Resolving the incorporation of He in silicate minerals

Lead Research Organisation: University of Bristol
Department Name: Chemistry


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.


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Pinilla C (2012) Interfacial storage of noble gases and other trace elements in magmatic systems in Earth and Planetary Science Letters

Description Trace elements are widely used to unravel magmatic processes and constrain how the Earth was formed.
Key to their use is understanding how they distribute between solid and liquid, and this in terms
hinges largely on the energetics of trace element incorporation into crystals. At equilibrium most trace elements such as noble gases
are incompatible in bulk magmatic minerals because of unfavourable incorporation energies. We have used
computational methods to explore the role that crystal interfaces can play in trace element incorporation
in minerals. We demonstrate that differences between bulk and interface incorporation energies can be
very large and lead to concentration differences of many orders of magnitude, consistent with experimental
evidence for interface enrichment. By emphasising the importance of adsorption/incorporation at interfaces,
we account for the competing effects of bulk equilibrium and interface segregation operating during melting
and crystallisation. For all species studied larger than Mn2+,
which includes the noble gases, segregation to interfaces is highly exothermic. We have simulated in particular the take-up of noble gases (not just helium).
In contrast to earlier work, a recent experimental study concluded that Ar is highly compatible
in mantle minerals. In contrast our calculations indicate bulk Ar solubility is very small and suggests
incorporation of Ar, as for He, at mineral interfaces is overwhelmingly favourable.
Exploitation Route Earth scientists and all those interested in the deep Earth must take account of interfaces as reservoirs for trace elements including all the noble gases. The importance of surface incorporation had not previously been appreciated by the Earth Sciences community.
Sectors Chemicals,Education,Culture, Heritage, Museums and Collections

Description Results now influence the interpretation of incompatible trace-element behaviour in the Earth are considerable: rates of melting and crystallisation as well as grain size can exercise profound controls on partitioning. (b) The more general conclusions of surface segregation of trace elements and noble gas incorporation are also crucially important in applications involving ceramics in materials science and in particular in problems involving radiation damage. These are currently being explored in conjunction with the Bristol Interface Analysis Centre and also with the modelling team at AWE.
First Year Of Impact 2008
Sector Aerospace, Defence and Marine,Education,Environment
Impact Types Cultural,Societal