Metallurgy at Extreme Conditions: Molten Iron-Alloy Constraints on the Light Elements in Earth's Core

地核的组成是地球科学中一个突出的谜团。从地震和宇宙化学约束我们知道，核心是由近纯铁(~95%Fe+5%Ni)组成的，内部核心是固体的，外部核心是熔融的。然而，根据我们对铁水在地核极端压力和温度条件下的行为的了解，很明显，在熔融的外核中也一定有一些其他的轻元素溶解。轻元素被认为与外核的对流有关，因此对产生地球磁场很重要。轻元素的性质和丰度也将决定在地幔硅酸盐和熔融的金属核心之间可能发生的反应的种类。在过去的半个世纪里，核中轻元素的主要候选者包括H、O、S、C和Si。尽管有好莱坞电影，我们永远无法直接对核心进行采样，因此需要其他方法来推断光元素的身份。基本上，这种方法一直是通过实验和理论来尝试和确定哪些元素可以在核心条件下溶解到铁水中。仔细阅读关于这个主题的大量文献就会发现，个别元素和元素的鸡尾酒已经进入、离开并重新受到时间的青睐。不同的实验和理论方法往往导致对光元素的同一性有非常不同的解释。在这里，我们提出了一种结合实验、热力学模拟和地震观测的方法来推断岩芯中的轻元素。地震数据限制了纵波在铁水中传播的速度和密度。它们还可以检测岩心液体是否已分离成一种以上的液体(不混溶)。原则上，如果一个人知道各种铁合金-轻元素混合物的相同性质，就可以推断出核心的成分。地震观测是可用的。核心条件下熔融合金性质的内部一致性模型并非如此。然而，热力学关系允许通过熔融合金的状态方程来确定物理性质。根据铁-轻元素合金成分的熔化曲线，可以推导出建立热力学模型所需的参数。在这里，我们建议测量FeO、Fe3C、FeS、FeH和FeSi等二元(二元)合金的熔化曲线，以求出热力学模型所需的量。我们在实验室中开发了强大的技术，可以使用激光加热的钻石顶锤在非常高的压力和温度下测量熔点。此外，我们还发展了一种令人兴奋的新的X射线成像技术，利用它我们可以直接测量铁-轻元素体系中的最小熔化成分(共晶)。这些数据进一步有助于约束热力学模型。总而言之，我们将使用一种实验方法来测量铁-轻元素合金如何熔化，并根据这些数据开发一个多组分热力学模型，使我们能够预测各种可能的核心液体的地震波速度和密度。然后，我们将把模型与实际观测结果进行比较，以推断熔融的外核中难以捉摸的轻元素的同一性。

One of the outstanding mysteries in the Earth sciences is the composition of the core. We know from seismic and cosmochemical constraints that the core is made of a nearly pure iron (~95% Fe + 5% Ni), and that the inner core is solid and the outer core is molten. However, based on our knowledge of the behavior of molten iron at the extreme pressure and temperature conditions of the core, it is apparent that there must be some other light element or elements dissolved in the molten outer core as well. It is thought that the light element is related to convection in the outer core and is therefore important for spawning the Earth's magnetic field. The nature and abundance of the light element will also determine the kinds of reactions that might occur at the boundary between mantle silicate and the molten metal core. For the last half-century the primary candidates for the light elements in the core have included H, O, S, C, and Si. We will never be able to sample the core directly, hollywood movies notwithstanding, so other approaches are required to deduce the identity of the light elements. Basically, the approach has been to try and determine which elements can dissolve into molten iron at core conditions using experiment and theory. A perusal of the vast literature on this subject reveals that individual elements and cocktails of elements have come into, out of, and back into favor with time. Different experimental and theoretical approaches often lead to very different interpretations as to the identity of the light elements. Here we propose a method for deducing the light element in the core that relies on a combination of experiment, thermodynamic modeling, and seismic observations. Seismic data constrain the velocity at which compressional waves can move through molten iron as well as the density. They can also detect whether the core liquid has separated into more than one liquid (immiscibility). In principle, if one knows the same properties for various iron alloy - light element mixtures, one can deduce the composition of the core. The seismic observations are available. An internally consistent model for the properties of molten alloys at core conditions is not. However, thermodynamic relationships allow the physical properties to be determined through the equation of state of molten alloys. The parameters required to develop the thermodynamic model can be deduced through the melting curves of iron - light element alloy compositions. Here, we are proposing to make measurements of the melting curves of two-component (binary) alloys such as FeO, Fe3C, FeS, FeH and FeSi in order to derive the quantities required for the thermodynamic model. We have developed robust techniques in our lab for measuring melting points to very high pressures and temperatures using the laser-heated diamond anvil cell. Further, we have developed an exciting and novel new X-ray imaging technique with which we can measure directly the minimum melting compositions (eutectics) in iron - light element systems. These data further help constrain the thermodynamic models. In summary, we will use an experimental approach to measure how iron - light element alloys melt and from this data we will develop a multi-component thermodynamic model that will allow us to predict the seismic wave velocities and density of a wide range of possible core liquids. We will then compare the model with actual observations to deduce the identity of the elusive light elements in the molten outer core.