Small-Scale Mantle Plumes: Imaging and Geodynamic Aspects

Abstract.

Columnar, upwelling masses of hot rock in the Earth’s mantle, called mantle plumes, are spectacular features of our planet because they supply the melts for the major intraplate volcanic sites. Although geodynamic modelling provides some constraints on the geometrical, physical, and fluid-dynamic properties of mantle plumes, there are still many uncertainties concerning the nature of mantle plumes, as it is difficult to image them inside the real Earth. These difficulties arise because mantle plumes are at depths of several tens to hundreds of kilometres, because they are relatively narrow (100-200 km), and because they have a small seismic velocity contrast (< 5%) relative to the surrounding mantle. Only with the help of specifically designed seismological experiments it was possible to image and identify a few mantle plumes in the last 10 years (e.g. Iceland, Massif Central, Eifel). Despite these advances a lot of uncertainties and unsolved questions remain about the physical properties of mantle plumes.

Current models for mantle plumes indicate that the buoyancy flux or mass deficit of the upwelling material varies by almost three orders of magnitude ($\sim$0.01-10 Mg/s). Besides the massive upward flows underneath large igneous provinces and major hotspots, there is now also evidence for small-scale mantle plumes. The later are related with very low or even no volcanic activity. Here I derive the physical properties (size, excess temperature, buoyancy flux, and heat flux) of the small-scale Eifel plume (Germany) based on seismological models. The Eifel plume has a width of about 100 km and extends from the upper asthenosphere ($\sim$70-80 km depth) down to at least the transition zone. Its excess temperature $\Delta T$ reaches about 100-150 K and the estimated buoyancy flux B is about 0.05$\pm$0.04 Mg/s. For the neighbouring small-scale mantle plume underneath the French Massif Central a B of about 0.09-0.7 Mg/s is determined based on existing integrated seismological and petrophysical models ($\Delta T\sim$150-200 K, $r\sim$60-75 km).

Small-scale mantle plumes may contribute significantly to the mass and heat flow in the Earth.s mantle, if they exist in a great number (e.g. more than 5000), as recently proposed by Malamud & Turcotte (1999). A major challenge for geoscientists in the next decades is to identify more of these small-scale plumes and provide unique models that are based on observations. Complementary fluid- and geodynamic simulations and petrophysical modelling is necessary to fully understand the dynamics of small-scale mantle plumes as well as their contribution to the mixing of mantle material.