Earth's Radiogenic Heat Production and the Composition of the Deep Continental Crust
Sammon, Laura Grace
McDonough, William F
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Much of the continental crust, the 40+ km thick plates of rock that make up the outer shell of our planet, is inaccessible to us living on its surface. Thus its composition is a mystery. We lack the technology to sample it directly at depths past 5 km, aside from a few deep (expensive) drill holes, so we must come up with a clever alternative for establishing its composition. The deep crust, the lower two-thirds of the continent, serves as a supporting root. When continents collide, they make mountain ranges, or when pulled apart they make rift valleys and basins. The composition of the deep crust, and specifically its silica, molecular water, and heat producing element (HPE: K, Th, U) contents, directly influence the crust's rheology during tectonic events and its potential for deadly earthquakes. Its chemical makeup is the sum of 4.5 billion years of crustal evolutionary processes that continuously shape and reshape the platform upon which society sits. An accurate description of the deep crust, however, requires careful integration of many different data sources. My research combines geochemistry with thermodynamics, geophysics, mineral-physics, seismology, and even particle physics to produce self-consistent models for the crust’s composition. Using thermodynamic calculations, I generate densities and seismic sound wave speeds from a range of chemical compositions. Matching these forecasted models to Earth’s seismic and gravity data allows me to translate the deep crust's physical properties into chemical compositions on both the regional and the global scale. Importantly, by quantifying not only the compositions, but also the uncertainties and the misfit in these results, I can better define the differences between competing models for crust deformation and evolution. Charting the distribution of Earth's geochemical resources has led to our collaborations with particle physicists, who need our expertise to determine the frequency of radioactive decay and therefore the amount of HPE decay emissions (known as geoneutrinos) in the crust; this geoneutrino flux is the background signal in their nuclear physics experiments. Their global flux measurements constrain our models for heat production and the amount of radiogenic energy that heats the Earth – which provides power to mantle convection, plate tectonics, and the destruction and creation of more continental crust.Our main sources of data are threefold. First, we have critically compiled geochemical analyses of >10,000 rock samples from pre-existing literature (Earthchem.org and affiliates). Second, we use geophysical data provided by sources such as the United States Geological Survey, the Earthscope USArray, and others to determine which of our geochemical samples could produce Earth’s observed seismic and density signals. Third, we partner with particle physicists in the United States, Canada, Italy, Japan, and China to jointly interpret data from three international geoneutrino detectors. By focusing on Earth as a whole system we seek a comprehensive understanding of its natural hazards and resources. Using multidisciplinary constraints, my goal is to build compositional models of the continental crust, with quantifiable uncertainties, that can be applied regionally and at larger scales. These findings will provide predictive insights on the strength and response of the continents when subjected to the dynamic processes of plate tectonics.