Oxidation of magmas by redox exchanges of hydrogen with mantle peridotite
Résumé
In concentrations ranging from ppm to wt% hydrogen is always present in natural minerals or melts in the form of a variety of species with different redox states. The identification of the nature and the mobility of the different H-species is a key to understanding the Earth`s mantle. In olivine and NAM in general, transport of H-species is dominated by ionic motion whereas in silicate melts, H migrate in the molecular form H2 and H2O. In spite of these fundamental contrasts, similarities can be outlined. In olivine, incorporation/extraction has been revealed to operate through a two-stage process with different activation energies. A faster stage is related to redox exchanges between protons and electronic defects and a slower process is associated with intrinsic defects mobility. In magmas, a model, elucidating the rate of redox exchanges between hydrogen and iron, has been recently articulated. This model shows that, in response to a transfer of redox potential, water-derived species can be incorporated in/extracted from a melt at rate faster than molecular H2O migration. Therefore, for melts as for olivine, H transport operates through two processes: A fast one associated with redox exchanges and slower one, which do not involve redox transfer. Using the available kinetics models, we simulate a redox exchange through H transfer during percolation of magma in a simplified peridotite. Calculation reveals that the rate of H exchanges in melts is much faster than for olivine. Therefore, the extent of redox exchange will be primarily rate-limited by the flux of H in the peridotite. The key parameters for the modelling are 1- H diffusion in olivine and 2- the melt fraction. In range 1 to 15 percent of melt, incorporation of H in olivine by redox exchanges between protons and electronic defects induces a significant oxidation of the magma. The extent of this oxidation event could explain the redox feature of subduction-derived magmas. On the other hand, the percolated mantle should become more water-rich and reduced.