Magma generation and transport throughout the Earth's mantle: ab initio simulation of silicate melts

硅酸盐液体是地球化学和热演化的主要媒介。由于硅酸盐液体及其源区在密度、化学扩散率、粘度和体积组成上的差异，岩浆的产生和输送是最有效的质量和热量输送的地质手段之一。岩浆作用负责海洋和大陆地壳的起源和持续形成，并将我们以捕虏体形式了解内部成分的主要线索之一带到地表。硅酸盐液体的物理性质在岩浆形成过程中甚至在今天的地球（高达~100 km或3 GPa）也会发生很大的变化，这些变化预计会对硅酸盐液体在地球化学和地球动力学过程中的作用产生重要影响。液体具有更大的可压缩性，因此与共存的固体相比密度减小，这被认为是由压力引起的液体结构变化所调节的，包括主要阳离子配位数的增加，尽管高压下液体的实验数据有限。压力引起的液体结构变化涉及输运性质的变化和固体-熔体元素随压力的分配。地球热史模型、对古熔岩和深部地幔包体的分析使我们考虑到的压力和温度范围比当今岩浆成因的范围要宽得多，而且熔体成分的范围可能与当前的原始地幔熔体有很大的不同。早期地球可能有一个深岩浆海洋，可能包含了整个地幔（至2890公里或136 GPa）。捕虏体是由深度达400公里（14 GPa）或可能更深的熔体带到地表的。科马长岩熔岩可能是由深至~800 km （~30 GPa）的地幔熔融产生的。地震学调查已经发现了地幔底部有一个超低速带的证据，人们认为这个带部分是熔融的，这可能为地球广泛熔融的过去提供重要线索。尽管它们对理解地球的热和化学演化很重要，但人们对几乎整个地幔压力状态下的硅酸盐液体知之甚少。在环境压力附近的音量和声速测量是丰富的，但不允许超越几个GPa的独特外推。熔化平衡包括固体和液体的温度和液体成分，已经测量到大约25 GPa。对预热样品的动态压缩研究已达到40 GPa（即地幔底部的三分之一），而在Hugoniot上产生熔融的研究已达到200 GPa，尽管仅针对少数成分。到目前为止，对液体结构的现场测量仅限于几GPa。第一原理模拟，主要来自我们的小组，已经取得了重要的进展，但只能研究一些相对简单的液体成分。因此，阻碍进一步进展的一个关键障碍是缺乏关于大部分地幔压力-温度状态下硅酸盐液体的关键特性的信息。我们希望解决的问题可能集中在提议的研究将测试的三个假设：1。地幔底部是否有硅酸盐熔融，如果有，有多少？2. 地球很可能在其早期历史的某个时候大部分或完全熔融，这可能是月球形成撞击的结果。一个完全熔融的地幔是如何演化的？深度是多少？结晶是像人们普遍认为的那样自下而上开始的，还是在中地幔深处开始的？3. 玄武岩倾向于向深部地幔分离；这与在核幔边界附近观测到的地震复杂性一致吗？

Silicate liquids are primary agents of chemical and thermal evolution of the Earth. Because of the contrast in density, chemical diffusivity, viscosity, and bulk composition between silicate liquids and their source regions, the generation and transport of magma is one of the most efficient geological means of mass and heat transport. Magmatic processes are responsible for the origin and ongoing formation of the oceanic and continental crust, and for bringing to the surface one of our primary clues to the composition of the interior in the form of xenoliths. The physical properties of silicate liquids are expected to vary substantially over the magma genetic regime even in the present day Earth (up to ~100 km or 3 GPa), and these variations are expected to have important consequences for the role of silicate liquids in geochemical and geodynamical processes. The greater compressibility of liquids, and therefore diminishing density contrast with coexisting solids is thought to be accommodated by pressure-induced changes in liquid structure, including increases in the coordination number of major cations, although experimental data on liquids at elevated pressure is limited. Pressure-induced changes in liquid structure have been implicated in variations of transport properties and solid-melt element partitioning with pressure. Models of the Earth's thermal history, analysis of ancient lavas, and of deep mantle xenoliths lead us to consider a range of pressure and temperature much broader than that of present day magma genesis, and a range of melt compositions that may differ substantially from that of current primary mantle melts. The early Earth probably had a deep magma ocean that may have encompassed the entire mantle (to 2890 km or 136 GPa). Xenoliths have been brought to the surface by melts from depths as great as 400 km (14 GPa) or possibly much deeper. Komatiitic lavas may have been produced by mantle melting starting as deep as ~800 km depth (~30 GPa). Seismological investigations have found evidence for an ultra-low velocity zone at the base of the mantle that is thought to be partially molten, and which may provide important clues to the Earth's extensively molten past. Despite their importance in understanding Earth's thermal and chemical evolution, very little is known of silicate liquids throughout almost the entire mantle pressure regime. Measurements near ambient pressure of the volume and sound speeds are abundant, but do not permit unique extrapolation beyond a few GPa. Melting equilibria including solidus and liquids temperatures and liquid compositions, have been measured up to about 25 GPa. Dynamic compression studies on pre-heated samples have reached 40 GPa (i.e. one third that at the base of the mantle), and studies that produce melting on the Hugoniot have reached 200 GPa, although only on a small number of compositions. In situ measurements of liquid structure are so far limited to a few GPa. First principles simulations, primarily from our group, have made important progress, but have only been able to study a few relatively simple liquid compositions. Thus a key stumbling block to further progress is a lack of information regarding crucial properties of silicate liquids across most of the mantle pressure-temperature regime. The issues that we wish to address may be focused around three hypotheses that the proposed research will test: 1. Is there silicate melt at the base of the mantle, and if so, how much? 2. It is likely that Earth was largely or completely molten at some time during its early history, possibly as a result of the moon-forming impact. How would the evolution of a completely molten mantle proceed? And at what depth? Did crystallisation begin bottom-up as is generally thought, or at mid-mantle depths? 3. Basalt tends to segregate into the deep mantle; is this consistent with the seismic complexity observed near the core-mantle boundary?