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 Fe 3+ /Fe Total(BR) (=0.55) and magnetite modal content (=2.8–4.3 wt%). Micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy measurements of serpentines at the Fe–K edge show that antigorite has a lower Fe 3+ /Fe Total ratio (=0.48) than oceanic lizardite/chrysotile assemblages. The onset of Atg-serpentinites dehydration is marked by the crystallization of a Fe 3+ -rich antigorite (Fe 3+ /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 Fe 3+ into serpentine rather than into olivine. The Atg-breakdown is marked by a decrease in Fe 3+ /Fe Total(BR) (=0.34–0.41), the crystallization of Fe 2+ -rich phases and the quasi-disappearance of magnetite (=0.6–1.4 wt.%). The observation of Fe 3+ -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 f o 2 inside the portion of the subducted mantle. Serpentinite dehydration could thus produce highly oxidized fluids in subduction zones and contribute to the oxidization of the sub-arc mantle wedge.