The second continent: Existence of granitic continental materials around the bottom of the mantle transition zone

Kenji Kawai, Shinji Yamamoto, Taku Tsuchiya, Shigenori Maruyama

    Research output: Contribution to journalArticle

    70 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Pages (from-to)1-6
    Number of pages6
    JournalGeoscience Frontiers
    Volume4
    Issue number1
    DOIs
    Publication statusPublished - Jan 1 2013

    Keywords

    • Continental growth
    • First principles
    • Geodynamics
    • Granite
    • Mantle transition zone
    • Tectonics

    ASJC Scopus subject areas

    • Earth and Planetary Sciences(all)

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