions can be made.
(i) Bulk heat production in granitic rocks of all ages is ca. 2.0 μW/m3. It is very low in Archean-Early Proterozoic
granitic rocks (1.67 ± 1.49 and 1.25 ± 0.83 μW/m3, respectively) and there is a remarkable peak in heat
production in Middle Proterozoic granites (presently 4.36 ± 2.17 μW/m3) followed by a gradual decrease towards
Cenozoic granites (3.09 ± 1.62 μW/m3). Low heat production in the ancient continental crust may be
important for preservation of cratonic lithosphere. (ii) There is no systematic correlation between the tectonically
controlled granite-type and bulk heat production, although A-type (anorogenic) granites are the most
radioactive, and many of them were emplaced in Middle Proterozoic. (iii) There is no systematic correlation
between heat flow and concentrations of radiogenic elements. (iv) The present-day global average Th/U value is
4.75 ± 4.27 with a maximum in Archean-Early Proterozoic granites (5.75 ± 5.96) and a minimum in Middle-
Late Proterozoic granites (3.78 ± 2.69). The Th/U ratio at the time of granite emplacement has a minimum in
Archean (2.78). (v) The present-day K/U ratio is close to a global estimate for the continental crust only for the
entire dataset (1.46 ± 1.63) ×104, but differs from the global ratio for each geological time, and all anomalously
high values are observed only in Archean-Early Proterozoic granites. (vi) We do not observe a systematic
difference in radiogenic heat production between Archean and post-Archean granites, but rather recognize a
sharp change in radiogenic concentrations and ratios from the Early Proterozoic to Middle Proterozoic granites.
The Proterozoic anomaly may be caused by major plate reorganizations possibly related to the supercontinent
cycle when changes in the granite forming processes may be expected, or it may even indicate a change in global
thermal regime, mantle dynamics and plate tectonics styles. (vii) Our results provide strong evidence that secular
change in the Urey ratio was not monotonous, and that plate motions may have been the fastest in Middle
Proterozoic and have been decreasing since then. (viii) We estimate the total present-day heat production in the
granitic crust as 5.8–6.8 TW and in the continental crust as 7.8–8.8 TW.
| by Artemieva et al., 2017 Earth Science Reviews, v. 172, 1-26 |
Heat production in granitic rocks at the time of their emplacement
based on the GRANITE2017 database.
Horizontal size of colored boxes – time span; vertical size - mean value ± 1σ.
On the top: red curve – crustal growth (Condie, 1998); color boxes –
supercontinents. Pink
shading corresponds to a global-scale Middle Proterozoic collisional event;
gray shadings
– collisional events associated with the assemblage stage of other
supercontinent.
We propose that the Middle Proterozoic peak in heat production in granites
may be related to
the supercontinent cycle with a change in the granite forming processes.
In the middle: black line – mean values of heat production (shown at the
bottom plot); blue line - globally average plate velocity (Korenaga, 2006),
assuming the Urey ratio of 0.15–0.30.
Plate velocity model is constrained assuming nearly constant intervals of
~800 Myr for the Wilson cycles.
Therefore plate velocities prior to a ~2.7 Ga supercontinent are not
constrained.
based on the GRANITE2017 database.
Horizontal size of colored boxes – time span; vertical size - mean value ± 1σ.
On the top: red curve – crustal growth (Condie, 1998); color boxes –
supercontinents. Pink
shading corresponds to a global-scale Middle Proterozoic collisional event;
gray shadings
– collisional events associated with the assemblage stage of other
supercontinent.
We propose that the Middle Proterozoic peak in heat production in granites
may be related to
the supercontinent cycle with a change in the granite forming processes.
In the middle: black line – mean values of heat production (shown at the
bottom plot); blue line - globally average plate velocity (Korenaga, 2006),
assuming the Urey ratio of 0.15–0.30.
Plate velocity model is constrained assuming nearly constant intervals of
~800 Myr for the Wilson cycles.
Therefore plate velocities prior to a ~2.7 Ga supercontinent are not
constrained.
