les éléments volatils dans le manteau terrestre.

Volatiles in Earth´s mantle

Résumé:

I will review some new experimental data on the behavior of water,
carbon dioxide and noble gases in Earth´s mantle. The water solubility
in upper mantle minerals was believed to be low compared to transition
zone minerals. Recent experiments have shown that pyroxenes under very
low pressure can incorporate nearly one weight percent of water. This
produces a minimum in water solubility in the upper mantle that
precisely coincides with the seismic low velocity zone. I therefore
suggest that at this depth, not all water can be stored in minerals
anymore; the excess water forms a partial melt. Partial melting in the
asthenosphere is consistent with the observed anisotropy of electrical
conductivity. Unlike water, carbon is almost completely insoluble in
mantle minerals and therefore forms a separate carbonate phase. CO2
has a major effect on the melting point of mantle peridotite, but the
mechanism of dissolution of CO2 in silicate melts is not well
understood. I show new synchrotron infrared absorbance spectra
obtained from silicate melt directly at high pressure and high
temperature. The data show that at typical magmatic temperatures,
molecular CO2 dominates in the melts over carbonate. Noble gases are
generally believed to be extremely incompatible in mantle minerals. I
show new experimental results that demonstrate that argon is highly
soluble in perovskite, the main constituent of the lower mantle, with
solubilities approaching 1 wt. percent. Xenon solubility, on the other
hand, appears to be negligible. These data suggest some obvious
solution for the Earth´s "missing xenon".