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

CSEDI：全地幔对流和过渡区水过滤器

• 批准号: 0330745
• 负责人: David Bercovici
• 金额: $ 78万
• 依托单位: Yale University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-07-01 至 2009-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0330745&HistoricalAwards=false
• 关键词: CSEDI; Whole; Mantle; Convection; Transition

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.

关于地球内部的一个主要问题是，为什么地幔看起来既混合又不混合。 地球化学分析表明，两种主要的地幔对流上升（分别为Cretemid-Ocean Ridge玄武岩MORB和Ocean-Island玄武岩OIB）具有不同的化学特征，因而来自地幔中不同的孤立层。 然而，地震学研究表明，冷的下沉板块（称为俯冲板块）下沉到整个地幔，因此会搅动地幔并破坏任何分层。因此，关于地幔如何对流的证据似乎完全矛盾。 然而，最近的研究表明，地幔410 km深处的熔融作用可以作为一个过滤器，将不相容元素隔离到深部（410 km以下）地幔中，但允许主要组分的全地幔循环，从而满足地球化学和地球物理约束。 当大部分的环境膨胀地幔从高水溶性过渡带上升到410 km以上的低水溶性上地幔时，它被水过饱和，并经历脱水熔融，过滤掉不相容元素。过滤后的固相将干燥的贫化材料提供给MORB源区。 然而，该过滤器被抑制为更热，更快的地幔柱，因此deliverenriched OIBs。 这种“水过滤”模型提出了许多新的问题：（1）在410公里处假定的熔融层的预测结构是什么，如何在地震学上进行检验？ (2)过滤机制对地幔对流和化学循环有什么影响？(3)在上地幔深部条件下含水熔体的性质是什么？解决这些问题和其他许多重要的新问题需要一个跨学科的方法，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，这是一个高度跨学科的项目，涉及地球动力学、矿物物理学、地震学和地球化学。 滤水器模型还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EarthScope，OMD（OceanMantle Dynamics）和COMPRES等大型观测和实验计划提供重点和假设检验。