Magma generation and transport throughout the Earth's mantle: ab initio simulation of silicate melts

岩浆在地幔中的生成和输送：硅酸盐熔体的从头计算模拟

• 批准号: NE/F017871/1
• 负责人: Lars Stixrude
• 金额: $ 36.77万
• 依托单位: University College London
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF017871%2F1
• 关键词: Magma; generation; transport; throughout; Earth

Silicate liquids are primary agents of chemical and thermal evolution of the Earth. Because of the contrast in density, chemical diffusivity, viscosity, and bulk composition between silicate liquids and their source regions, the generation and transport of magma is one of the most efficient geological means of mass and heat transport. Magmatic processes are responsible for the origin and ongoing formation of the oceanic and continental crust, and for bringing to the surface one of our primary clues to the composition of the interior in the form of xenoliths. The physical properties of silicate liquids are expected to vary substantially over the magma genetic regime even in the present day Earth (up to ~100 km or 3 GPa), and these variations are expected to have important consequences for the role of silicate liquids in geochemical and geodynamical processes. The greater compressibility of liquids, and therefore diminishing density contrast with coexisting solids is thought to be accommodated by pressure-induced changes in liquid structure, including increases in the coordination number of major cations, although experimental data on liquids at elevated pressure is limited. Pressure-induced changes in liquid structure have been implicated in variations of transport properties and solid-melt element partitioning with pressure. Models of the Earth's thermal history, analysis of ancient lavas, and of deep mantle xenoliths lead us to consider a range of pressure and temperature much broader than that of present day magma genesis, and a range of melt compositions that may differ substantially from that of current primary mantle melts. The early Earth probably had a deep magma ocean that may have encompassed the entire mantle (to 2890 km or 136 GPa). Xenoliths have been brought to the surface by melts from depths as great as 400 km (14 GPa) or possibly much deeper. Komatiitic lavas may have been produced by mantle melting starting as deep as ~800 km depth (~30 GPa). Seismological investigations have found evidence for an ultra-low velocity zone at the base of the mantle that is thought to be partially molten, and which may provide important clues to the Earth's extensively molten past. Despite their importance in understanding Earth's thermal and chemical evolution, very little is known of silicate liquids throughout almost the entire mantle pressure regime. Measurements near ambient pressure of the volume and sound speeds are abundant, but do not permit unique extrapolation beyond a few GPa. Melting equilibria including solidus and liquids temperatures and liquid compositions, have been measured up to about 25 GPa. Dynamic compression studies on pre-heated samples have reached 40 GPa (i.e. one third that at the base of the mantle), and studies that produce melting on the Hugoniot have reached 200 GPa, although only on a small number of compositions. In situ measurements of liquid structure are so far limited to a few GPa. First principles simulations, primarily from our group, have made important progress, but have only been able to study a few relatively simple liquid compositions. Thus a key stumbling block to further progress is a lack of information regarding crucial properties of silicate liquids across most of the mantle pressure-temperature regime. The issues that we wish to address may be focused around three hypotheses that the proposed research will test: 1. Is there silicate melt at the base of the mantle, and if so, how much? 2. It is likely that Earth was largely or completely molten at some time during its early history, possibly as a result of the moon-forming impact. How would the evolution of a completely molten mantle proceed? And at what depth? Did crystallisation begin bottom-up as is generally thought, or at mid-mantle depths? 3. Basalt tends to segregate into the deep mantle; is this consistent with the seismic complexity observed near the core-mantle boundary?

硅酸盐液体是地球化学和热演化的主要代理。由于硅酸盐液体及其源区域之间的密度，化学扩散，粘度和散装组成的对比，岩浆的产生和运输是质量和热传输的最有效的地质手段之一。岩浆过程负责海洋和大陆地壳的起源和持续形成，并以异种石的形式将我们的主要线索之一带到内部的组成。即使在当今的地球（最多约100 km或3 GPa），硅酸盐液体的物理特性也有望在岩浆遗传状态下有很大差异，并且这些变化对硅酸盐在地球化学和地球动力学过程中的作用有重要影响。液体的较高可压缩性，因此与共存固体形成鲜明对比的密度降低被认为是由压力诱导的液体结构变化所容纳的，包括主要阳离子的协调数量增加，尽管对升高压力下液体的实验数据受到限制。压力诱导的液体结构的变化与转运性能的变化和与压力分配的固体熔体分配有关。地球的热历史模型，对古代熔岩的分析和深幔异种石的模型使我们考虑了一系列的压力和温度范围要比当今的岩浆起源宽得多，以及一系列熔体组合物可能与当前原发地幔熔体的融化成分有很大差异。地球早期可能有一个深岩浆海洋，可能涵盖了整个地幔（至2890公里或136 GPA）。异种石的深度从400公里（14 GPA）或可能更深得多的熔体带到了表面。从地幔熔化的深度（约800 km）（〜30 GPA）开始，可能是通过地幔熔化产生的。地震学研究发现了地幔底部一个超低速度区域的证据，被认为是部分熔融的，并且可能为地球广泛融化的过去提供重要的线索。尽管它们在理解地球的热和化学演化方面的重要性，但几乎整个地幔压力状态中，硅酸盐液体几乎不知道。在体积和声速的环境压力附近的测量值丰富，但不允许在几个GPA以外的独特外推。熔化的平衡包括固体和液体温度和液体成分，已测量约25 GPa。对预热样品的动态压缩研究已达到40 GPA（即，在地幔底部的三分之一），在雨果式上产生熔化的研究达到了200 GPA，尽管仅在少量组成上。到目前为止，原位测量液体的限制仅限于几个GPA。第一原理模拟主要来自我们小组，已经取得了重要进展，但只能研究一些相对简单的液体组成。因此，进一步进步的关键绊脚石是缺乏有关大多数地幔压力温度制度中硅酸盐液体至关重要特性的信息。我们希望解决的问题可能集中在提出的研究将测试的三个假设围绕：1。是否在地幔底部有硅酸盐融化，如果是，多少？ 2。在早期历史上的某个时候，地球很可能是由于月球形成的撞击而导致的。完全熔融地幔的演变将如何进行？在什么深度？结晶是否按照通常的想法开始自下而上？ 3。玄武岩倾向于将其隔离为深陆壁。这与在核心壳边界附近观察到的地震复杂性一致吗？

项目成果

First-principles calculation of the elastic moduli of sheet silicates and their application to shale anisotropy

• DOI: 10.2138/am.2011.3558
• 发表时间: 2011
• 期刊: American Mineralogist
• 影响因子: 3.1
• 作者: B. Militzer;H. Wenk;Stephen Stackhouse;Lars Stixrude

First-principles study of diffusion and viscosity of anorthite (CaAl2Si2O8) liquid at high pressure

• DOI: 10.2138/am.2011.3646
• 发表时间: 2011-05-01
• 期刊: AMERICAN MINERALOGIST
• 影响因子: 3.1
• 作者: Karki, Bijaya B.;Bohara, Bidur;Stixrude, Lars
• 通讯作者: Stixrude, Lars

Theoretical Computation of Diffusion in Minerals and Melts

• DOI: 10.2138/rmg.2010.72.22
• 发表时间: 2010-01-01
• 期刊: DIFFUSION IN MINERALS AND MELTS
• 作者: de Koker, Nico;Stixrude, Lars
• 通讯作者: Stixrude, Lars

Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt

• DOI: 10.1007/s00269-009-0315-1
• 发表时间: 2010-02-01
• 期刊: PHYSICS AND CHEMISTRY OF MINERALS
• 影响因子: 1.4
• 作者: Karki, Bijaya B.;Bhattarai, Dipesh;Stixrude, Lars
• 通讯作者: Stixrude, Lars