@article{fdi:010061146,
  title = {{M}u {XANES} study of iron redox state in serpentine during oceanic serpentinization},
  author = {{A}ndreani, {M}. and {M}unoz, {M}arguerite and {M}arcaillou, {C}. and {D}elacour, {A}.},
  editor = {},
  language = {{ENG}},
  abstract = {{S}erpentinization of ultramafic rocks at mid-ocean ridges generates significant amounts of {H}-2, {CH}4, and supports specific biological communities. {T}he abiotic {H}-2 production is attributed to the reduction of {H}2{O} during serpentinization, which balances oxidation of ferrous iron contained in primary minerals (mainly olivines and pyroxenes) to ferric iron contained in secondary minerals (mainly serpentines and magnetite). {M}agnetite has thus far been considered as the sole {F}e3+-carrier for estimating bulk {H}-2 production, notably because the valence of iron in serpentine minerals and its relationship with both magnetite abundance and serpentinization degree are usually not determined. {W}e show that the serpentine contribution to the {F}e and {F}e3+ budget has a significant effect on {H}-2 production. {W}e performed mu-{XANES} analysis at the {F}e {K}-edge on thin sections of peridotites with various degrees of serpentinization from {ODP} {L}eg 153 ({MARK} region, 23 degrees {N}). {F}e3+/{F}e-{T}ot in oceanic serpentines is highly variable (from similar to 0.2 to 1) at the thin section scale, and it is related non-linearly to the local degree of serpentinization. {A} typical value of 0.7 is observed above 60% serpentinization. {T}he highest values of {F}e3+/{F}e-{T}ot observed within or close to late veins suggest that the {F}e3+/{F}e-{T}ot in serpentine record the local water-rock ({W}/{R}) ratio, as previously proposed from thermodynamic modeling. {W}e estimate that the ({W}/{R}) ratio increased from similar to 0.6 to 25 during serpentinization at {MARK}, and locally reached similar to 100 in veins. {M}ass balance calculations combining all mineral and bulk rock analyses provide the distribution of {F}e and {F}e3+ as serpentinization progresses. {S}erpentine dominates the {F}e3+ budget of the rock over magnetite during the first 75% of serpentinization, contributing up to 80% of the total {F}e3+. {A}t later stages, serpentine contribution to the {F}e3+ budget decreases down to similar to 20%, while magnetite formation exponentially increases. {I}ron transfer from serpentine to magnetite balances the bulk {F}e3+ content of the rock that increases almost linearly with the advance of the reaction. {F}ormation of serpentine accounts for the majority of {F}e3+ and {H}-2 production at early stages of serpentinization at a depth >2 km at {MARK} where the concentration of {H}-2 can reach more than 100 m{M} according to the low {W}/{R}. {H}-2 production values and depths can vary from one site to another, depending on the evolution of the temperature, {W}/{R} ratio, inlet fluid composition, and favored formation of serpentine vs. magnetite. {A}t {MARK}, {F}e3+ in serpentine represents 15-27% of the total {F}e contained in a rock serpentinized to more than 80%, and accounts for 25% of the total {H}-2 production that is estimated at 325-335 mmol/kg of rock. {T}he absence of magnetite does not necessarily mean a negligible {H}-2 production, even at low {T} conditions (<150-200 degrees {C}) under which the {F}e- and {F}e3+-richest serpentines have been observed. {S}erpentine minerals are important {F}e3+-carrier in the altered ocean lithosphere, and may affect mantle redox state while dehydrating at depth in subduction zones.},
  keywords = {{S}erpentine ; {I}ron ; {R}edox ; {H}ydrogen ; {W}/{R} ; {M}id-ocean ridge ; {ATLANTIQUE}},
  booktitle = {},
  journal = {{L}ithos},
  volume = {178},
  numero = {},
  pages = {70--83},
  ISSN = {0024-4937},
  year = {2013},
  DOI = {10.1016/j.lithos.2013.04.008},
  URL = {https://www.documentation.ird.fr/hor/fdi:010061146},
}