Resolving the incorporation of He in silicate minerals

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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.