Iron oxidation state in serpentines and magnesian chlorites of subduction-related rocks

Abstract. The ferric iron content in hydrothermally altered
ultrabasic rocks and their major minerals, serpentines and Mg-chlorites, is
important for establishing the oxidation state budget from oceanic ridges to
subduction zones, in carbonaceous chondrites, and for modeling phase
equilibria. A compilation of literature Mössbauer spectroscopic data on
serpentines and magnesian chlorites from high-pressure ophiolites yields much
lower ferric-to-total-iron ratios (Fe3+ / Fetotal) than those
obtained on similar samples by X-ray absorption near-edge spectroscopy (XANES), leading to contradictory estimates of
the ferric iron budget of subduction zones. New Mössbauer analysis of
antigorite and Mg-chlorite samples from suites of high-pressure ophiolitic
terrains of various Phanerozoic ages confirms the low and homogeneous values
previously obtained by this technique, while lizardite inherited from
oceanic hydrothermal alteration is ferric iron rich. We argue that XANES
values may be biased by photo-oxidation when samples have a high Mg content,
which is the case for serpentines and chlorites from subduction zones.
Photo-oxidation is less important in Fe-poor phyllosilicates of the mica and
talc families and does not affect the Fe-rich serpentines (greenalite,
cronstedtite) of meteorites or Fe-rich terrestrial phyllosilicates.
Mössbauer Fe3+ / Fetotal ratios of serpentine confirm the
occurrence of a major redox change at the lizardite–antigorite transition
near 300–400 ∘C rather than at the dehydration of antigorite at
500–650 ∘C in serpentinites from high-pressure ophiolites.
