Redox state of iron during high-pressure serpentinite dehydration

The Cerro del Almirez massif (Spain) represents a unique fragment of serpentinized oceanic lithosphere that has been first equilibrated in the antigorite stability field (Atg-serpentinites) and then dehydrated into chlorite-olivine-orthopyroxene (Chl-harzburgites) at eclogite facies conditions during subduction. The massif preserves a dehydration front between Atg-serpentinites and Chl-harzburgites. It constitutes a suitable place to study redox changes in serpentinites and the nature of the released fluids during their dehydration. Relative to abyssal serpentinites, Atg-serpentinites display a low Fe3+/Fe-Total(BR) (=0.55) and magnetite modal content (=2.8-4.3 wt%). Micro-X-ray absorption near-edge structure (mu-XANES) spectroscopy measurements of serpentines at the Fe-K edge show that antigorite has a lower Fe3+/Fe-Total ratio (=0.48) than oceanic lizardite/chrysotile assemblages. The onset of Atg-serpentinites dehydration is marked by the crystallization of a Fe3+-rich antigorite (Fe3+/Fe-Total =0.6-0.75) in equilibrium with secondary olivine and by a decrease in magnetite amount (=1.6-2.2 wt%). This suggests a preferential partitioning of Fe3+ into serpentine rather than into olivine. The Atg-breakdown is marked by a decrease in Fe3+/Fe-Total(BR) (=0.34-0.41), the crystallization of Fe2+-rich phases and the quasi-disappearance of magnetite (=0.6-1.4 wt.%). The observation of Fe3+-rich hematite and ilmenite intergrowths suggests that the O-2 released by the crystallization of Fe(2+)rich phases could promote hematite crystallization and a subsequent increase in fo(2) inside the portion of the sub-ducted mantle. Serpentinite dehydration could thus produce highly oxidized fluids in subduction zones and contribute to the oxidization of the sub-arc mantle wedge.