The Earth's interior radiates approximately 46 terawatts (TW, 1012 J s-1) of heat (total combined production of nuclear power today is about 1 TW of energy). The Earth's thermal energy comes from the cooling of the planet, minor contributions from tidal friction and inner core growth, and significantly from the decay of radioactive elements. The exact proportional contribution of these heat supplies is unknown. Plate tectonics is the primary manifestation of heat transport within the Earth today. Large tectonic plates are produced at mid-ocean ridges and transported across the surface of the globe to deep ocean trenches, where they plunge into the Earth's interior sending cold slabs into the interior and cooling the planet. In order to better understand the dynamic processes that occur both within the solid Earth as well as on the planet's surface, we must develop a comprehensive model of Earth composition and structure that is internally consistent with observations from the fields of geology, geochemistry, geophysics and particle physics. The abundance and distribution of naturally-occurring radioactive elements is integral to the understanding of Earth dynamics, as radiogenic heat provides a key source of energy that serves to drive mantle convection, plate tectonics and the evolution of the entire planet.
The primary objectives of the proposed research are to: (1) understand the nature and three-dimensional distribution of naturally-occurring radioactive elements in the Earth, particularly potassium, thorium and uranium, which produce >99% of all radiogenic heat in the Earth; (2) develop an internally consistent model of the Earth, including the thermal constitution and evolution of the planet; and, (3) build Earth models that can be tested against data from newly developed antineutrino detectors. (Antineutrinos are neutral nuclear particles produce during beta-decay of radioactive elements.) The proposed work plan involves dynamic modeling to determine the geometry, distribution and chemical composition of the Earth's major reservoirs, namely the continental crust, mantle and metallic core. Model results will be interpreted in conjunction with antineutrino data in order to develop a comprehensive, three-dimensional model of Earth composition and structure.
In this collaborative project with scientists from University of Maryland and University of Hawaii, our team from University of Colorado at Boulder made two contributions. First, we helped calculating geoneutrino flux at the surface of the Earth for geoneutrinos generated from radioactive decays of U and Th within the Earth's mantle. We built mantle compositional models that are constrained by seismic and geochemical observations and contain distributions of U and Th in the mantle. The geonuetrino flux from the mantle is the maximum above the the central Pacific. Our study provided guidance for building future neutrino detectors to determine the distributions of U and Th in the mantle, which should have significant implications for understanding the overall bulk composition of the Earth, its heat/energy budget and dynamic evolution. This work was published in Sramek et al. [2013, Earth Planet. Sci. Lett.]. Second, we determined temporal and spatial distributions of heat flux at the Earth's surface and at the core-mantle boundary (i.e., the heat escape from the Earth's mantle and from the core) from modeling global mantle convection using geologically constrained plate motion history. Our calculations showed that the Earth's surface heat flux was at its maxima at ~120 Ma or after Pangea breakup, and that the core-mantle boundary heat flux was at a minimum at 270 Ma shortly after the assembly of Pangea. We proposed that the core-mantle boundary heat flux minimum had significant effects on the core dynamo and caused long-term stable polarity of earth magnetic field, i.e., superchrons. This study thus has significant implications for understanding the heat budget of the Earth and also the generation of Earth's magnetic field. This work was published in Zhang and Zhong [2011, Earth Planet. Sci. Lett.]. In summary, we have contributed to the success of this collaborative project.
