Mantle plumes - both deep and shallow

Mantle plumes are persistent upwellings of hot mantle material that are typically associated with mafic volcanism at hot spots such as Hawaii.  A clear understanding of the nature and depth of mantle plumes has been limited by our general lack of knowledge of the characteristics of the deep mantle, and by the difficulties in resolving seismic signals that pass through the lower mantle.

Geoscientists from Princeton University have developed a new mathematical approach for processing seismic data, enabling them to model the lower mantle more precisely than before (Montelli et al., 2004).  They have processed data from over 1,500,000 seismic records to create tomographic images of temperature anomalies in the mantle, with the specific objective of determining the diameter and depth of mantle plumes.

All of the 32 plumes studied appear to have diameters in the range 200 to 800 km, and at least 6, and possibly as many as 12 of the plumes extend down as far as the D” layer in the lower mantle.  On the other hand, as many as 8 of the plumes appear to be restricted to the upper mantle, and are interpreted to originate near to the 660-670 km deep mantle phase-change boundary.  (see http://www.mala.bc.ca/~earles/mantle-nov99.htm)  Some of the remaining plumes studied may originate at intermediate depths, although in many cases the results are ambiguous and some of these plumes could have deeper sources (Montelli et al., 2004).

Locations of some of the relatively deep and relatively shallow mantle plumes are shown on the map below.  The ones depicted here as being deep are described by Montelli et al. (2004) as having velocity anomalies down to the 2800 km depth level, while those labelled as "shallow" have maximum anomaly depths of 650 to 1000 km.

Velocity anomaly representations of both deep and shallow mantle plumes are shown below.  Areas with relatively low wave velocities (ie. warm mantle) are shown in yellows and reds.

Velocity anomalies for two "deep" mantle plumes Velocity anomalies for two "shallow" mantle plumes

Montelli et al. (2004) conclude that the deep plumes originate near to the CMB, and that at least some of the shallow plumes originate near to the 670 km phase-change boundary.   Although there is little evidence for a thermal boundary at 670 km, the authors suggest that this boundary could affect mantle flow dynamics such that plumes are generated in this region.  They also state that their results are consistent with mantle plume velocities of less than 10 cm/y.

Reference

Montelli, R, Nolet G, Dahlen, F, Masters G, Engdahl, E and Hung, S., 2004, Finite-frequency tomography reveals a variety of mantle plumes, Science, Vol. 303, p. 338-343. (January 2004)

Steven Earle, 2004. Malaspina University-College, Geology Department, Return to Earth Science News