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

地幔对流很重要，因为它驱动(I)板块构造(地震活动和造山背后的终极过程)；以及(Ii)熔融(对火山活动和产生地壳、低温/水圈和大气至关重要)，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)物质从底部移动到表面，在那里冷却并下沉，再次重新启动循环，提供了一种非常有效的热传递机制。驱动水流的浮力的不同可能是由于温度和/或组成的不同，导致较轻的物质上升和密度较大的物质下沉。令人惊讶的是，固体地幔在地质时间尺度上通过蠕变而变形，使得地幔也因对流而失去热量。虽然我们知道海洋板块是地球上这一周期的表层元素的表现，我们从地震成像中对这一过程的今天几何图形的了解不完整，但我们没有过去几何图形的直接证据。利用地幔对流模型的改进、地球物理和地球化学观测资料的成熟以及矿物物理的限制，地幔对流领域现在已经准备好产生重大进展。地幔中的对流比简单系统中的对流要复杂得多，例如平底锅中的水，因为当热的地幔到达地表时，它就会融化。熔体上升到表面形成地壳，脱气形成大气层和水圈，并留下残留物。这些过程的结合使建模变得更加有趣，因为地壳和残留物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这一过程的手段。例如，熔化和脱气的速度与对流的强度有关。大气中收集的已知数量的Ar 40在地幔中以已知的速度从Potassium40中产生，这对脱气速度提供了一个综合的限制。我们还将使用原始氦3和产生HE4的阿尔法粒子的通量作为进一步的限制。我们还将研究铅的同位素，它们是放射性U和Th父母的稳定子体。这些是关于地幔对流的更有用的秒表，但不同于惰性气体，因为它们不是脱气的，而是回收的。它们被返回到地幔，在那里对流将地壳、残留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，人们需要合适的几何模型(我们将把它正确地建模为一个球壳)和合适的活力(我们可以达到与地球相似的活力，即使是在早期的地球)。地球物理证据表明，今天的地幔作为一个整体对流，而地球化学证据表明，需要古老的孤立的储集层。有大量的假设在发挥作用(通常是由一个学科单独驱动的)，试图协调这些限制。我们将检验这些假设。我们将使用的地球化学数据集是由全球无数研究团队几十年来收集的，使用的数据收集价值轻松超过10亿GB(&gt；2000*500k)。了解地幔对流是固体地球科学的零级问题，提出的项目将使我们取得重大的、长期的进展。数值地球动力学方法允许以定量的方式施加最广泛的约束--物理、地球物理(包括诸如内核大小等综合守恒定律和非常高的空间分辨率地震层析成像)和地球化学观测的基本守恒定律；提供了对这种假说激增的最平均检验。

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.