Resolving the incorporation of He in silicate minerals

尽管稀有气体（氦、氩、氪、氙、氡）在天然岩石和矿物中含量极低，但它们是地幔地球化学演化的潜在极有价值的示踪剂。目前，我们还不能真正利用它们，因为我们并不完全了解它们在地幔岩石融化时的行为。我们之前对氦以外的稀有气体的研究，它们以与许多其他原子相似的方式成为矿物晶体结构的一部分。问题仍然是氦，这个系列中最轻、可能最有用的成员，在地幔融化时将如何表现。他有不同原子质量的形态。较轻的一种不再在地球内部产生；它在很早的历史上就与地球结合在一起了。另一方面，较重的一种是在地球上通过放射性衰变不断产生的，主要是铀和钍。这项拨款的主要目的是解决所谓的“氦悖论”，即不同类型的氦的低比例（相对于较重的较轻）的地幔衍生熔体具有高的氦浓度，而高比例的熔体具有低的氦浓度。这种长期的观察很难与“标准模型”相一致，在标准模型中，氦比铀或钍更有可能进入熔融的地幔物质。相反，他们指出了一个“替代模型”，在这个模型中，相反的情况是正确的：氦倾向于留在固体中。另一种模型的实验证据越来越多，但还没有全面的解释为什么它是正确的。在这个应用中，我们希望利用我们在计算原子融入熔融硅酸盐和结晶硅酸盐晶体结构的方式方面的丰富经验来研究氦的融入。这尤其具有挑战性，因为氦原子很小，在原子尺度上有许多可能的结合机制。为了理解氦悖论背后的基本物理化学，我们将计算氦、铀和钍在各种地幔矿物中的相对溶解度。我们还将使用计算机模拟技术来比较相对于大块材料（如果天然样品由小晶体组成，这一点很重要）界面上氦的掺入，并使用模拟长时间尺度的方法来观察致密硅酸盐中的氦扩散。

Despite being present in miniscule amounts in natural rocks and minerals the noble gases (helium, argon, krypton, xenon, radon) are potentially incredibly valuable tracers of the geochemical evolution of the Earth's mantle. At represent, we cannot really use them because we do not entirely understand how they behave when a mantle rock melts. Our previous work on the noble gases other than helium they become part of the crystal structure of minerals in a similar way to many other atoms The question remains as to how helium, the lightest and potentially the most useful member of the series, will behave when the mantle melts. He has forms, with different atomic masses. The lighter one is no longer produced within the Earth; it was incorporated in the Earth very early in its history. The heavier one, on the other hand is continually produced in the Earth by radioactive decay, mostly of uranium and thorium. The principal objective of this grant is to resolve the so-called 'helium paradox' wherein mantle-derived melts with low ratios of the different types of helium (lighter with respect to heavier) have high helium concentrations, whereas melts with high ratios have low helium concentrations. This long-standing observation is hard to reconcile with the 'standard model' in which helium is more likely to pass into molten mantle material than uranium or thorium. Instead, they point to an 'alternative model' in which the reverse is true: helium prefers to remain in the solid. There is emerging experimental evidence for the alternative model, but as yet no comprehensive explanation of why it is correct. In this application we wish to use our extensive experience of calculating the way that atoms become incorporated into the crystal structure of molten and crystalline silicates to study helium incorporation. This is particularly challenging as there are many possible mechanisms of incorporation on the atomic scale because the helium atom is small. To understand the fundamental physical chemistry behind the helium paradox we shall calculate the relative solubilities of helium, uranium and thorium in a wide range of mantle minerals. We will also use our computer simulation techniques to compare the incorporation of helium at interfaces relative to the bulk material (important if natural samples consist of small crystals), and also use methods for simulating long timescales to look at helium diffusion in dense silicates.