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Melt retention and segregation beneath mid-ocean ridges

Nature volume 410, pages 920–923 (2001)Cite this article

Abstract

Geochemical models of melting at mid-ocean ridges—particularly those based on trace elements and uranium-decay-series isotopes—predict that melt segregates from the matrix at very low porosities1,2,3,4,5,6,7,8, of order 0.1%. Some of these models also require that the melt ascends rapidly3,5. But these predictions appear to conflict with seismic data obtained by the mantle electromagnetic and tomography (MELT) experiment9. These data reveal, beneath the East Pacific Rise (at 17 °S), a region of low velocities several hundred kilometres wide, which is best explained by the presence of 1–2% melt, distributed on a grain scale in disk-shaped geometries10. Here I show that these apparently contradictory constraints can be reconciled by taking into account the geometry and resulting permeability of the intergranular network of melt, together with the changing character of the melt as it ascends. A deep, volatile-rich melt with low viscosity and density is mobile at 0.1% porosity, but basaltic melt only becomes mobile at a porosity above 1%. While the volumetric contribution of the volatile-rich melt to the erupted basalts is small, the isotopic disequilibria (except for radium) generated by porous flow of this melt are preserved if melt transport is rapid at the onset of high-productivity melting. Also, because of incomplete extraction, some melt is retained in a broad zone, consistent with the MELT observations.

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Figure 1: Permeability and resulting maximum porosity in a one-dimensional melting column.
Figure 2: Comparison of isotopic disequilibria produced by different permeabilities at a solid upwelling velocity of 3 cm yr-1 and with distribution coefficients for an enriched source (Table 1).
Figure 3: Sketch of the distribution of melt beneath a mid-ocean ridge.

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Acknowledgements

Discussions with S. Eggins, D. Green and H. O'Neill are gratefully acknowledged, as well as pre-publication access to a G3 article by M. Spiegelman. Reviews by T. Elliot and M. Spiegelman helped to shape this manuscript.

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  1. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

    Ulrich H. Faul

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Correspondence to Ulrich H. Faul.

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Faul, U. Melt retention and segregation beneath mid-ocean ridges. Nature 410, 920–923 (2001). https://doi.org/10.1038/35073556

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