INcorporation and DIffusion of noble gases in Grain bOundaries – INDIGO

Sintering and doping by high pressure techniques: piston cylindre and Paterson press
Nobles bases spectrometry
SEM, EBSD and TEM

- First publication (EPSL, 2015) by the research group INDIGO.
- Invited Talk, S. Demouchy and P. Brunard inDeécembre 2015 (AGU fall meeting).
- Diffusion coefficients for He and Ar in olivine grain boundaries (Delon et al., soumis, Chemical Geology).

For the next years we will focus on doping of deformed olivine aggregates as well as minéral mixutr e( olivine + diopside).

1. Burnard P, Demouchy S, Delon R ., Arnaud N,. Marrocchi, Y., Cordier P, Addad A. (2015) The role of grain boundaries in the storage and transport of noble gases in the mantle. Earth Planet. Sci. Lett. 430, 260-270. doi : 10.1016/j.epsl.2015.08.024
2. Delon R ., Demouchy S, Marrocchi Y., Boudhifd M., A., Barou F., Cordier P., Addad, A., Burnard P, A.. Helium incorporation and diffusion in polycrystalline olivine. Submitted to Chem Geol. August 2017.

The noble gases (NG) are key tracers of the evolution of the terrestrial mantle-atmosphere system. The large and well-identified NG isotopic and abundance heterogeneities in the Earth’s mantle fundamentally influence our understanding of mantle geodynamics; for example, Xe isotopic compositions of mantle rocks have recently been used to demonstrate that some part of the mantle has preserved material that is at least 4.45 Ga old (Mukhopadhyay, Nature 2012). However, in order to determine how these primordial (or near-primordial) NGs have been preserved, we need constraints on the fundamental behaviour of NGs in the mantle such as: Where noble gases are sited? How do they partition during melting? And how they might be transported?

At present, these parameters are seriously under constrained because a) it is technically challenging to perform experiments with (noble) gases at high temperature and pressure and b) NG behaviour in silicate materials is complex with different potential sites and incorporation mechanisms. INDIGO aims to perform a series of highly innovative experiments with which to constrain noble gas behaviour in solid, polycrystalline mantle compositions. These experiments will provide the basis for thermodynamic models applicable to mantle conditions. Despite being innovative, recent preliminary results by the project proposers demonstrate that the methods proposed are effective and practical for studying NGs at mantle-relevant conditions (Burnard et al, EPSL, submitted).

An important novel aspect to our approach is that NG behaviour (partitioning, diffusion) in polycrystalline material will be addressed in addition to simple pure minerals; it has recently been shown (Hiraga, nature 2004) that intergranular interfaces provide potential storage sites for incompatible elements, including the noble gases. Interfaces will certainly influence NG behaviour in nature; the experiments proposed in INDIGO are designed to simulate the mantle as a well-equilibrated assemblage rather than as isolated minerals. This is a fundamental innovation relative to previous work investigating noble gas behaviour in the mantle.

Recently, the influence of intergranular interfaces on geologically important properties (from accommodation of strain during deformation to diffusion of chemical species) has received significant focus from petrophysicists worldwide. However, the noble gases themselves can be used as tools for investigating the nature of the intergranular interface. Being inert, the noble gases do not form chemical bonds at these conditions, and, as a result, the noble gases are passive tracers for the environment surrounding the atom. As noble gas atomic radii vary considerably and systematically with mass, these variations can be used to probe the structure of non-crystalline solids. For example, noble gas solubilities in silicate liquids depend on the distribution of vacancy sizes: interfaces are also vacancy-rich phases and noble gas incorporation in grain boundaries can similarly elucidate the structure of the grain boundary itself. This has never previously been addressed.

Noble gas distribution in rocks and minerals are highly dependent on defects, yet this has never been quantified. The INDIGO experimental protocol permits defect population densities to be varied. Thermodynamic models of NG solubility will include both intergranular interfaces and defect populations.

The work proposed involves carrying out innovative experiments using existing high pressure, high temperature presses then analysing the run products with standard noble gas mass spectrometric techniques. The main financial demands are for personnel (1 Ph.D. student + 1 post-doc; an additional Ph.D. student has already been funded). Both intergranular interfaces and noble gas geochemistry are very “current” topics with many extremely well cited papers in high visibility journals.