Interfacial storage of noble gases and other trace elements in magmatic systems

Abstract

Trace elements are widely used to unravel magmatic processes and constrain the chemical differentiation of the Earth. Key to their use is understanding the controls on fractionation between solid and liquid, which hinge on the energetics of trace element incorporation into crystals. At equilibrium most trace elements are incompatible in bulk magmatic minerals because of unfavourable incorporation energies. We use 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. Computational results are presented for divalent cations and Ar in bulk forsterite, at the {010} surface and at the {100}/{010} stepped grain boundary. For all species studied larger than Mn2+, segregation to the interface is highly exothermic. We discuss in particular the take-up of noble gases where, in contrast to earlier work, a recent experimental study concluded that argon is highly compatible in mantle minerals. In contrast our calculations indicate bulk Ar solubility is very small and suggests incorporation at mineral interfaces is overwhelmingly favourable.