Chromite Oxidation Patterns Associated to Serpentinization: Case Studies from the Mid-Atlantic Ridge, the Alter do Chão Massif (NE Alentejo, Portugal) and the Ronda Massif (Spain)

Chromite oxidation during serpentinization of host peridotites is a well-documented process. Detailed compositional characterization chromites and Cr-rich spinels from three geotectonic settings provided the basis for this study, focused on comparison their patterns as means to evaluate relative mobility spinel components serpentinization-related in those different contexts, namely: (i) tectonic exposures serpentinized oceanic upper-mantle Azores sector Mid-Atlantic Ridge (MAR); (ii) Cabeço de Vide (CV, Alter do Chão Massif, Portugal); (iii) subcontinental mantle Ronda Massif (Spain). Electron microprobe data show that: irrespective setting original composition, Cr-spinels follow similar trends; early Cr-spinel oxidation, involving significant Mg- Al-depletion leading formation ferritchromit rims, has been more intense CV serpentinites than recent MAR serpentinites; with possible exception Zn, trace (Mn, Ti, V, Ni Co) seem be relatively immobile thus becoming slightly enriched rims; (iv) extreme responsible incomplete outer rims magnetite several grains. Time, fluid pH, fluid/rock ratios main factors controlling intensity extension chromite oxidation.