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.

地幔连接很重要，因为它驱动（i）板块构造（地震性和山区建筑背后的最终过程）；（ii）熔化（对于火山和生产地壳，冷冻/水圈和大气至关重要），但我们不知道它是如何工作的。在转化率中，例如在锅中加热的水，光（热）材料从基部到表面的运动，在那里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构。随着温度和/或组成的差异，驱动流动的浮力差异和材料下沉的差异可能是由于温度和/或组成的差异所致。令人惊讶的是，在地质时间尺度上蠕变稳固的地幔变形，使地幔也通过建筑物损失了热量。虽然我们知道海板是地球上该周期的表面元素的表现，并且我们从地震成像中对此过程的几何形状有不完整的知识，但过去我们没有直接证据表明几何形状。现在，使用地幔构造建模，地球物理学和地球化学可观察物以及矿物质物理学约束的改进的结合，地幔转换领域已准备好实现重大进步。地幔中的对流比简单系统（例如平底锅中的水）中的构造更为复杂，因为当热地幔到达融化的表面时。升到表面，形成地壳，并获得大气和水圈，并留在退休后。这些过程的组合使建模更加有趣，因为外壳和残基与起始材料具有不同的浮力。值得注意的是，它还为我们提供了限制流程的手段。例如，熔化和脱气的速度与建筑的活力有关。在大气中收集的已知的argon40量，从钾40的地幔中以已知速率产生，对脱气速率提供了综合限制。我们还将使用Primordial Helium3和Alpha颗粒的通量作为进一步的约束。我们还将研究铅的同位素，这是放射性U和TH父母的稳定女儿。这些在地幔转化率上是进一步有用的秒表，但与惰性气体不同，因为它们没有脱气，但被回收。他们被送回地幔，在那里，运输工具将地壳，居住和起始材料融合在一起。当它们再次熔化时，它们的铅同位素签名取决于各个组件的比例，自上次融化以来的搅拌和时间已经过去。为了了解地幔搅动一个需要右几何形状的模型（我们将正确地将其建模为球形外壳），并且以正确的活力（即使在早期地球也可以达到地球状的活力）。表明当今地幔对流全身，而地球化学证据需要古老的孤立水库。试图调和这些限制因素有很多假设（通常仅由一门学科动机）。我们将检验这些假设。我们将使用的地球化学数据集已被全球无数的研究团队收集了数十年，使用其收集价值的数据容易超过10亿英镑（> 2000*500k）。了解地幔转换是固体地球科学的零订单问题，该项目提出的项目将使我们能够做出重大的长期进步。数值地球动力学方法允许以定量的方式带来最广泛的约束 - 物理学的基本保护定律，地球物理学（包括综合的核心，例如内核的大小和非常高的空间分辨率地震层析成像）和地球化学观察力；提供了这种假设扩散的最平均检验。