Thermal constraints on the role of hydrated oceanic mantle lithosphere in the genesis of intermediate-depth seismicity

虽然地球上的大部分地震发生在地表附近，但也有相当一部分地震发生在更深的地方。这些地震是在构造板块中产生的，这些板块通过一种称为俯冲的过程下沉到地幔中。俯冲作用通过将水和碳等挥发性物质循环回到地球内部，在地球的演化过程中发挥着重要作用。这些挥发物中的一部分及时从冰板释放出来，通过火山返回地表，但也有一部分被俯冲到更深的地方。我们不知道的是这些相对分数是什么。部分困难在于追踪挥发物在哪里释放。研究人员假设，在“中等深度”(或地球表面以下约70至300公里)的下行板块发生的地震是由俯冲板块内部深处的水释放引起的。下沉板块地壳中的水很可能是在较浅的深度释放出来的，但该团队正在研究的水位于下沉板块(被称为“地幔岩石圈”)的地壳下方，在正确的条件下，有可能被俯冲到更深的地方。研究人员已经找到了一个地质位置，在那里他们可以测试中深地震期间地幔岩石圈中的水是否正在释放。在这个项目中，他们将为选定的试验区中存在的复杂俯冲带开发热模型，以检查地震发生地点是否具备释放水的条件。如果是这样的话，这将表明这些挥发物不会被俯冲到地球更深的地方，而是很可能最终返回地球表面。这对于我们理解地球的化学演化以及它在地质时期内维持挥发性物质丰富的大气的能力是很重要的。该团队开发的热模拟方法将向科学界公开。其他科学家可以利用它在其他地方研究复杂楼板的热性质，以进一步加深我们在更广泛范围内的理解。该项目将支持一名职业生涯早期的科学家，并将吸引本科生暑期实习生参与研究可视化努力。俯冲带中深部地震活动的发生通常归因于向下的大洋板块内矿物相的变质脱水。水在板块形成时在海脊形成时被引入，并在俯冲之前通过外部隆起断层引入，然而引入的水量及其在中深度地震活动中的作用仍然不确定。南美俯冲带的两个平坦地段在空间和时间上都具有强烈的板片几何变化和会聚特征。研究人员假设，这些变化导致了深部温度的变化，从而控制了中等深度的不同地震活动。他们将通过高分辨率的3D和随时间变化的有限元模型预测俯冲岩石圈的热结构来检验这一假设，他们可以从这些模型预测变质条件和发生脱水的地方。将以多种形式提供完整的热模型和开源建模能力，以便广泛的研究人员可以使用它们，从岩石学和地球化学研究生到地球动力学建模专家。因此，该项目将为软件基础设施做出贡献，并利用国家科学基金会在俯冲带研究方面的重大投资。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。

While most of the Earth’s earthquakes happen near the surface, a significant subset occurs at greater depths. These earthquakes are generated within tectonic plates that are sinking into the Earth’s mantle through a process called subduction. Subduction plays an important role in the evolution of our planet by cycling volatile materials like water and carbon back into the planet’s interior. Some of these volatiles are released from the slab in time to be returned to the surface through volcanoes, but some fraction are subducted to much greater depths. What we don’t know is what those relative fractions are. Part of the difficulty is in tracing where volatiles are released. The investigators hypothesize that earthquakes that happen in downgoing plates at “intermediate depths” (or between about 70 and 300 km below the Earth’s surface) are caused by the release of water from deep within the subducting plates. Water brought down in the crust of the downgoing plate is probably released at shallower depths, but the water the team is studying, located below the crust of the downgoing plate (known as the “mantle lithosphere”) has the potential to be subducted to much greater depths given the correct conditions. The investigators have found a geological location where they can test whether or not water in the mantle lithosphere is being released during intermediate depth earthquakes. In this project, they will develop thermal models for the complex subduction zones that exist in the selected test areas in order to examine whether the conditions are right for the release of water at the locations where earthquakes are seen. If so, then it will show that these volatiles do not get subducted to greater depths in the Earth, but rather are likely to eventually be returned to the Earth’s surface. This is important for our understanding of the chemical evolution of our planet and its ability to maintain a volatile-rich atmosphere over geologic time. The thermal modeling approach the team develops will be made public to the scientific community. Other scientists can use it to study the thermal properties of complex slabs elsewhere to further our understanding on a broader scale. This project will support an early-career scientist, and will also engage undergraduate summer interns in research visualization efforts. The occurrence of intermediate-depth seismicity in subduction zones is commonly attributed to the metamorphic dehydration of mineral phases within the downgoing oceanic plate. Water is introduced to the plate upon its formation at the ridge and by outer-rise faulting just before subduction, yet the amount of water introduced and its role in intermediate-depth seismicity remains uncertain. Two flat segments in the South American subduction zone are characterized by strong variations in slab geometry and convergence both in space and in time. The investigators hypothesize that these variations lead to temperature variations at depth that control the variable seismicity at intermediate depths. They will test this hypothesis by predicting the thermal structure of the subducted lithosphere through high-resolution 3D and time-dependent finite element models from which they can predict the metamorphic conditions and where dehydration takes place. The full thermal models and open-source modeling capability will be made available in multiple forms so that they can be used by a wide range of researchers ranging from graduate students in petrology and geochemistry to specialists in geodynamical modeling. This project will therefore contribute to software infrastructure and leverage significant investments by the National Science Foundation in subduction zone research.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.