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

驱动地幔对流和地球板块构造的能量是地球吸积剩余的热量、内核结晶产生的热量和天然放射性同位素（主要是钾（K）、钍（Th）和铀（U），即产热元素（HPE））的放射性衰变产生的热量的组合。所有的HPE都强烈集中在大陆地壳中。因此，记录它们在地壳中的浓度对于定量计算地球深部的热量生产至关重要，但事实证明这种记录是难以捉摸的。本项目将寻求开发一种方法，用于量化大陆地壳中HPE的预算，因为大陆地壳高度不均匀，而且大多数情况下无法进入，因此经常尝试使用地表热流数据来确定地壳的HPE丰度。然而，从地球上流出的热量？地球表面的热量是地壳内部产生的热量和来自下伏地幔的热量的结合，区分这两种热源一直很困难。我们的项目试图通过对莫霍面（地壳和地幔之间的边界）的温度进行限制来确定地壳的产热。为了做到这一点，我们将分析矿物，虽然广泛存在，但在大多数地壳岩石中的丰度较低-称为副相。选择的副相在结晶时含有U，但不包括Pb（U衰变的最终子产物）。我们将从玄武岩岩浆（捕虏体）中迅速带到地表的下地壳样品中提取含U的副相。由于Pb在每个辅助阶段的扩散温度从一个矿物种类到下一个不同，我们可以使用Pb同位素数据来确定温度的捕虏体坐在他们夹带之前的玄武质宿主熔岩。结合对样品平衡压力（因此，推导深度）和地表热流的估计，我们可以使用我们的数据来确定给定地区大陆地壳的HPE含量。该项目将作为可行性的示范，如果成功，可以应用于其他地区，以绘制各大洲的HPE浓度图。

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.