Letter | Published:

Seismic evidence of effects of water on melt transport in the Lau back-arc mantle

Nature volume 518, pages 395398 (19 February 2015) | Download Citation

Abstract

Processes of melt generation and transport beneath back-arc spreading centres are controlled by two endmember mechanisms: decompression melting similar to that at mid-ocean ridges and flux melting resembling that beneath arcs1. The Lau Basin, with an abundance of spreading ridges at different distances from the subduction zone, provides an opportunity to distinguish the effects of these two different melting processes on magma production and crust formation. Here we present constraints on the three-dimensional distribution of partial melt inferred from seismic velocities obtained from Rayleigh wave tomography using land and ocean-bottom seismographs. Low seismic velocities beneath the Central Lau Spreading Centre and the northern Eastern Lau Spreading Centre extend deeper and westwards into the back-arc, suggesting that these spreading centres are fed by melting along upwelling zones from the west, and helping to explain geochemical differences with the Valu Fa Ridge to the south2, which has no distinct deep low-seismic-velocity anomalies. A region of low S-wave velocity, interpreted as resulting from high melt content, is imaged in the mantle wedge beneath the Central Lau Spreading Centre and the northeastern Lau Basin, even where no active spreading centre currently exists. This low-seismic-velocity anomaly becomes weaker with distance southward along the Eastern Lau Spreading Centre and the Valu Fa Ridge, in contrast to the inferred increase in magmatic productivity1. We propose that the anomaly variations result from changes in the efficiency of melt extraction, with the decrease in melt to the south correlating with increased fractional melting and higher water content in the magma. Water released from the slab may greatly reduce the melt viscosity3 or increase grain size4, or both, thereby facilitating melt transport.

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Acknowledgements

We thank P. J. Shore, Y. J. Chen and the captains, crew and science parties of the RVs Roger Revelle and Kilo Moana for data collecting; D. W. Forsyth, Y. Yang, G. G. Euler, D. Heeszel, X. Sun and W. Shen for helping with data processing; N. Harmon, C. Rychert, P. Skemer and B. M. Mahan for discussions; and N. Hu for support. IRIS PASSCAL and OBSIP provided land-based seismic instrumentation and OBSs, respectively. This work was supported by the Ridge 2000 Program under NSF grants OCE-0426408 (D.A.W. and J.A.C.), EAR-0911137 (D.A.W.), OCE-0426369 (S.C.W.), OCE-0430463 (D.K.B.) and OCE-0426428 (R.A.D.).

Author information

Affiliations

  1. Department of Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA

    • S. Shawn Wei
    •  & Douglas A. Wiens
  2. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

    • Yang Zha
    • , Terry Plank
    •  & Spahr C. Webb
  3. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA

    • Donna K. Blackman
  4. Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA

    • Robert A. Dunn
  5. Department of Geology, Southern Illinois University, Carbondale, Illinois 62901, USA

    • James A. Conder

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Contributions

S.S.W., advised by D.A.W., analysed the seismic data. T.P. downloaded and analysed the geochemical data. S.S.W. and D.A.W. took the lead in writing the manuscript, and all authors discussed the results and edited the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. Shawn Wei.

Raw seismic data are available at the Data Management Center of the Incorporated Research Institutions for Seismology (http://www.iris.edu/dms/nodes/dmc), under network IDs YL, Z1 and XB.

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  1. 1.

    Supplementary Tables

    This file contains Supplementary Table 1, which shows all the geochemical data used for calculating the melting paths.

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https://doi.org/10.1038/nature14113

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