Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life(?)

The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50-300 degrees C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and dihydrogen gas (+/- brucite) are products of the reaction. At higher temperatures (400-600 degrees C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.