A weakly serpentinized Opx-rich harzburgite from New Caledonia shows that magnetite formation during serpentinization is related to gradients in silica activity. The primary assemblage consists of Opx megacrysts (XMg = 0.87) in a matrix of fine-grained Opx (XMg = 0.903) and olivine (XMg=0.900). Numerous serpentinite veins cut the Opx and extend into surrounding olivine. Where the veins occur in the Opx, the serpentine is Fe-rich (XMg = 0.87) and the veins lack magnetite, whereas where the veins cut olivine serpentine is magnesian (XMg=0.923-0.958) and magnetite is abundant. There is no talc where the Opx is undergoing serpentinization, nor is there brucite in the olivine-rich domains. In addition to being richer in iron, the serpentine associated with the Opx is also richer in silica than that associated with the serpentinization of olivine.
Our analyses indicate that the Opx was hydrated by a reaction in which the Fe and Mg were transferred directly to the serpentine. Assuming that all the iron is ferrous, this reaction can be modeled as:
1.5 Fe0.26Mg1. 74Si2O6 + 2 H2O = Fe0.39Mg2.61Si2O5(OH)4 + SiO2
If some of the Fe was ferric a bit less silica would be produced by this reaction. The silica released by this cell of the reaction was consumed by the serpentinization of the olivine, which followed this reaction:
3.084 Mg1.8Fe0.2SiO4 + 0.916 SiO2 + 5.89 H2O =
2 Mg2.775Fe0.225Si2O5(OH)4 + 0.056 Fe3O4 + 1.89 H2
We contend that the variation in serpentine chemistry between the Opx-dominated and olivine-dominated areas and the absence of magnetite in the serpentinized Opx is a reflection of silica activity. In areas of high silica activity, Fe was dissolved into the serpentine (either in the ferrous or ferric state), whereas in areas of low silica activity, silica was extracted from the serpentine, leaving Fe to form magnetite.