Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Metastable garnet in oceanic crust at the top of the lower mantle

Abstract

As oceanic tectonic plates descend into the Earth's lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite—the ‘post-garnet’ and ‘post-spinel’ transformations, respectively. Recent phase equilibrium studies1,2 have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at 800 km depth as earlier studies indicated3,4,5,6, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation7. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

Your institute does not have access to this article

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

Figure 1: The variation of the transformed volume fraction with duration at the desired temperature.
Figure 2: Transmission electron micrographs showing microstructures of the reaction zone in the post-garnet transformation of pure pyrope at 30.8 GPa and 1,273 K.
Figure 3: Comparison between the rates of the post-spinel and post-garnet transformations under subduction-zone conditions.
Figure 4: Effects of transformation kinetics on the density of oceanic crust (mid-ocean-ridge basalt, MORB) descending into the lower mantle.

References

  1. Hirose, K., Fei, Y., Ma, Y. & Mao, H. K. The fate of subducted basaltic crust in the Earth's lower mantle. Nature 397, 53–56 (1999)

    ADS  CAS  Article  Google Scholar 

  2. Akaogi, M. & Ito, E. Calorimetric study on majorite-perovskite transition in the system Mg4Si4O12-Mg3Al2Si3O12: transition boundaries with positive pressure-temperature slopes. Phys. Earth Planet. Inter. 114, 129–140 (1999)

    ADS  CAS  Article  Google Scholar 

  3. Irifune, T. & Ringwood, A. E. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600-800 km in the mantle. Earth Planet. Sci. Lett. 117, 101–110 (1993)

    ADS  CAS  Article  Google Scholar 

  4. Ringwood, A. E. Role of the transition zone and 660 km discontinuity in mantle dynamics. Phys. Earth Planet. Inter. 86, 5–24 (1994)

    ADS  CAS  Article  Google Scholar 

  5. Kesson, S. E., Fitz Gerald, J. D. & Shelley, J. M. G. Mineral chemistry and density of subducted basaltic crust at lower-mantle pressures. Nature 372, 767–769 (1994)

    ADS  CAS  Article  Google Scholar 

  6. Faust, J. & Knittle, E. The stability and equation of state of majoritic garnet synthesized from natural basalt at mantle conditions. Geophys. Res. Lett. 23, 3377–3380 (1996)

    ADS  CAS  Article  Google Scholar 

  7. Kubo, T. et al. Mechanisms and kinetics of the post-spinel transformation in Mg2SiO4 . Phys. Earth Planet. Inter. 129, 153–171 (2002)

    ADS  CAS  Article  Google Scholar 

  8. Kawai, N. & Endo, S. The generation of ultrahigh hydrostatic pressures by a split sphere apparatus. Rev. Sci. Instrum. 41, 1178–1181 (1970)

    ADS  CAS  Article  Google Scholar 

  9. Ohtani, E. et al. High pressure generation by a multiple anvil system with sintered diamond anvils. Rev. Sci. Instrum. 60, 922–925 (1989)

    ADS  Article  Google Scholar 

  10. Kondo, T. et al. Ultrahigh-pressure and high-temperature generation by use of the MA-8 system with sintered-diamond anvils. High Temp. High Press. 25, 105–112 (1993)

    CAS  Google Scholar 

  11. Kato, T., Ohtani, E., Kamaya, N., Shimomura, O. & Kikegawa, T. High Pressure Research in Mineral Physics: Application to Earth and Planetary Sciences (eds Syono, Y. & Manghnani, M. H.) 33–36 (Geophysical Monograph 67, American Geophysical Union, Washington DC, 1992)

    Google Scholar 

  12. Kato, T. et al. In situ X ray observation of high-pressure phase transitions of MgSiO3 and thermal expansion of MgSiO3 perovskite at 25 GPa by double-stage multianvil system. J. Geophys. Res. 100, 20475–20481 (1995)

    ADS  Article  Google Scholar 

  13. Hirose, K., Fei, Y., Ono, S., Yagi, T. & Yagi, T. In situ measurements of the phase transition boundary in Mg3Al2Si3O12: implications for the nature of the seismic discontinuities in the Earth's mantle. Earth Planet. Sci. Lett. 184, 567–573 (2001)

    ADS  CAS  Article  Google Scholar 

  14. Oguri, K. et al. Post-garnet transition in a natural pyrope: a multi-anvil study based on in situ X-ray diffraction and transmission electron microscopy. Phys. Earth Planet. Inter. 122, 175–186 (2000)

    ADS  CAS  Article  Google Scholar 

  15. Miyajima, N., Fujino, K., Funamori, N., Kondo, T. & Yagi, T. Garnet-perovskite transformation under conditions of the Earth's lower mantle: an analytical transmission electron microscopy. Phys. Earth Planet. Inter. 116, 117–131 (1999)

    ADS  CAS  Article  Google Scholar 

  16. Poirier, J. P., Peyronneau, J., Madon, M., Guyot, F. & Revcolevschi, A. Eutectoid phase transformation of olivine and spinel into perovskite and rock salt structures. Nature 321, 603–605 (1986)

    ADS  CAS  Article  Google Scholar 

  17. Kubo, T. et al. Formation of metastable assemblages and mechanisms of the grain-size reduction in the postspinel transformation of Mg2SiO4 . Geophys. Res. Lett. 27, 807–810 (2000)

    ADS  CAS  Article  Google Scholar 

  18. Cahn, J. W. The kinetics of grain boundary nucleated reactions. Acta Metall. 4, 449–459 (1956)

    CAS  Article  Google Scholar 

  19. Kirby, S. H., Stein, S., Okal, E. A. & Rubie, D. C. Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere. Rev. Geophys. 34, 261–306 (1996)

    ADS  Article  Google Scholar 

  20. Riedel, M. R. & Karato, S. Grain-size evolution in subducted oceanic lithosphere associated with the olivine-spinel transformation and its effects on rheology. Earth Planet. Sci. Lett. 148, 27–43 (1997)

    ADS  CAS  Article  Google Scholar 

  21. Ono, S., Ito, E. & Katsura, T. Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle. Earth Planet. Sci. Lett. 190, 57–63 (2001)

    ADS  CAS  Article  Google Scholar 

  22. Karato, S. On the separation of crustal component from subducted oceanic lithosphere near the 660 km discontinuity. Phys. Earth Planet. Inter. 99, 103–111 (1997)

    ADS  Article  Google Scholar 

  23. Kawakatsu, H. & Niu, F. Seismic evidence for a 920-km discontinuity in the mantle. Nature 371, 301–305 (1994)

    ADS  Article  Google Scholar 

  24. Niu, F. & Kawakatsu, H. Depth variation of the mid-mantle seismic discontinuity. Geophys. Res. Lett. 24, 429–432 (1997)

    ADS  Article  Google Scholar 

  25. Pawley, A. R., McMillan, P. F. & Holloway, J. R. Hydrogen in stishovite, with implications for mantle water content. Science 261, 1024–1026 (1993)

    ADS  CAS  Article  Google Scholar 

  26. Irifune, T. et al. Properties of Earth and Planetary Materials at High Pressure and Temperature (eds Manghnani, M. H. & Yagi, T.) 1–8 (Geophysical Monograph 101, American Geophysical Union, Washington DC, 1998)

    Book  Google Scholar 

  27. Anderson, O. L., Issak, D. G. & Yamamoto, S. Anharmonicity and the equation state for gold. J. Appl. Phys. 6, 1534–1543 (1989)

    ADS  Article  Google Scholar 

  28. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981)

    ADS  Article  Google Scholar 

Download references

Acknowledgements

We thank D. H. Green and B. Hibberson for discussions and information on the larger ADC anvil, and K. Fujino for providing natural pyrope crystal and comments. This work was partially supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Tomoaki Kubo.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Kubo, T., Ohtani, E., Kondo, T. et al. Metastable garnet in oceanic crust at the top of the lower mantle. Nature 420, 803–806 (2002). https://doi.org/10.1038/nature01281

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature01281

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing