New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 23

mantle boundary (e.g., Nakagawa and Tackley, 2004; McNamara and Zhong, 2005).
Mineral physics experiments and theory now allow thermal and chemical
heterogeneity of these provinces to be estimated based on predictions of elastic
parameters (e.g., Murakami et al., 2004; Mao et al., 2006; Ohta et al., 2008; Duffy,
2008; Shim, 2008). Next-generation experimental facilities will provide the ability to
characterize textures throughout the mantle pressure-temperature ( P-T ) range, such as
crystal-liquid wetting angles and shape-preferred orientations—features that provide
direct constraints on mantle evolution. The locations of large igneous provinces
(LIPs) reconstructed for plate motions suggest that the deep-mantle LLSVPs may
have persisted for at least 300 My, constituting a long-term connection between deep
dynamics and surface geology (Burke et al., 2008; Torsvik et al., 2006). Many
questions about the composition and dynamics of these huge chemical heterogeneities
remain to be resolved, and petrological and geochemical investigations of surface
materials are needed to evaluate possible deep compositions, but their discovery has
driven models for mantle evolution in totally new directions.
Figure 2.6 Seismic tomography indicates that the present-day lower mantle is dominated
by large low-velocity provinces beneath southern Africa and the south-central Pacific,
plus high-velocity regions beneath the Pacific Rim, as shown in Figure 2.5. The evolution
of these structures with time is critical to deciphering the origin and composition of
mantle reservoirs and their fluxes. This figure shows a simulation of whole-mantle
convection with thermal and chemical heterogeneity and reconstructed plate motions
since 450 Ma. Left: Calculated mantle structure at 230 Ma with reconstructed plate
boundaries in black. Right: Present-day mantle structure with continent outlines in black
from the same simulation. Positive and negative temperature anomalies are shown in
yellow and blue, respectively; dense chemical heterogeneity is shown in green; the core-
mantle boundary is shown in pink. This simulation predicts that a Paleozoic Gondwana
LLSVP split to form the African and Pacific structures. It illustrates how plate and
continent reconstructions can be combined with seismic tomography, LIPS paleo-
reconstructions, and geodynamical modeling to trace the evolution of present-day mantle
structure into the deep past. SOURCE: Zhang et al. (2010).