权益分类	功能权益	普通用户	{{item.name}}会员
{{category.name}}	{{benefitItem.name}}

Testing mantle dynamics : Constraining high resolution numerical spherical convection models with geochemistry and geophysics

测试地幔动力学：用地球化学和地球物理学约束高分辨率数值球形对流模型

基本信息

批准号：
NE/H006559/1
负责人：
J Davies
金额：
$ 39.68万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2011
资助国家：
英国
起止时间：
2011 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FH006559%2F1
关键词：
Testing mantle dynamics Constraining resolution

项目摘要

Mantle convection is important since it drives (i) plate tectonics (the ultimate process behind seismicity and mountain building); and (ii) melting, (critical for volcanism and producing crust, cryo/hydrosphere and atmosphere) but we do not know how it 'works'. In convection, for example water heated in a saucepan, the movement of the light (hot) material from the base to the surface where it cools and sinks back down to restart the cycle again provides a very efficient heat transfer mechanism. The differences in buoyancy that drive flow, with lighter material rising and denser material sinking, can be due to differences in temperature and/or composition. Amazingly the solid mantle deforms by creep on the geological time-scale allowing Earth's mantle to also lose its heat by convection. While we know that the ocean plate is the manifestation of the surface element of this cycle on Earth, and we have incomplete knowledge from seismic imaging for the present-day geometry of this process, we have no direct evidence of the geometry in the past. The field of mantle convection is now ready to yield a significant advance using the combination of the improvements in mantle convection modelling, the maturity of geophysics and geochemical observables, and mineral physics constraints. Convection in the mantle is more complex than convection in simple systems, such as water in a saucepan, since as hot mantle reaches the surface it melts. The melt rises to the surface forming a crust, and degasses to give an atmosphere and hydrosphere, and leaves behind a residue. The combination of these processes make the modelling more interesting since the crust and residue have a different buoyancy to the starting material. Significantly it also gives us the means to constrain the process. For example the rate of melting and degassing is related to the vigour of convection. The known amount of Argon40 that has collected in the atmosphere, produced at a known rate in the mantle from Potassium40, provides an integrated constraint on the rate of degassing. We will also use the flux of primordial Helium3 and alpha particle produced He4 as further constraints. We will also look at the isotopes of lead which are the stable daughters of radioactive U and Th parents. These are further useful stopwatches on mantle convection, but are different to the inert gases since they are not degassed but are recycled. They are returned to the mantle where the convection stirs the crust, residue and starting material together. When they are melted again their Lead isotopic signature is dependent on the proportion of the various components, the stirring and the time that has elapsed since it last melted. To understand mantle stirring one needs models in the right geometry (we will model it correctly as a spherical shell) and at the right vigour (we can reach Earth-like vigour even for early Earth). The geophysics evidence suggests that present-day the mantle convects as a whole body, while geochemical evidence requires ancient isolated reservoirs. There are a large number of hypotheses in play (usually motivated by one discipline alone) trying to reconcile these constraints. We will test these hypotheses. The geochemical data-sets we will use have been collected over very many decades, by countless research teams across the globe, utilizing data whose value at collection easily exceeds £1 billion (>2000*500k). Understanding mantle convection is a zeroth order problem for solid Earth science and the project proposed will allow us to make a significant long-lasting advance. The numerical geodynamic approach allows the broadest range of constraints to be brought to bear in a quantitative manner - the basic conservation laws of physics, geophysics (including integrative ones such as size of inner core - and very high spatial resolution seismic tomography) and geochemistry observables; providing the meanest test of this proliferation of hypotheses.

地幔对流是重要的，因为它驱动（i）板块构造（地震活动和造山运动背后的最终过程）;和（ii）熔化，（火山活动和产生地壳，低温/水圈和大气的关键），但我们不知道它是如何“工作”的。在对流中，例如在平底锅中加热的水，轻（热）材料从底部到表面的移动，在那里它冷却并下沉以再次重新启动循环，提供了非常有效的传热机制。驱动流动的浮力的差异，较轻的材料上升和较致密的材料下沉，可能是由于温度和/或成分的差异。令人惊讶的是，固体地幔在地质时间尺度上通过蠕变变形，使地球的地幔也通过对流失去热量。虽然我们知道海洋板块是地球上这一周期的表面元素的表现，而且我们对这一过程的现代几何形状的地震成像知识不完整，但我们没有过去几何形状的直接证据。地幔对流领域现在已经准备好产生一个显着的进步，使用地幔对流建模的改进，地球物理学和地球化学观测的成熟度，矿物物理学的限制相结合。地幔中的对流比简单系统（如平底锅中的水）中的对流更复杂，因为当热地幔到达表面时，它会融化。熔融物上升到表面形成地壳，并脱气以形成大气层和水圈，并留下残留物。这些过程的结合使建模更有趣，因为地壳和残余物具有与起始材料不同的浮力。重要的是，它还为我们提供了约束这一进程的手段。例如，熔化和脱气的速率与对流的强度有关。在大气中收集的已知数量的氩40，在地幔中以已知的速率从钾40产生，提供了对脱气速率的综合约束。我们还将使用原始氦3和α粒子产生的氦4作为进一步的约束。我们还将研究铅的同位素，它们是放射性铀和钍母体的稳定子体。这些是地幔对流的有用秒表，但与惰性气体不同，因为它们没有脱气，而是循环使用。它们被送回地幔，在那里对流将地壳、残余物和起始物质搅拌在一起。当它们再次熔化时，它们的铅同位素特征取决于各种成分的比例，搅拌和自上次熔化以来所经过的时间。为了理解地幔的搅动，我们需要正确的几何模型（我们将它正确地建模为一个球壳）和正确的活力（我们甚至可以在早期地球上达到类似地球的活力）。地球物理学的证据表明，现今的地幔作为一个整体进行对流，而地球化学的证据则需要古老的孤立储层。有大量的假设在起作用（通常由一个学科单独驱动）试图调和这些约束。我们将测试这些假设。我们将使用的地球化学数据集是由地球仪的无数研究团队在几十年内收集的，收集的数据价值很容易超过10亿英镑（>2000* 500 k）。了解地幔对流是固体地球科学的零阶问题，提出的项目将使我们能够取得重大的长期进展。数值地球动力学方法允许最广泛的约束，以量化的方式承担-物理学的基本守恒定律，地球物理学（包括一体化的，如内核的大小-和非常高的空间分辨率地震层析成像）和地球化学观测;提供最平均的测试这种扩散的假设。