EGU General Assembly 2008
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  Information - IS20 - GMPV40/GD29/SM19/TS18 Plume-like instabilities in the mantle - hotspots, wetspots or displaced material from the transition zone? (co-organized by GMPV, GD, SM, & TS; co-sponsored by EAG & ILP)

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This session brings together an interdisciplinary group of researchers interested in the problem of how convective instabilities in the upper mantle originate and what their relationship is to magma generation processes (“hotspots”) and lithosphere geodynamics. In recent years a number of high-resolution integrated seismic projects across regions of Tertiary to recent volcanism in central Europe have, in collaboration with detailed geochemical studies, demonstrated the existence of a number of small-scale, almost cylindrical, upwellings of low-velocity mantle material (~ 100-150 km in diameter). These “diapiric instabilities” have some characteristics in common with those of “classical” mantle plumes (e.g. thermal and geochemical anomalies, associated basement uplift), but a number of distinct differences:
1. They are much smaller in size than classical plumes
2. They do not appear to “have” a plume head
3. They appear to originate in the Transition Zone (410-660 km depth)
The existence of these small-scale plume structures suggests that there might exist a number of different classes of mantle plumes, originating from different depths within the mantle (e.g. the Transition Zone, the lower mantle or the CMB). So far such structures have only been postulated to exist beneath the European continent (e.g. the Massif Central, the Eifel and possibly the Bohemian Massif), but it is highly likely that similar structures exist beneath other continents.

The following observations can be made concerning the origin of these upper mantle plumes:
· They are small-scale convective instabilities within the upper mantle beneath Europe which appear to originate in the Transition Zone (410-660km depth)
· There is a strong correlation between the location of the “upwellings” and lithospheric architecture – suggesting some form of top-down control.
· The upwellings appear to be concentrated around the edge of a region of subducted slabs at the base of the upper mantle.
· Basaltic magmas derived by decompression partial melting of the upwelling mantle “diapirs” have the distinctive geochemical signature of a common mantle source component – the European Asthenospheric Reservoir (EAR). The EAR could be the product of outflow from one or more lower mantle plumes.

The location of these upper mantle instabilities could be controlled by a number of factors, such as:
· The regional stress field
· Inherited lithospheric structures (e.g. sutures and weak zones)
· The upwelling of hot or volatile-rich material from the deep mantle
· Dynamic mantle upwelling in response to delamination of subducted (or thickened continental) lithosphere – so-called “splash plumes”

In our session, we are particularily interested in presentations which could broaden out PLUME to other areas of the world where the Transition Zone (410-660 km) is seismically fast and might also be a "slab graveyard". We welcome especially presentations of researchers working on the mantle Transition Zone worldwide - particularly if they can see a link between recent surface volcanism and seismically fast materials in the TZ. Another area of particular interest does involve researchers interested in high pressure metamorphism of subducted oceanic crust and sediments - in the context of how much water might be recycled (in hydrous minerals) into the TZ.

Preliminary List of Solicited Speakers


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