TY - JOUR
T1 - The second continent
T2 - Existence of granitic continental materials around the bottom of the mantle transition zone
AU - Kawai, Kenji
AU - Yamamoto, Shinji
AU - Tsuchiya, Taku
AU - Maruyama, Shigenori
N1 - Funding Information:
We thank Yukio Isozaki, Bob Geller and an anonymous reviewer for constructive comments, which greatly improved our manuscript. This work was completed under the support of a JSPS Fellowship for Young Scientists to K.K., the Grant-in-Aid for Scientific Research from JSPS ( Grant No. 20001005 ) to T.T., and the Grant-in-Aid for Scientific Research from JSPS ( Grant No. 20244083 ) to S.M.
PY - 2013/1
Y1 - 2013/1
N2 - It has been thought that granitic crust, having been formed on the surface, must have survived through the Earth's evolution because of its buoyancy. At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction, subduction erosion, and continental subduction. Granitic rocks, the major constituent of the continental crust, are lighter than the mantle at depths shallower than 270 km, but we show here, based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone, and thus can be subducted in the depth range of 270-660 km. This suggests that there can be two reservoirs of granitic material in the Earth, one on the surface and the other at the base of the mantle transition zone (MTZ). The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust. Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle. This could explain the anomalous seismic-wave velocities observed around 660 km depth. The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation, chemical discontinuities between granitic material and the surrounding mantle, or a combination thereof.
AB - It has been thought that granitic crust, having been formed on the surface, must have survived through the Earth's evolution because of its buoyancy. At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction, subduction erosion, and continental subduction. Granitic rocks, the major constituent of the continental crust, are lighter than the mantle at depths shallower than 270 km, but we show here, based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone, and thus can be subducted in the depth range of 270-660 km. This suggests that there can be two reservoirs of granitic material in the Earth, one on the surface and the other at the base of the mantle transition zone (MTZ). The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust. Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle. This could explain the anomalous seismic-wave velocities observed around 660 km depth. The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation, chemical discontinuities between granitic material and the surrounding mantle, or a combination thereof.
KW - Continental growth
KW - First principles
KW - Geodynamics
KW - Granite
KW - Mantle transition zone
KW - Tectonics
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U2 - 10.1016/j.gsf.2012.08.003
DO - 10.1016/j.gsf.2012.08.003
M3 - Article
AN - SCOPUS:84871454113
VL - 4
SP - 1
EP - 6
JO - Geoscience Frontiers
JF - Geoscience Frontiers
SN - 1674-9871
IS - 1
ER -