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.

驱动地球上地幔对流并进而驱动板块构造的能量是地球吸积剩余热量、内核结晶产生的热量以及天然放射性同位素（主要是钾 (K)、钍 (Th) 和铀 (U)，即发热元素 (HPE)）放射性衰变产生的热量的组合。所有 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，但事实证明这样的记录是难以捉摸的。该项目将寻求开发一种可用于量化大陆地壳中 HPE 预算的方法。由于大陆地壳高度不均匀，且大多难以接近，因此通常尝试使用地表热流数据来确定地壳的 HPE 丰度。然而，从地球表面流出的热量是地壳内产生的热量和从底层地幔流出的热量的组合，区分这两种热源一直很困难。我们的项目旨在通过限制莫霍面（地壳和地幔之间的边界）的温度来确定地壳热量的产生。为此，我们将分析大多数地壳岩石中分布广泛但丰度较低的矿物（称为副相）。选择结晶时含有 U 的辅助相，但排除 Pb（U 衰变的最终子产物）。我们将从玄武质岩浆（捕虏体）中快速带到地表的下地壳样本中提取含铀副相。由于铅在每个副相中扩散的温度因一种矿​​物种类而异，因此我们可以使用铅同位素数据来确定捕虏体在被夹带到玄武岩宿主熔岩中之前所处的温度。结合样本平衡压力（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论证，如果成功，可以应用于其他地区，以绘制各大洲的 HPE 集中度地图。

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.