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的含量很小，但它们可能是地幔地球化学演化的宝贵示踪剂。目前，我们不能真正使用它们，因为我们不完全了解它们在地幔岩石熔化时的行为。我们以前对氦以外的惰性气体的研究，它们以类似于许多其他原子的方式成为矿物晶体结构的一部分。问题仍然是氦，这个系列中最轻和潜在的最有用的成员，在地幔熔化时将如何表现。他有不同的形式，有不同的原子质量。较轻的一个不再在地球内部产生;它在地球历史的早期就被纳入地球。另一方面，较重的中子则是在地球上通过放射性衰变不断产生的，主要是铀和钍的衰变。这项资助的主要目的是解决所谓的“氦悖论”，即不同类型氦（轻相对于重）比例低的地幔熔体具有高氦浓度，而比例高的熔体具有低氦浓度。这一长期的观察结果很难与“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，情况正好相反：氦更喜欢保持在固体中。有新的实验证据支持替代模型，但还没有全面的解释为什么它是正确的。在本申请中，我们希望利用我们计算原子并入熔融和结晶硅酸盐的晶体结构中的方式的丰富经验来研究氦的并入。这是特别具有挑战性的，因为在原子尺度上存在许多可能的掺入机制，因为氦原子很小。为了理解氦佯谬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。我们还将使用我们的计算机模拟技术来比较氦在相对于大块材料的界面处的结合（如果天然样品由小晶体组成，则这一点很重要），并且还将使用模拟长时间尺度的方法来观察氦在致密硅酸盐中的扩散。

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.