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

俯冲是构造板块下沉到地球深部--地幔的过程。俯冲至关重要，因为它驱动(I)板块构造(地震活动和造山背后的最终过程)；(Ii)熔融(对火山活动至关重要，并产生地壳和大气)和(Iii)地幔环流。然而，我们还不完全了解它是如何“工作”的。俯冲板块(‘板块’)形成了地幔对流的下流分支。地幔对流在几个重要方面与人们所熟知的炉子上平底锅里沸腾的水的对流不同。首先，地幔岩石是坚固的，但它们可以在很长的时间尺度上蠕动。其次，在向下400-800公里深的“过渡带”中，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，使冷俯冲板块比周围温暖的地幔坚硬得多。准确的蠕变方式随着压力和应力等因素的变化而变化，并控制板材在下沉过程中升温时失去强度的速度。楼板再次变形的难易程度会影响其下沉速度。蠕变的方式也受到相变时矿物结构和颗粒尺寸变化的影响。过渡带中蠕变和相变之间的相互作用使板块从上地幔俯冲到过渡带下方地幔的过程复杂化。特别重要的是约660公里深度的转变，在那里地幔粘度增加了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楼板停滞不前。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(包括水和二氧化碳，它们影响了气候的演化)。观察到的板块运动的快速变化表明，有时板块很容易下沉通过过渡带(阀门打开)，而另一些时候它们会失速并堆积在那里(阀门关闭)。根据地震记录重建的地球内部的地震层析成像图像显示，目前许多板块，包括位于过渡地带的汤加、日本和苏门答腊池以下的板块，以及其他一些板块，例如中美洲下方，直接下降到很深的地方。人们提出了不同的解释。一个端元假说(由合著者戈斯博士提出)是，最古老、最冷的板块最坚硬，往往会在过渡区的底部变平，而不是直接下沉，而年轻的温暖板块形成的堆积物更容易在过渡区下沉。与之形成对比的是，卡拉托合作伙伴提出的假设是，在400公里处的主要相变过程中，板坯由细小而脆弱的新颗粒组成。在年轻的板坯中，温暖的温度促进了颗粒的生长，板坯迅速恢复力量，使它们能够通过，但旧的板坯仍然薄弱，因此无法打开阀门。最近，co-i Davies与Imperial的同事开发了一种数值代码，允许具有适应模型复杂性规模的网格的模型，即，在具有小尺度变化的区域中的高分辨率，如相或蠕变机制的近变化，以及在计算上较便宜的区域，在变化性较低的区域中，分辨率较高。这使我们能够首次模拟热、相和蠕变对俯冲岩屑的影响之间的复杂相互作用。我们将制作一组俯冲模型，其中包括相变性质(来自co-I Lithgow-Berelloni)和蠕变规律(来自合作伙伴Karato)的最新数据。通过将模型预测与地球物理观测进行比较，我们将能够确定是否两个末端成员假设或组合或替代机制中的任何一个解释了过渡区‘阀门’的关键工作原理。

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'.