CSEDI: Whole-Mantle Convection and the Transition-Zone Water Filter

关于地球内部的一个主要问题是，为什么地幔同时呈现出混合良好和未混合的状态。地球化学分析表明，两种主要类型的地幔对流上升流（分别产生洋中脊玄武岩MORB和洋岛玄武岩OIB）具有不同的化学特征，因此来自地幔中不同的孤立层。然而，地震学研究表明，冷的下沉流（称为俯冲板块）穿过整个地幔，因此会搅动地幔并破坏任何分层。因此，关于地幔对流的证据似乎是完全矛盾的。然而，最近有人提出，地幔410公里深度的熔融可以作为一个过滤器，将不相容的元素隔离到深层（低于410公里）的地幔中，但允许主要成分的整个地幔循环，从而满足地球化学和地球物理的限制。当大部分环境上升流地幔从高水溶性过渡带上升到410km以上的低溶解度上地幔时，它变得水过饱和并经历脱水熔融，过滤掉了不相容的元素。过滤后的固相为MORB源区提供干燥、耗尽的材料。然而，对于更热、更快的地幔柱来说，过滤器被抑制了，从而产生了丰富的oib。这种“水过滤器”模型提出了许多新问题：(1)410公里处假定的熔融层的预测结构是什么，如何从地震上进行测试？(2)过滤机制对地幔对流和化学循环的影响是什么？(3)深部上地幔条件下含水熔体的性质是什么？回答这些问题以及许多其他重要的新问题需要跨学科的方法，因此需要一个由地球动力学家、实验矿物物理学家、地震学家和地球化学家组成的团队。更广泛的影响：该项目涉及两名研究生的教育，涉及地球动力学，矿物物理学，地震学和地球化学的高度跨学科项目。水过滤器模型也提出了新的问题和研究领域，这将推动学科之间的相互作用，并为更大的观测和实验项目（如EarthScope， OMD（海洋地幔动力学）和compes）提供焦点和假设测试。

A major question about the Earth's interior is why the mantle appearsboth well-mixed and unmixed at the same time. Geochemical analyses implythat the two main types of mantle convective upwellings (which createmid-ocean ridge basalts MORB and ocean-island basalts OIB, respectively)have distinct chemical signatures and thus come from different isolatedlayers in the mantle. However, seismological studies suggest thatcold downwellings (called subducting slabs) sink across the entiremantle and would therefore stir the mantle and destroy any layering.Thus there appears to be completely contradictory pieces of evidenceof how the mantle convects. However, it has been recently suggestedthat melting at 410 km depth in the mantle can act as a filter thatsequesters incompatible elements into the deep (sub-410km) mantle, butallows whole-mantle circulation of major components, thereby satisfyingboth geochemical and geophysical constraints. As most of the ambientupwelling mantle rises out of the high-water-solubility transitionzone into the low-solubility upper mantle above 410km, it becomeswater-supersaturated and undergoes dehydration melting that filtersout incompatible elements. The filtered solid phase supplies dry,depleted materials to the MORB source region. The filter, however,is suppressed for hotter, faster mantle plumes which therefore deliverenriched OIBs. This "water-filter" model raises many new questions: (1)What is the predicted structure of the putative melt layer at the 410km,and how can this be tested seismologically? (2) What is the influenceof the filter mechanism on mantle convection and chemical circulation?(3) What is the nature of hydrous melt at deep upper-mantle conditions?Answering these and a host of other important new questions demands aninterdisciplinary approach and therefore involves a team comprised ofa geodynamicist, an experimental mineral physicist, a seismologist anda geochemist.Broader impact:This project involves the education of two graduate students in a highlyinterdisciplinary project entailing geodynamics, mineral physics,seismology and geochemistry. The water-filter model also raises newquestions and fields of research that will drive interaction betweendisciplines and provide focus and hypothesis-testing for the largerobservational and experimental programs such as EarthScope, OMD (OceanMantle Dynamics) and COMPRES.