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

下地壳包体的 U-Pb 热年代学：估算莫霍面温度以限制地壳产热

• 批准号: 1650260
• 负责人: Roberta Rudnick
• 金额: $ 39.45万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-02-15 至 2023-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1650260&HistoricalAwards=false
• 关键词: Pb; Thermochronology; Lower; Crustal; Xenoliths

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.

驱动地幔对流和地球板块构造的能量是地球吸积的余热、内核结晶产生的热量和自然产生的放射性同位素(主要是产热元素钾（K)、钍（Th）和铀(U)）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处的热量产生至关重要，但这样的记录被证明是难以捉摸的。该项目将寻求开发一种方法，可用于量化大陆地壳中hpe的预算。由于大陆地壳是高度非均质的，而且大多难以接近，因此通常尝试使用表面热流数据来确定地壳的HPE丰度。然而，从地球流出的热量呢？地球表面是地壳内部产生的热量和从下面的地幔流出的热量的结合，区分这两种热源一直很困难。我们的项目试图通过限制莫霍面（地壳和地幔之间的边界）的温度来确定地壳的产热。为了做到这一点，我们将分析那些虽然分布广泛，但在大多数地壳岩石中丰度很低的矿物，这些矿物被称为副相。选择的副相在结晶时含有U，但不含Pb （U衰变的最终产物）。我们将从玄武岩岩浆（捕虏体）中快速带到地表的下地壳样品中提取含铀的附属相。由于铅在每个辅助相中的扩散温度因矿物种类而异，因此我们可以使用Pb同位素数据来确定捕虏体在被玄武岩宿主熔岩夹带之前所处的温度。结合对样品的平衡压力（因此，推导深度）和表面热流的估计，我们可以使用我们的数据来确定给定区域大陆地壳的HPE含量。该项目将作为可行性的示范，如果成功，可以应用于其他区域，以便制定各大洲的HPE浓度图。

项目成果

Heat Transfer and Production in Cratonic Continental Crust: U‐Pb Thermochronology of Xenoliths From the Siberian Craton

• DOI: 10.1029/2022gc010497
• 发表时间: 2022-10
• 期刊: Geochemistry
• 影响因子: 3.7
• 作者: F. Apen;R. Rudnick;D. Ionov;J. Cottle;Jean‐Franҫois Moyen;A. Golovin;A. Korsakov

Four-dimensional thermal evolution of the East African Orogen: accessory phase petrochronology of crustal profiles through the Tanzanian Craton and Mozambique Belt, northeastern Tanzania

• DOI: 10.1007/s00410-020-01737-6
• 发表时间: 2020-10
• 期刊: Contributions to Mineralogy and Petrology
• 影响因子: 3.5
• 作者: F. Apen;R. Rudnick;J. Cottle;A. Kylander‐Clark;M. Blondes;P. Piccoli;G. Seward

Apatites for destruction: Reference apatites from Morocco and Brazil for U-Pb petrochronology and Nd and Sr isotope geochemistry

• DOI: 10.1016/j.chemgeo.2021.120689
• 发表时间: 2022-02-20
• 期刊: CHEMICAL GEOLOGY
• 影响因子: 3.9
• 作者: Apen, Francisco E.;Wall, Corey J.;Seward, Gareth G. E.
• 通讯作者: Seward, Gareth G. E.