Mass transfer in the oceanic lithosphere: Serpentinization is not isochemical

Whereas the serpentinization reaction leads to stark differences in the physical properties of mantle rocks at mid-ocean ridges, the chemical changes associated with this reaction are thought to be restricted to the addition of water and the generation of hydrogen (isochemical reaction). Here, I compile a geochemical dataset of serpentinized peridotites at mid-ocean ridges evidencing that a decrease by up to 11% of the MgO/SiO2 ratio is associated with serpentinization. This MgO/SiO2 decrease is consistent with the calculated distribution of Mg in the minerals since, during isochemical serpentinization, 10% of the Mg should be contained in brucite, an Mg-hydroxide not commonly observed in serpentinized peridotites, which are typically composed of serpentine (Mg3Si2O5(OH)(4)) and magnetite (Fe3O4). This latter mineralogical assemblage and a decrease of the MgO/SiO2 ratio were only reproduced in numerical models of peridotite reacting with fluids containing aqueous silica at fluid to rock (FIR) ratios greater than 20. At higher F/R ratios, talc (Mg3Si4O10(OH)(2)) was found to be stable, in agreement with observations in extremely altered samples found at mid-ocean ridges. The potential sources for aqueous silica in the fluid are the alteration of mafic units intruding mantle rocks at slow-spreading ridges. The mineralogical and chemical changes associated with SiO2 gain during serpentinization at mid-ocean ridges will have consequences on abiotic hydrogen production, contribute to a volume increase of 50% and decrease water incorporation during serpentinization by more than 10% compared to isochemical serpentinization. These changes will also increase the depth at which fluids are released by dehydration reactions in subduction zones by more than 20 km. (C) 2015 Elsevier B.V. All rights reserved.