Collaborative Research: Constraining the thermal conditions of the subduction interface by integrating petrology and geodynamics

俯冲带是地球上的一个板块俯冲到另一个板块之下的地方。它们是许多与社会有关的灾害的所在地，包括地球上最致命的地震的发生，如2011年东北地震和相关海啸，以及火山喷发，如圣海伦斯山(1980年)和皮纳图博山(1991年)。导致这些地震和火山爆发的过程最终取决于俯冲带的热结构--也就是巨大深度的温度。从古俯冲带挖出的变质岩包含了它们在被挖出之前沿着俯冲带旅行时所目睹的独特的温度记录。地球动力学模型和地球物理观测提供了对当今俯冲带热结构的估计。有趣的是，岩石记录表明，与现代俯冲带预测的条件相比，温度要高得多。这项拟议的工作将调查这种差异的原因，这可能在于我们解释岩石条件的方式，它们被挖掘出来的方式，或者甚至是我们如何比较模型预测和岩石记录。这项建议旨在通过两个关键问题解决岩石模型差异：1)我们目前对岩石记录的解释是否准确地代表了相关古代俯冲带的热结构？2)岩石是从平均比现代俯冲带更热的古俯冲带中剥离出来的吗？为了回答问题1，该小组将使用相对较新的分析方法确定从代表一系列热结构的五个特征良好的地点挖掘出的变质岩的P-T条件：痕量元素温度计(例如金红石中的锆石)和矿物包裹体气压计(例如石榴石中的石英气压计)。为了回答问题2，他们将建立以出土岩石为代表的古代俯冲带的地球动力学模型。这些模型将纳入区域特定俯冲动力学的影响，例如板块年龄和俯冲速率随时间的变化、俯冲开始、脊俯冲和板块断裂(如果适用)。模型预测的俯冲界面上的P-T条件将与新产生的P-T估计进行比较，以重新评估它们的差异。岩石学观测和地球动力学模拟相结合的双管齐下的方法允许对俯冲带的热演化进行定量勘探。与俯冲有关的变质岩是来自板块界面的唯一直接物质样本；使用最新的温压方法评估P-T条件将为团队提供更准确和精确的方法来解开他们所经历的复杂历史。通过地球动力学模拟，可以隔离各个参数对俯冲带热结构的影响，这种有针对性的方法将允许评估对出土岩石记录的温暖条件的可能解释。将双管齐下的方法应用于选定的古代俯冲地点，将使研究人员能够确定这种差异是否可以被调和。这项工作的结果对许多过程都有重要的影响，包括挥发分的地球化学循环，大陆地壳的构造，以及导致弧火山活动的条件。这项工作将使研究生和早期职业科学家参与不同学科的科学家(岩石学家和地球动力学家)以及美国和海外不同机构之间的新合作。作为该项目的一部分，将在加利福尼亚州(Franciscan和Catalina)创建EarthCacheTM网站。这些地点将通过流行的地球藏宝活动让公众参与到地球科学中来，并将通过与从俯冲带挖掘出的地质特征的直接观测相关的信息来传播关于俯冲带地质的信息。该奖项反映了NSF的法定使命，并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。

Subduction zones are places on Earth where one of Earth's tectonic plates dives beneath another. They are the location of many societally-relevant hazards, including the generation of Earth's deadliest earthquakes, such as the 2011 Tohoku earthquake and associated tsunami, and volcanic eruptions such as those at Mt St Helens (1980) and Mt Pinatubo (1991). The processes that lead to these earthquakes and volcanoes are ultimately dependent on the thermal structure of subduction zones - that is how hot it is at great depths. Metamorphic rocks exhumed from ancient subduction zones contain unique records of the temperatures that they witnessed as they traveled down a subduction zone before being exhumed. Geodynamic models and geophysical observations provide estimates of the thermal structures of present-day subduction zones. Interestingly, the rock record suggests significantly warmer conditions than those predicted for modern subduction zones. The proposed work will investigate the reasons for this discrepancy, which may lie in the way we interpret conditions from the rocks, the way they are exhumed, or even in how we compare model predictions and the rock record.This proposal aims to address the rock-model discrepancy through two key questions: 1) Does our current interpretation of the rock record accurately represent the thermal structure of the associated ancient subduction zone? and 2) Did rocks get exhumed from ancient subduction zones that were hotter on average than modern subduction zones? To address Question 1 this team will determine P-T conditions for metamorphic rocks exhumed from five well-characterized localities that represent a range of thermal structures using relatively new analytical methods: trace element thermometers (e.g. Zr-in-rutile) and mineral inclusion barometers (e.g. quartz-in-garnet barometry). To address Question 2, they will develop geodynamical models of the ancient subduction zones represented by the exhumed rocks. These models will incorporate the effects of region-specific subduction dynamics, such as variations in slab age and subduction rate with time, subduction initiation, ridge subduction, and slab breakoff, when applicable. The model-predicted P-T conditions along the subduction interface will be compared with the newly produced P-T estimates to re-evaluate their disparity. The two-pronged approach of combining petrological observations and geodynamical modeling allows quantitative exploration of the thermal evolution of subduction zones. Subduction-related metamorphic rocks are the only direct samples of material from the plate interface; evaluating the P-T conditions using the latest thermobarometric approaches will provide the team with a more accurate and precise way to untangle the complex history they have experienced. Through geodynamical modeling, the effects of individual parameters on the thermal structure of subduction zones can be isolated, and this targeted approach will allow evaluation of possible explanations for the warm conditions that are recorded by exhumed rocks. The application of the two-pronged approach to the selected ancient subduction localities will allow researchers to determine whether the disparity can be reconciled. The results of this work have important implications for many processes, including geochemical cycling of volatiles, construction of continental crust, and the conditions that lead to arc volcanism. This work will engage graduate students and early career scientists in new collaborations between scientists of different disciplines (petrologists and geodynamicists) and at different institutions both in the US and abroad. EarthCache (TM) sites will be created in California (Franciscan and Catalina) as part of the project. These sites will engage the public in geoscience through the popular activity of geocaching and will disseminate information about subduction zone geology through information tied to direct observations of geologic features exhumed from subduction zones.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.