Understanding how the mantle transition-zone 'valve' controls slab fate

俯冲是构造板块下降到地球深处的过程，即地幔。俯冲非常重要，因为它驱动了板块构造（地震活动和造山活动背后的最终过程）；（ii）熔融（对火山活动至关重要，并产生地壳和大气）和（iii）地幔循环。然而，我们并不完全理解它是如何“工作”的。俯冲板块（“板块”）形成地幔对流的下行分支。地幔对流在几个重要方面不同于我们所熟悉的在炉子上用平底锅烧开水的对流。首先，地幔岩石是固体，但它们可以在很长的时间尺度上蠕动。其次，在“过渡带”中，地幔下400至800公里处3000公里深处，地幔矿物经历高压相变，形成更紧密、更致密的结构。蠕变随温度变化强烈，使得冷俯冲板块比周围温暖的地幔要硬得多。确切的蠕变类型因压力和应力等因素而异，并控制板在下沉过程中因升温而失去强度的速度。板坯变形的难易程度会影响其下沉速度。蠕变类型还受相变时矿物结构和晶粒尺寸变化的影响。过渡带中蠕变和相变的相互作用使上地幔板块向过渡带下地幔的俯冲变得复杂。特别重要的是660公里深度附近的转变，地幔粘度增加了10-100倍，冷板块相变的延迟使它们暂时比地幔轻。这可能会导致过渡区的板坯失速。通过这种方式，过渡带控制着热量和物质在地幔中循环的效率（包括影响气候演变的水和二氧化碳）。观察到的板块运动的快速变化表明，在某些情况下，板块很容易下沉通过过渡带（“阀门”打开），而在其他情况下，它们在那里停滞并堆积（“阀门”关闭）。根据地震记录重建的地球内部地震层析成像显示，目前，许多板块，包括汤加、日本和苏门答腊下面的板块，都在过渡区，而其他一些板块，如中美洲下面的板块，则直接下沉到很深的地方。人们提出了不同的解释。一个端元假说（由co-I Goes博士提出）认为，最古老、最冷的板块是最坚硬的，往往在过渡带的底部变平，而不是直接下沉，而年轻的温暖板块形成的堆更容易下沉。与此相反，合伙人卡拉托则假设，在400公里处的主要相变中出现了由小而弱的新颗粒组成的板块。在年轻的石板中，温暖的温度会促进颗粒的生长，石板很快就会恢复强度，从而使它们能够通过，而旧的石板仍然很弱，因此无法打开阀门。最近，co-I Davies与Imperial的同事一起开发了一种数字代码，该代码允许具有网格的模型适应模型复杂性的尺度，即在小尺度变化的区域（如相位或蠕变机制的近变化）具有高分辨率，并且在低可变性区域计算成本更低，分辨率更低。这使我们能够首次对俯冲板块的热、相和蠕变效应之间复杂的相互作用进行建模。我们将结合最新的相变特性数据（来自co-I Lithgow-Bertelloni）和蠕变规律数据（来自合作伙伴Karato），建立一套俯冲模型。通过将模型预测与地球物理观测结果进行比较，我们将能够确定两种端元假设中的任何一种，或者组合或替代机制是否解释了过渡区“阀”的关键工作。

Subduction is the process where tectonic plates descend into Earth's deep interior, the mantle. Subduction is critically important since it drives (i) plate tectonics (the ultimate process behind seismicity and mountain building); (ii) melting, (critical for volcanism, and producing crust and atmosphere) and (iii) mantle circulation. Yet, we do not fully understand how it 'works'.Subducting plates ('slabs') form the downwelling limb of mantle convection. Mantle convection differs in several important ways from the familiar convection of water boiling in a saucepan on a stove. Firstly, mantle rocks are solid, but they can creep on long time scales. Secondly, in the 'transition zone', 400 to 800 km down into the 3000 km deep mantle, mantle minerals undergo high-pressure phase changes to more tightly-packed and denser structures.Creep varies strongly with temperature, making cold subducting plates much stiffer than the surrounding warm mantle. Exact creep style varies with, amongst others, pressure and stress, and controls how rapidly slabs lose their strength as they heat up while sinking. How easily a slab deforms again influences its sinking speed. The style of creep is also affected by the changes in mineral structure and grain size that occur at phase transitions. The interaction between creep and phase changes in the transition zone complicate the subduction of plates from the upper mantle into the mantle below the transition zone. Of special importance is the transition around 660 km depth where mantle viscosity increases by a factor of 10-100 and a delay of the phase transformation in the cold slabs makes them temporarily lighter than the mantle. This can lead to stalling of slabs in the transition zone. In this way, the transition zone controls how efficiently heat and material are cycled through the mantle, (including water and CO2 which have affected the evolution of climate).Observed rapid changes in plate motions indicate that there are episodes in which slabs sink through the transition zone quite readily ('valve' open), and others in which they stall there and pile up ('valve' shut). Seismic tomography images of the Earth's interior, reconstructed from seismogram recordings, show that at the moment, many slabs, including those below Tonga, Japan and Sumatra pool in the transition zone, while a few others, for example below Central America, descend straight to great depths.Different explanations have been proposed. One end-member hypothesis (put forward by co-I Dr. Goes) is that the oldest, coldest plates are stiffest and tend to flatten at the base of the transition zone rather than sink straight through, while young warm slabs form piles that sink through the transitions more easily. Partner Karato in contrast hypothesises that slabs emerge from the major phase transition at 400 km consisting of small, weak new grains. While in young slabs, warm temperatures encourage grain growth and the slabs quickly regain strength allowing them to push through, old slabs remain weakened and are hence unable to open the valve.Recently, co-I Davies, together with colleagues at Imperial developed a numerical code that allows models with grids that adapt to the scale of model complexity, i.e. high resolution in regions with changes over small scales, like near changes in phase or creep mechanism, and, computationally-less-expensive, coarser resolution in regions with low variability. This allows us to model for the first time, the complex interplay between the thermal, phase and creep effects on subducting slabs.We will make a set of subduction models incorporating the most recent data on phase change properties (from co-I Lithgow-Bertelloni) and creep laws (from partner Karato). By comparing model predictions with geophysical observations we will be able to determine if either of the two end-member hypotheses or combined or alternative mechanism explains the crucial workings of the transition zone 'valve'.