Nature 369, 474 - 476 (09 June 1994); doi:10.1038/369474a0

Seismic evidence for silicate melt atop the 410-km mantle discontinuity

J. Revenaugh^* & S. A. Sipkin^†

^*Institute of Tectonics, University of California at Santa Cruz, Santa Cruz, California 95064, USA
^†US Geological Survey, Golden, Colorado 80401, USA

LABORATORY results demonstrating that basic to ultrabasic melts become denser than olivine-rich mantle at pressures above 6 GPa (refs 1–3) have important implications for basalt petrogenesis, mantle differentiation and the storage of volatiles deep in the Earth. A density cross-over between melt and solid in the extensively molten Archaean mantle has been inferred from komatiitic volcanism^4–6 and major-element mass balances⁷, but present-day evidence of dense melt below the seismic low-velocity zone is lacking. Here we present mantle shear-wave impedance profiles obtained from multiple-ScS reverberation mapping for corridors connecting western Pacific subduction zone earthquakes with digital seismograph stations in eastern China, imaging a 5.8% impedance decrease roughly 330 km beneath the Sea of Japan, Yellow Sea and easternmost Asia. We propose that this represents the upper surface of a layer of negatively buoyant melt lying on top of the olivine-phase transition (the 410-km seismic discontinuity). Volatile-rich fluids expelled from the partial melt zone as it freezes may migrate upwards, acting as metasomatic agents^8,9 and perhaps as the deep 'proto-souree' of kimberlites^10,11. The remaining, dense, crystalline fraction would then concentrate above 410 km, producing a garnet-rich layer that may flush into the transition zone.

References

1.	Rigden, S., Ahrens, T. J. & Stolper, E. M. Science 226, 1071−1074 (1984). | ISI | ChemPort |
2.	Agee, C. B. & Walker, D. J. geophys. Res. 93, 3437−3449 (1988). | ISI | ChemPort |
3.	Miller, G. H., Stolper, E. M. & Ahrens, T. J. geophys. Res. 96, 11831−11848 (1991).
4.	Stolper, E., Walker, D., Hager, B. H. & Hays, J. F. J. geophys. Res. 86, 6261−6271 (1981). | ISI | ChemPort |
5.	Ohtani, E. Earth planet. Sci. Lett. 67, 261−272 (1984). | Article | ISI | ChemPort |
6.	Miller, G. H., Stolper, E. M. & Ahrens, T. J. geophys. Res. 96, 11849−11864 (1991).
7.	Agee, C. B. & Walker, D. Earth planet. Sci. Lett. 90, 144−156 (1988). | Article | ChemPort |
8.	Jordan, T. H. Phil. Trans. R. Soc. A301, 359−373 (1981). | ChemPort |
9.	Menzies, M. A. & Hawkesworth, C. J. (eds) Mantle Metasomatism (Academic, London, 1987).
10.	Eggler, D. H. & Wendlandt, R. F. in Kimberlites, Diatremes and Diamonds: Their Geology, Petrology, and Geochemistry (eds Boyd, F. R. & Mayer, H. 0. A.) 330−338 (Am. Geophys. Un., Washington DC, 1979). | ChemPort |
11.	Canil, D. & Scarfe, C. M. J. geophys. Res. 95, 15805−15816 (1990).
12.	Williams, Q. & Jeanloz, R. Science 239, 902−905 (1988). | ISI | ChemPort |
13.	Stixrude, L. & Bukowinski, M. S. T. Science 250, 541−543 (1990). | ChemPort |
14.	Revenaugh, J. & Jordan, T. H. J. geophys. Res. 94, 5787−5813 (1989).
15.	Revenaugh, J. & Jordan, T. H. J. geophys. Res. 96, 19749−19762 (1991).
16.	Revenaugh, J. & Jordan, T. H. J. geophys. Res. 96, 19763−19780 (1991).
17.	Revenaugh, J. & Jordan, T. H. J. geophys. Res. 96, 19781−19810 (1991). | ISI |
18.	Gasparik, T. Contr. Miner. Petrol. 102, 389−404 (1989). | Article | ChemPort |
19.	Sipkin, S. A. & Revenaugh, J. J. geophys. Res. (in the press).
20.	Anderson, D. L. Theory of the Earth (Blackwell Scientific, Boston, Massachusetts, 1989).
21.	van der Hilst, R. D., Engdahl, E. R. & Spakman, W. Geophys. J. Int. 115, 264−302 (1993).
22.	Dziewonski, A. M., Hales, A. L. & Lapwood, E. R. Phys. Earth planet. Inter. 10, 12−48 (1975). | Article | ISI |
23.	O'Connell, R. J. & Budiansky, B. J. geophys. Res. 82, 5719−5735 (1977).
24.	Mavko, G. M. J. geophys. Res. 85, 5173−5189 (1980).
25.	Toramaru, A. & Fujii, N. J. geophys. Res. 91, 9239−9252 (1986). | ChemPort |
26.	Spiegelman, M. & McKenzie, D. Earth planet. Sci. Lett. 83, 137−152 (1987). | Article | ISI |
27.	Dziewonski, A. M. & Anderson, D. L. Phys. Earth planet. Inter. 25, 297−356 (1981). | Article | ISI |
28.	Ohtani, E. Earth planet. Sci. Lett. 67, 261−272 (1984). | Article | ISI | ChemPort |
29.	O'Nions, R. K., Evensen, N. M. & Hamilton, P. J. J. geophys. Res. 84, 6091−6101 (1979). | ChemPort |
30.	Jacobsen, S. B. & Wasserburg, G. J. J. geophys. Res. 84, 7411−7427 (1979). | ISI | ChemPort |
31.	DePaolo, D. J. Geochim. cosmochim. Acta 44, 1185−1196 (1980). | Article | ISI | ChemPort |
32.	McKenzie, D. Earth planet. Sci. Lett. 95, 53−72 (1989). | Article | ISI |