Collaborative Research: Experimental deformation of monazite and titanite: Implications for interpretation of petrochronologic data

岩石的变形行为控制着岩石圈的强度、地震的位置和强度、地球对撞击事件的反应以及矿石矿物的聚集和沉积。为了了解岩石在地质历史中变形的条件和持续时间，并为了更好地了解地震过程、矿产资源识别和地壳强度模拟，我们将进行一系列对地质年龄测定重要的矿物实验。这些变形实验将在模拟地壳深处环境的条件下进行。主要研究人员将制作嵌入天然矿物颗粒的合成岩石样本，并使用岩石变形设备来施加地球深部剪切带典型的高应力和应变条件。这些实验将集中于副相矿物钛铁矿和独居石的变形行为，这两种矿物通常用于岩石年代学，这是一种使用放射性年龄测年来确定古代变质反应和变形事件的时间的技术。主要造岩矿物(如石英、长石、橄榄石)的变形行为已经得到了很好的研究，但我们的实验将是第一批研究副相矿物变形行为的实验之一。副相矿物，如独居石，是稀土元素(REE)的储集层，我们的实验结果将提供更好的了解稀土在断裂带内的活化。该项目的社会效益包括对研究生和本科生以及博士后研究员进行实验和分析技能方面的直接培训，这些技能在许多高水平的政府、国防和工业实验室中很有价值。外展工作将包括为国际扶轮普罗维登斯小学开发地球科学课程。布朗大学地球、环境和行星科学系有一个名为科学教学和教育计划(STEP)的积极推广计划，该计划与当地教师合作，在他们的课堂上开发地球科学模块。该项目还将有助于扩大STEM中任职人数不足的群体。该项目的目标是通过实验使选定的副相矿物变形，并使用多种先进的显微分析技术来研究各种变形和重结晶机制如何影响对地质年代学重要的元素和同位素的分布。研究人员将进行一个多步骤的实验方案，包括在Griggs-Rig固体介质变形仪中变形，然后对部分样品进行高温静态退火。具有良好特征的微量元素含量的副相独居石和钛铁矿将以斑岩的形式嵌入石英、合成石英或长石的基质中，并在规定的温度、压力和应变速率条件下变形。多种先进的分析技术将使我们能够研究变形样品的组成和结构，从聚集体到原子尺度。主要研究人员将量化晶格缺陷对杂质元素迁移率的影响以及重结晶的后果，以解释变形矿物的地球化学。我们的结果将为辅助相矿物的流变行为提供基本约束，并使辅助相地质年代学和地质测温更有信心地应用于变形岩石。该奖项反映了美国国家科学基金会的法定使命，并已通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The deformation behavior of rocks controls the strength of the lithosphere, the location and intensity of earthquakes, the response of Earth to impact events, and the concentration and deposition of ore minerals. To understand the conditions and duration under which rocks deformed in the geologic past, and to improve understanding of earthquake processes, mineral resource identification, and crustal strength modelling, we will conduct a series of experiments on minerals important for geologic age dating. These deformation experiments will be conducted at conditions which simulate environments deep within the Earth’s crust. The principal investigators will fabricate synthetic rock samples embedded with natural mineral grains and use a rock deformation apparatus to impose high stress and strain conditions typical of deep Earth shear zones. The experiments will focus on the deformation behavior of accessory phase minerals titanite and monazite, which are routinely used for petrochronology, a technique which uses radiometric age dating to determine the timing of ancient metamorphic reactions and deformation events. The deformation behavior of the major rock-forming minerals (e.g., quartz, feldspar, olivine) has been well-studied, but our experiments will be among the first to investigate the deformation behavior of accessory phase minerals. Accessory phase minerals, such as monazite, are reservoirs for Rare Earth Elements (REEs) and the results of our experiments will provide better understanding of REE mobilization within fault zones. Societal benefits of the project include direct training of graduate and undergraduate students and a postdoctoral researcher in experimental and analytical skills that are valuable in many high-level government, defense, and industrial laboratories. Outreach efforts will involve developing earth science curricula for Providence, RI elementary school. The Department of Earth, Environmental and Planetary Sciences at Brown has an active outreach program known as the Science-Teaching and Education Program (STEP), which partners with local teachers to develop earth science modules in their classes. The project will also contribute to the broadening of underrepresented groups in STEM. The goal of this project is to experimentally deform a selection of accessory phase minerals and use multiple advanced microanalytical techniques to examine how various deformation and recrystallization mechanisms affect the distribution of elements and isotopes important for geochronology. The researchers will conduct a multi-step experimental protocol consisting of deformation in a Griggs-rig solid medium deformation apparatus followed by high temperature static annealing of a subset of samples. Accessory phases monazite and titanite with well-characterized trace element contents will be embedded as porphyroclasts in a matrix of quartz, synthetic quartz, or feldspar and deformed under prescribed temperatures, pressures, and strain rate conditions. Multiple advanced analytical techniques will allow us to investigate the composition and structure of deformed samples from the aggregate to atomic scale. The principal investigators will quantify the influence of lattice defects on the mobility of impurity elements and consequences of recrystallization for interpreting the geochemistry of deformed minerals. Our results will provide fundamental constraints on the rheological behavior of accessory phase minerals and enable more confident applications of accessory phase geochronology and geothermometry to deformed rocks.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.