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Institut de minéralogie, de physique des matériaux et de cosmochimie

Mantle minerals and silicate melts

Given their role in mantle dynamics, it is important to understand the mechanisms of incorporation of volatile elements in minerals, either as trace or major components. In planetary interiors, most minerals are nominally anhydrous and the presence of water as hydroxyl groups has profound consequences on the physical properties of the mineralogical assemblages, affecting processes such as seismic wave propagation. Indeed, volatiles strongly modify the physical properties of crystals, but also their melting curves, and dramatically influence melt density and viscosity. However, though extensive knowledge of these effects at depth is established for water, it is still very rudimentary for some volatiles, and nonexistent for others.

Volatiles are also key tracers of terrestrial processes (planetary differentiation, magmatism, recycling). Two major examples are halogens that trace recycling from the surface to the deep Earth, and noble gases whose isotopes are used to date and quantify planetary dynamics.

Volatile elements as trace elements in minerals

We have developed a strong expertise in the experimental incorporation of volatile elements in mantle minerals (MP3). Their quantification remains a key issue, and the solubility of H, Cl, F and noble gases can be evaluated by nanoSIMS (COSMO, ROCKS) combined with other ion beam analysis methods such as ERDA, PIXE, NRA (MP3). Although significant progress has been made, the crystal chemistry of volatiles in nominally anhydrous phases is still poorly understood. An approach combining first-principles modeling of molecular species in crystalline hosts, prediction of equilibrium isotopic fractionation factors, vibrational spectroscopy by experiment and theory is developed, aiming at clarifying the mechanisms of volatile element incorporation in minerals (MINENV).

Volatile elements as main components of high-pressure minerals: Diamonds

Carbon is able to form minerals at depth, as carbonates or diamond (a mineral made of pure carbon).  The latter is a good natural example of the existence of deep volatile cycling. The study of diamonds from the Earth’s mantle benefits from a comparison between diamonds synthesized in the laboratory and natural gems, possible with access to the mineralogical collections of UPMC, MNHN and MINES ParisTech. This gives us the possibility of studying natural gems and carbonados, in order to understand diamond formation in the Earth and to give us insights on the deep carbon cycle (MP3, ROCKS).

Volatile elements in silicate melts

The effect of volatiles on the properties of liquid silicates is significant: volatile species (H2O, CO2, halogens) affect magma density, viscosity, partial melting, crystallization, ascent and volcanic dynamics. When released to the atmosphere, volatiles may impact climate and ozone layer chemistry. At depth, volatiles also influence the physical properties of melts (i.e. lithosphere-asthenosphere boundary, transition zone, core mantle boundary). Two strategies are developed: experimental studies mostly aiming at the characterization of volatile-enriched silicate melts at high pressures and temperatures through in situ characterizations (diamond anvil cells, Paris-Edinburgh presses, multi anvil presses) combined either with laboratory spectroscopy (Raman, FTIR, X-Rays…) or with synchrotron investigations (XRD, EXAFS, XANES, SXRF, FTIR…), and the study of natural samples such as melt inclusions trapped in minerals (MP3, ROCKS, COSMO).

Cécile Duflot - 20/12/17

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