New look at mantle plumes

Mantle plumes are considered to be important components of the global mantle convection. Traditionally they are thought to represent hot and therefore buoyant volumes of mantle material having similar bulk chemical composition as the rest of the mantle. This view has been recently revised after study of the Hawaii plume, which is the most productive active mantle plume with the magma flux of ca. 0.3 cubic km of magma per year. Modelling shows that such enormous magma productivity cannot be explained by anomalously high plume temperature alone (ca. 300 C higher than “normal” mantle temperature), as had been suggested in previous studies. A new explanation is provided by a combined geochemical-geophysical study (Sobolev et al., 2005). Geochemists from the Max Plank Institute in Mainz have shown that the unusually high nickel and silica contents of most Hawaiian magmas require that more than half of the melts came from a source dominated by the so-called recycled oceanic crust. This is oceanic crust, which was subducted into the mantle and converted into eclogite long ago, and was then entrained and re-melted by the ascending mantle plume. The physical consequences of this geochemical model were analyzed in the GFZ. It was shown that it predicts a magma flux 2.5 times higher than for a high-temperature plume containing no recycled oceanic crust. The new model (Fig.1) explains the recent high magmatic productivity of the Hawaiian plume very well and is also consistent with anomalous seafloor topography in the region known as Hawaiian Swell. Moreover, the predicted high degree of partial melting in recycled oceanic crust at 130-170 km depth within the Hawaiian plume explains remarkably well the seismic low velocity zone previously observed at these depths beneath the southern part of the Hawaii Island by the GFZ research team (Li et al., 2000). Large (few tens percent) amount of the heavy recycled oceanic crust in mantle plumes reduces their buoyancy by several tens of percent, which must influence mantle convection and must be considered in global convection models.

References
 

• Sobolev, A. V., Hofmann, A. W., Sobolev, S. V., Nikogosian, I. K. (2005): An olivine-free mantle source of Hawaiian shield basalts. - Nature, 434, 590-597.
• Li, X., R. Kind, K. Priestley, S.V. Sobolev, F. Tilmann and M. Weber (2000): Mapping the Hawaiian plume conduit with converted seismic waves, Nature, 405, 938-941.

Cartoon illustrating new model of the Hawaii plume