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

虽然地球上的大多数地震都发生在地表附近，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内部产生的，这些板块通过一个被称为俯冲的过程沉入地幔。俯冲作用通过循环挥发性物质，如水和碳，回到地球内部，在我们星球的进化中起着重要作用。这些挥发物中的一部分及时从板块中释放出来，通过火山回到地表，但还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多少。部分困难在于追踪挥发物的释放位置。研究人员假设，发生在“中深度”（或地球表面以下约70至300公里）下沉板块的地震是由俯冲板块深处的水释放引起的。下沉板块地壳中的水可能在较浅的深度被释放，但该团队正在研究的位于下沉板块地壳下方的水（被称为“地幔岩石圈”）有可能在适当的条件下被俯冲到更深的深度。研究人员已经找到了一个地质位置，在那里他们可以测试在中深度地震中地幔岩石圈中的水是否被释放出来。在这个项目中，他们将为选定的测试区域中存在的复杂俯冲带开发热模型，以检查在看到地震的位置释放水的条件是否合适。如果是这样，那么它将表明这些挥发物并没有被俯冲到地球的更深的地方，而是最终有可能回到地球表面。这对我们了解地球的化学演化及其在地质时期维持挥发物丰富的大气的能力非常重要。该团队开发的热建模方法将向科学界公开。其他科学家可以用它来研究其他地方复杂板的热特性，以进一步扩大我们对更广泛范围的理解。该项目将支持一名早期职业科学家，也将吸引本科生暑期实习生参与研究可视化工作。俯冲带中深度地震活动的发生通常归因于下沉大洋板块内矿物相的变质脱水作用。水是在板块在山脊处形成时被引入的，在俯冲之前通过外升断层被引入，但引入的水量及其在中深度地震活动中的作用仍不确定。南美洲俯冲带的两个平段在空间和时间上都具有很强的板块几何变化和收敛的特征。研究人员假设，这些变化导致了深度的温度变化，从而控制了中深度的可变地震活动性。他们将通过高分辨率3D和时间相关的有限元模型来预测俯冲岩石圈的热结构，以此来测试这一假设，他们可以预测变质条件和脱水发生的地方。完整的热模型和开源建模能力将以多种形式提供，以便广泛的研究人员使用，从岩石学和地球化学的研究生到地球动力学建模的专家。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。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.