SCIENTIFIC MOTIVATION

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It has been more than 40 years since the wide-spread acceptance of plate tectonics theory, but no definitive agreement has yet been reached among geoscientists on the fundamental nature of the global dynamic processes that drive the plate motions. There are still vigorous debates about the proportion of heat coming from the core versus internal radiogenic heating in the mantle; about the degree to which the mantle is chemically, rheologically and/or viscously stratified and whether there is layered or whole mantle circulation; about the origin and even the existence of mantle thermal plumes rising beneath hot spot volcanic centers like Hawaii and Iceland; the chemical/thermal nature and origin of heterogeneity in the deepest mantle (figure 1); the nature and importance of mechanical coupling between the mantle and the core; the chemical composition of the core and its evolution; and the nature and importance of chemical coupling between the deep Earth and surface reservoirs, especially of water and CO2. Ultimately, fundamental understanding of the Earth's evolution and present dynamics is necessary to better address key issues of societal relevance: (1) natural hazards such as earthquakes and volcanic eruptions; (2) the whole-Earth budget of volatile elements, especially water and carbon dioxide; (3) the impact of internal dynamical processes on climate.

Figure 1


Figure 1 Cartoons representing different end member models for the location of chemical reservoirs in the mantle and their relationship to dynamics. Blue: oceanic plates/slabs. Red: hot plumes. Purple: "primitive" mantle.DMM: Depleted Mantle; ERC: enriched Recyled Crust. (A) Typical geochemical model layered at 660 km depth. (B) Homogeneous mantle except for some mixture of ERC and primitive material at the base. (C) Primitive blob model with added ERC layer; (D) Complete recycling model. (E) Primitive piles model. (F) Deep Primitive layer. From Tackley (2000).

Geological activity is characterized by the movement of materials and heat on scales ranging from regional to global. Indeed, convection of the mantle and core, the action of thermal plumes, movement of tectonic plates at the surface, and infiltration of magma and other fluids at depth offer dramatic examples of Earth's ongoing geological evolution. The geological record provides an integrated history of our planet's mass and heat fluxes. Through seismological, geodetic, heat-flow and magnetic observations, geophysics yields information about the current dynamics of the interior; paleomagnetic and the geo- chemical signatures of magma source regions and of direct samples (xenolith rocks and very deep diamonds brought up in volcanic eruptions) from the interior provide a means of tracking the time scales of these internal processes. A new source of deep materials (>300 km) recently recognized in ophiolites (ancient remnants of oceanic crust) offers a new window into the deep mantle with unknown implications. Combining information about the properties of Earth materials, as derived from petrology and mineral physics, the full range of geological, geophysical and geochemical observations are interpreted through geodynamical models.

Tracking of mass and energy (heat) fluxes lies at the heart of understanding how our planet has evolved over geological time. What is the differential motion of fluids at depth, leading to volcanism and metamorphism due to upward migration toward the surface? What is the potential for downward sequestration of hydrous (and other volatile-bearing) fluids, or even of dense oxide or metallic melts in the deep interior? How effectively is heat transmitted across the core and through the thickness of the mantle, providing the energy that sustains the geomagnetic field as well as the plate-tectonic processes observed at the Earth's surface? The types of problems that arise in deep Earth research are challenging to solve for several reasons: Extreme P-T conditions, impure materials, and complex systems with many interacting factors operating over many orders of magnitude space- and time-scales. Remote-sensing of structures, processes and compositions at great depth with no direct sampling requires sophisticated theories, geophysical inferences, inverse theory, and intensive computational modeling capabilities. Progress requires broad knowledge of multiple fields, more than any one expert can encompass. Individual fields tend to operate on their own, not always recognizing the need for communication across disciplines and often hampered by the lack of a common language. Intrinsic complexity of the internal systems and the constraints on our observations push the limits of both knowledge and technology.Meanwhile, a new generation of disciplinary tools, that will provide unprecedented views of the Earth's interior, is becoming available to the geoscience community, through major infrastructure efforts that are currently under way, or in the planning stages. For example,Earthscope, and more specifically theUSArray program provides seismologists with a high resolution "window" into the deep mantle and core with broadband seismic waveform data over the North American continent from densely spaced receivers. The COMPRESprogram allows mineral physicists to perform advanced high pressure and temperature measurements on mineral properties at conditions relevant to the Earth's deep interior and compare them with results of "first principles" calculations and provides an avenue for interactions with material science communities. Other initiatives aim at providing geodynamicists and seismologists with a unified, state of the art framework for mantle and core convection and seismic wave propagation computations (CIG), and researchers in geodesy and geomagnetism with satellite observations that are revolutionizing these fields (e.g. Oersted, Champ, GRACE and Swarm). Paleomagnetic data are being assembled into the MagIC PMAG database. In geochemistry, the enormous volumes of high quality chemical and isotopic data gathered over the past 25 years are now assembled into systematic and broadly accessible databases (GEOROC and PETDB, and ever-improving measurement techniques are providing new perspectives on mantle processes at scales from micrometers to thousands of kilometers. In general, our community has been building "big-science" data gathering tools but using only "small-science" approaches to deep Earth data interpretation. As a result, only partial return on these investments can be expected. In other fields of science, such as Astronomy and Atmospheric Sciences, this issue has been appreciated and addressed.

Given the enormous amount and diversity of observations becoming available, a quantum leap in the understanding of the constitution and evolution of our planet can be expected, if we can identify and focus on the key issues, and define how best to address them by fully utilizing complementary disciplinary data (seismology, geodesy, geo- and paleo-magnetism, measurements of material properties at high pressure and temperature, geochemistry) and modeling tools (seismic wave propagation, convection simulations, ab initio mineral physics computations).

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