Abstract The detailed mineralogical study of serpentinized peridotites collected at 13 localities at mid-ocean ridges, in ophiolites and ultramafic bodies shows the formation of two alteration zones around olivine grains. At the olivine contact, a fine-grained mixture of serpentine, Fe-brucite (MgMg+ Fe molar ratio comprised between 0. 66 and 0. 82) and awaruite occurs (Reaction Zone 1). X-ray mapping indicates limited mass transfer during Reaction Zone 1 formation, suggesting isochemical serpentinization except for the addition of water. The measured distribution of Fe and Mg between serpentine and brucite in Reaction Zone 1 is well reproduced with thermodynamic modelling incorporating the latest data for the Fe (OH) 2 endmember. Thermodynamic modelling also reveals that, at low water to rock ratio, awaruite formation limits H2 production to values more than one order of magnitude lower than previous estimates. The predicted H2, aq concentrations are comprised between 10−3 and 10−2 mol/kg, that is in the same range than the maximum values measured in fluids expelled at ultramafic-hosted hydrothermal sites. At a water to rock ratio of 1, the updated thermodynamic model only predicts magnetite formation after olivine at temperatures above 300 °C, that is above the temperature estimates for serpentinization based on published oxygen isotope data. Nevertheless, a second assemblage composed of Ni-bearing magnetite, serpentine and Mg-brucite (MgMg+ Fe molar ratio comprised between 0. 83 and 0. 98) can be found at the mesh rim (Reaction Zone 2). Reaction Zone 2 can display a symplectite microtexture. Transmission electron microscopy reveals the presence of porosity and Fe-brucite relics in the symplectite, suggesting Reaction Zone 2 formation after Reaction Zone 1 by dissolution-precipitation. Significant mass transfer is observed in Reaction Zone 2 at the mesh rim, probably associated with the solid volume decrease of 7 % during Reaction Zone 1 reaction to form Reaction Zone 2. The mineralogy and the composition of Reaction Zone 2 are reproduced with thermodynamic modelling of olivine serpentinization not by increasing the water to rock ratio but rather by removing H2 from the system. This indicates that H2 diffusion is the main driver for magnetite formation during serpentinization. The H2, aq concentrations at the equilibrium with Reaction Zone 2 fall in the 10−7 - 10−3 mol/kg range. The relative proportion of Reaction Zone 1 and Reaction Zone 2 in serpentinized peridotites has first-order impacts on H2 production.
Malvoisin et al. (Thu,) studied this question.