IV. Redox Processes

IV. Redox Processes

Nb-Ta
Evolution of the silicate Earth’s Nb/Ta ratio during accretion (after Cartier et al., 2014)

 

The effect of oxygen fugacity on element partitioning during planetary differentiation (with the development of fO2 proxies such as Nb/Ta and V/Yb ratios).  Our metal/silicate partitioning experiments at a range of oxygen fugacities show that the Nb/Ta ratio is a robust tracer of the prevailing oxygen fugacity during silicate/melt equilibria in the magma oceans that resulted from the giant impacts during the Earth’s accretion.

 

 


 

 

Debret-2015
Evolution of the Fe3+/Fetotal ratio in the subducting serpentinized mantle (after Alt et al., 2012 and Debret et al., 2015).

 

Role of serpentinites in the redox state of the mantle. Serpentinization of mantle rocks is an oxidizing process : serpentine can display Fe3+/Fetotal ratio up to 100%, as shown by the study of serpentinized mantle rocks from the Alps and Almirez (Spain). From our work on natural mantle samples, we can propose that the oxygen fugacity conditions that prevail during the breakdown of that major hydrous mineral in the slab are close to the magnetite-hematite buffer. This observation can help explain why arc basalts tend to be more oxidized than MORBs and OIBs.

 

 

 

 

Redox-inclusion
Left: Melt inclusion (with a thermal shrinkage bubble) trapped in an olivine phenocryst. Right: Fe K-edge XANES spectra: the pre-peak region of the spectra shown here allows the determination of the Fe speciation in the melt inclusion.

 

Redox conditions in basaltic melts. The study of primitive melt inclusions whose compositions are not extensively modified by differentiation processes that affect lavas, gives access to the redox conditions of the mantle in the different geodynamic settings.

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