New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 22

including geodynamics, petrology, mineral physics, geochemistry, and seismology.
New opportunities naturally arise from these interactions. For example, improved
resolution of mantle seismic heterogeneity provides better constraints on candidate
reservoirs and places limits on their compositions and geodynamic behavior. A
dramatic example of a recent interdisciplinary advancement on this topic is provided
by the discovery of two huge lower-mantle provinces with distinctive material
properties (see Figure 2.5). These are the Southern Pacific and African Large Low
Shear wave Velocity Provinces (LLSVPs) with several thousand-kilometer
dimensions extending upward from the core-mantle boundary hundreds of kilometers
(e.g., Ni et al., 2005; Wang and Wen, 2007). First detected by global seismic
tomography, over the past decade these LLSVPs have been found to have abrupt
lateral margins; stronger reductions of S-wave velocity than P-wave velocity,
indicating anomalously high incompressibility; and anomalously high density—all
suggestive of hot, chemically distinct material (Garnero et al., 2007; Garnero and
McNamara, 2008; Trønnes, 2009).
Figure 2.5 Pattern of S-wave velocity anomalies (dVs) at the core-mantle boundary
for model S20RTS (Ritsema and van Heijst, 2000). Red areas have lower than average
S-wave velocity, and blue areas have higher than average S-wave velocity. The green
curves outline the 20 percent of the core-mantle boundary area with the lowest S-wave
velocities, and this corresponds to the two LLSVPs beneath southern Africa and the
south-central Pacific that have been characterized by seismic tomography and
waveform modeling studies over the past two decades. SOURCE: Reprinted from
Thorne et al. (2004), with permission from Elsevier.
Geodynamical modeling (see Figure 2.6) suggests that such massive hot dense
piles of material can be localized by mantle circulation, with their margins possibly
serving as loci for thermal boundary layer instabilities that rise through the mantle as
well as accumulation zones for dense partially molten material right above the core-