Where does the Source of the Icelandic Plume lie? (Enigmatic Observations with a Dynamic Solution)

Details Where does the Source of the Icelandic Plume lie? (Enigmatic Observations with a Dynamic Solution) Where does the Source of the Icelandic Plume lie? (Enigmatic Observations with a Dynamic Solution)

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.t51d..07c&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #T51D-07

Physics

3900 Mineral Physics, 8121 Dynamics, Convection Currents And Mantle Plumes, 8180 Tomography

Scientific paper

Recent observations concerning the root of the mantle plume under the Iceland hotspot seem to be rather contradictory (Foulger and Pearson, Geophys. J. Int. 145, 2001, F1-F5). Most seismic tomographic studies find a distinct low-velocity anomaly beneath Iceland in a columnar form which extends down to the 660 km discontinuity and no deeper. This would indicate an upper-mantle plume originating above the 660 km boundary. Very high {3}He{4}He isotopic ratios and Sr/Nd/Pb characteristics measured in Icelandic lava rocks suggest however that they come from a primitive, undegassed mantle reservoir which is traditionally located in the lower mantle. In terms of classical plume dynamics it is very difficult to reconcile these observational discrepancies. We can explain this phenomenon by a newly found plume type (Cserepes and Yuen, Earth Planet. Sci. Lett. 183, 61-71, 2000): this comes from below the 660 km discontinuity with no plume stem in the lower mantle and no root situated in any thermal boundary layers of the mantle. Numerical convection modelling shows that this kind of plume ( we will designate this as a 'mid-mantle plume') can arise if the 660 km discontinuity is due to an endothermic phase transition and if it is partially penetrable by vertical convective flow. Our 3-D numerical solutions show explicitly that the spinel-perovskite transition can be such a boundary. In this case, in analogy with the avalanche-like subduction events which can cross the 660 km boundary in an episodic manner, upwellings can and must break through the discontinuity intermittently . These avalanche-like upwellings arise from a diffuse but rather limited volume below the 660 km boundary in the middle mantle, and form a genuine cylindrical plume above the depth of 660 km. Thus, these mid-mantle plumes come from the top part of the lower mantle and transport lower-mantle material to the surface, but they have no deep stem and therefore no observable seismic anomalies in the lower mantle: they appear in tomographic images as upper-mantle plumes.

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