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媒介。由于硅酸盐液体与其源区之间的密度、化学扩散率、粘度和体积组成的差异，岩浆的产生和输送是质量和热量输送的最有效的地质手段之一。岩浆作用是大洋地壳和大陆地壳起源和持续形成的原因，也是将我们以捕虏体形式了解内部组成的主要线索之一带到地表的原因。硅酸盐液体的物理性质预计会有很大的变化，在岩浆成因制度，甚至在今天的地球（高达~100公里或3 GPa），这些变化预计将有重要的影响，硅酸盐液体的作用，地球化学和地球动力学过程。更大的液体的可压缩性，并因此减少密度对比与共存的固体被认为是适应压力引起的液体结构的变化，包括主要阳离子的配位数的增加，虽然在高压下的液体的实验数据是有限的。压力引起的液体结构的变化已经牵连在输运性质和固-熔体元素分配与压力的变化。地球的热历史模型，古熔岩的分析，深地幔捕虏体导致我们考虑的压力和温度的范围比今天的岩浆成因，和一系列的熔体组合物，可能会有很大的不同，从目前的原生地幔熔体。早期的地球可能有一个很深的岩浆海洋，可能已经包围了整个地幔（2890公里或136 GPa）。捕虏体被从400公里（14 GPa）或更深的地方熔融带到地表。科马提质熔岩可能是由地幔熔融产生的，开始深度约为800 km（约30 GPa）。地震学研究发现了地幔底部超低速区的证据，该区域被认为是部分熔融的，这可能为地球过去广泛熔融提供重要线索。尽管硅酸盐液体在理解地球的热演化和化学演化中具有重要意义，但几乎在整个地幔压力体系中对硅酸盐液体知之甚少。测量接近环境压力的体积和声速是丰富的，但不允许唯一的外推超过几个GPa。熔融平衡，包括固相线和液体温度和液体组成，已被测量到高达约25 GPa。对预热样品的动态压缩研究已达到40 GPa（即地幔底部的三分之一），而对Hugoniot产生熔融的研究已达到200 GPa，尽管只是对少数成分进行了研究。到目前为止，液体结构的原位测量仅限于几个GPa。第一性原理模拟，主要来自我们的团队，已经取得了重要的进展，但只能研究一些相对简单的液体成分。因此，一个关键的绊脚石，以进一步的进展是缺乏信息的关键性质的硅酸盐液体在大多数地幔压力-温度制度。我们希望解决的问题可能集中在三个假设，拟议的研究将测试：1。地幔底部是否有硅酸盐熔体？如果有，有多少？2.很可能地球在其早期历史的某个时候大部分或完全熔化，可能是月球形成的影响的结果。一个完全熔融的地幔是如何演化的呢？在什么深度？结晶开始是像通常认为的那样自下而上，还是在中地幔深处？3.玄武岩倾向于分离到地幔深处，这是否与核幔边界附近观察到的地震复杂性相一致？

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

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