U-Pb Thermochronology of Lower Crustal Xenoliths: Estimating Moho Temperature in Order to Constrain Crustal Heat Production

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The energy that drives mantle convection and thus plate tectonics on Earth is a combination of heat remaining from Earth accretion, heat generated by crystallization of the inner core, and heat generated by radioactive decay of naturally occurring radioisotopes, primarily potassium (K), thorium (Th), and uranium (U), the heat-producing elements (HPE). All HPEs are strongly concentrated into the continental crust. Thus, documenting their concentrations in the crust is essential for any quantitative accounting of heat production within the deeper Earth, but such documentation has proven elusive. This project will seek to develop a method that can be used to quantify the budget of HPEs in the continental crust.Because the continental crust is highly heterogeneous, and mostly inaccessible, determining the HPE abundances of the crust has often been attempted using surface heat flow data. However, the heat flowing from the Earth?s surface is a combination of heat produced within the crust and heat flowing from the underlying mantle and distinguishing between these two heat sources has been difficult. Our project seeks to determine crustal heat production by placing constraints on the temperature at the Moho - the boundary between the crust and the mantle. To do this, we will analyze minerals that, though widespread, occur in low abundances in most crustal rocks - termed accessory phases. Select accessory phases contain U when they crystallize, but exclude Pb (the ultimate daughter product of U decay). We will extract U-bearing accessory phases from samples of the lower crust that were carried rapidly to the surface in basaltic magmas (xenoliths). Because the temperature at which Pb diffuses in each of the accessory phases varies from one mineral species to the next, we can use the Pb isotope data to determine the temperature at which the xenoliths were sitting prior to their entrainment in the basaltic host lavas. Combined with estimates of the pressure of equilibration of the samples (hence, depth of derivation), and the surface heat flow, we can use our data to determine the HPE content of the continental crust in a given region. This project will serve as a demonstration of feasibility, which, if successful, can be applied to other regions in order to develop a HPE concentration map of the continents.