What is the mechanism behind changes in oxidation state (of iron, for example) as magma rises?
My question is a more fundamental one, being, if this equilibrium (#11) governing the changes in oxidation states becomes more prevalent as the pressure decreases rapidly, what are the governing equations for when the magma is deeper in the earth?


My interpretation is that at the volcano being studied, the right side of equation 11 is favored at low pressure, and the left side is favored at high pressure (greater depth).  Deep magma there contains sulfide ($\ce{S^2-}$) in accordance with equation 11, rather than sulfate ($\ce{SO4^2-}$) which would involve equation 12.  So equation 11 applies to all depths being studied and equation 12 does not apply to this volcano at any depth.