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Ponded melt at the boundary between the lithosphere and asthenosphere

Nature Geoscience volume 6, pages 1041–1044 (2013)Cite this article

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Abstract

The boundary between Earth’s rigid lithosphere and the underlying, ductile asthenosphere is marked by a distinct seismic discontinuity1. A decrease in seismic-wave velocity and increase in attenuation at this boundary is thought to be caused by partial melt2. The density and viscosity of basaltic magma, linked to the atomic structure3,4, control the process of melt separation from the surrounding mantle rocks5,6,7,8,9. Here we use high-pressure and high-temperature experiments and in situ X-ray analysis to assess the properties of basaltic magmas under pressures of up to 5.5 GPa. We find that the magmas rapidly become denser with increasing pressure and show a viscosity minimum near 4 GPa. Magma mobility—the ratio of the melt–solid density contrast to the magma viscosity—exhibits a peak at pressures corresponding to depths of 120–150 km, within the asthenosphere, up to an order of magnitude greater than pressures corresponding to the deeper mantle and shallower lithosphere. Melts are therefore expected to rapidly migrate out of the asthenosphere. The diminishing mobility of magma in Earth’s asthenosphere as the melts ascend could lead to excessive melt accumulation at depths of 80–100 km, at the lithosphere–asthenosphere boundary. We conclude that the observed seismic discontinuity at the lithosphere–asthenosphere boundary records this accumulation of melt.

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Figure 1: Results of the experimental study.
Figure 2: Magma properties and mobility as a function of depth.
Figure 3: Schematic illustration of the lithosphere and asthenosphere boundary.

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Acknowledgements

This work was supported by Grant-in-Aid awards for Scientific Research to E.O. (numbers 16340164, 18104009 and 22000002) and S.U. (numbers 13440163, 16340170 and 20103003) from the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government. This work was also partially supported by the Global Center of Excellence program of Earth and Planetary Science, Tohoku University, Japan. T.S. was supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2004A0250, 2004B0049, 2005A0453, 2005B0011, 2006A3705, 2006B3710, 2002B0087, 2003A0057, 2003B0036, 2004A0384, 2004B0646, 2007A1095 and 2009A1661) and Japan Atomic Energy Agency (Proposal No. 2002-S-01, 2003-S-05, 2004-S-04, 2006A-E17 and 2006B-E23). Y.W. acknowledges support from the NSF (EAR 0711057). All authors acknowledge useful discussions with and suggestions by S. Labrosse.

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Author notes

  1. John W. Hernlund

    Present address: Present address: Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan,

Authors and Affiliations

  1. Department of Earth and Planetary Materials Science, Tohoku University, Sendai 980-8578, Japan

    Tatsuya Sakamaki, Akio Suzuki & Eiji Ohtani

  2. Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan

    Hidenori Terasaki

  3. Department of Earth Sciences, Okayama University, Okayama 700-8530, Japan

    Satoru Urakawa

  4. Japan Atomic Energy Agency, Hyogo 679-5148, Japan

    Yoshinori Katayama

  5. SPring-8, Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan

    Ken-ichi Funakoshi

  6. Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60637, USA

    Yanbin Wang

  7. Department of Earth and Planetary Science, University of California, Berkeley 94720, USA

    John W. Hernlund

  8. Department of Geology and Geophysics, School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

    Maxim D. Ballmer

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Contributions

T.S., S.U., A.S., E.O. and Y.K. performed the density measurements. A.S., H.T., E.O., T.S. and K-i.F. carried out the viscosity measurements. T.S., A.S., H.T., E.O. and K-i.F. performed the structure measurements. T.S., E.O., A.S., Y.W. M.D.B. and J.W.H. made arguments on geological applications of the experimental results, and prepared the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to Tatsuya Sakamaki.

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Sakamaki, T., Suzuki, A., Ohtani, E. et al. Ponded melt at the boundary between the lithosphere and asthenosphere. Nature Geosci 6, 1041–1044 (2013). https://doi.org/10.1038/ngeo1982

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