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

虽然地球上大多数地震发生在地表附近，但有一个重要的子集发生在更深的地方。这些地震是在构造板块内产生的，这些板块通过一个称为俯冲的过程沉入地幔。 俯冲在地球的演化过程中扮演着重要的角色，它将水和碳等挥发性物质循环回地球内部。这些挥发物中的一部分及时从板块中释放出来，通过火山返回到地表，但另一部分则俯冲到更深的地方。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地点。研究人员假设，发生在“中间深度”（或地球表面以下约70至300公里）的下行板块中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会在较浅的深度释放，但该团队正在研究的位于下沉板块地壳下方的水（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。研究人员发现了一个地质位置，在那里他们可以测试地幔岩石圈中的水是否在中深度地震中释放出来。在该项目中，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，以检查地震发生地点的水释放条件是否合适。如果是这样的话，那么它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到地球表面。这对于我们理解地球的化学演化及其在地质时期维持富含挥发物的大气层的能力非常重要。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块的热特性，以进一步在更广泛的范围内加深我们的理解。该项目将支持一个早期的职业科学家，也将从事研究可视化工作的本科暑期实习生。俯冲带中深地震活动的发生通常归因于下行大洋板块内矿物相的变质脱水。水是在板块形成时在海脊和俯冲前的外隆断层作用下引入的，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坦段的特征是板片几何形状的强烈变化和空间和时间上的会聚。研究人员假设，这些变化导致深度的温度变化，从而控制中间深度的可变地震活动。他们将通过高分辨率三维和随时间变化的有限元模型预测俯冲岩石圈的热结构来测试这一假设，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，以便从岩石学和地球化学研究生到地球动力学建模专家的广泛研究人员都可以使用。因此，该项目将有助于软件基础设施，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.