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

了解地幔过渡区“阀门”如何控制板块命运

基本信息

批准号：
NE/I024429/1
负责人：
J Davies
金额：
$ 26.08万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2012
资助国家：
英国
起止时间：
2012 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FI024429%2F1
关键词：
Understanding how mantle transition zone

项目摘要

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'.

俯冲作用是指构造板块下降到地球内部深处--地幔的过程。俯冲作用至关重要，因为它推动了（一）板块构造（地震和造山运动背后的最终过程）;（二）熔融（火山活动的关键，并产生地壳和大气）和（三）地幔环流。然而，我们并不完全了解它是如何“工作”的。俯冲板块（“板”）形成了地幔对流的下降翼。地幔对流在几个重要方面与我们熟悉的炉子上平底锅中沸腾的水的对流不同。首先，地幔岩石是固体，但它们可以在很长的时间尺度上蠕变。第二，在“过渡区”，即400至800公里深至3000公里深的地幔中，地幔矿物经历高压相变，形成更紧密、更致密的结构。蠕变随温度变化很大，使得冷俯冲板块比周围的暖地幔更坚硬。确切的蠕变类型随压力和应力等因素而变化，并控制板在下沉时受热时失去强度的速度。板块再次变形的难易程度影响其下沉速度。蠕变的类型也受到相变时矿物结构和晶粒尺寸变化的影响。蠕变和相变之间的相互作用使板块从上地幔向过渡带下方的地幔俯冲复杂化。特别重要的是660公里深度附近的过渡，地幔粘度增加了10-100倍，冷板的相变延迟使它们暂时轻于地幔。这可能导致过渡区中的板坯失速。通过这种方式，过渡带控制着热量和物质（包括影响气候演变的水和二氧化碳）在地幔中循环的效率。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通过过渡带下沉（“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）。根据地震图记录重建的地球内部地震层析成像图像显示，目前，许多板块，包括汤加、日本和苏门答腊下面的板块，在过渡区汇集，而其他一些板块，例如中美洲下面的板块，则直接下降到很深的地方。一个端元假说（由共同研究员戈斯博士提出）认为，最古老、最冷的板块是最坚硬的，倾向于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更容易通过过渡区下沉。与此相反，Karato的合伙人假设，板片是在400公里处从主要的相变中出现的，由小而弱的新颗粒组成。而在年轻的板坯中，温暖的温度会促进晶粒生长，板坯很快恢复强度，使它们能够通过，而老的板坯仍然很弱，因此无法打开阀门。最近，co-I Davies与帝国理工学院的同事一起开发了一种数值代码，允许模型具有适应模型复杂性尺度的网格，即在小尺度变化的区域具有高分辨率，例如相位或蠕变机制的接近变化，以及在具有低可变性的区域中计算上较便宜的较粗糙的分辨率。这使我们能够第一次对俯冲板块上的热、相和蠕变效应之间的复杂相互作用进行建模。我们将制作一套俯冲模型，其中包含相变特性（来自co-I Lithgow-Bertelloni）和蠕变定律（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，我们将能够确定两个端元假设中的任一个或组合或替代机制是否解释了过渡区“阀”的关键工作。