Onset and Progression of Two-Stage Serpentinization and Magnetite Formation In Olivine-Rich Troctolite, Core 227, IODP Hole U1309D

Early serpentinization in core 227, IODP Hole U1309D is an approximately isochemical (except for water) replacement of olivine by a mixture of antigorite (Mg#92) and Fe-rich (Mg#65) brucite. The early serpentinite veins (type 1) are thin (<0.05mm) and exploit pre-existing cracks in olivine. Early serpentinization was a high-temperature (>300oC), rock-dominated, event. The second (main) episode of serpentinization produced through-going, mm-scale lizardite(Mg#96)-magnetite veins (type 2). Type 2 veins reflect open system serpentinization whose mineralogy and bulk chemistry requires the addition of silica. Magnetite forms one or more distinct bands in the interior of type 2 veins. At the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine) a brucite-serpentine mixture, similar to that found in type 1 veins, is present. The magnetite-lizardite cores of type 2 veins form by oxidation of the Fe component of this marginal brucite to yield magnetite combined with silicification of the Mg component of brucite to yield a more magnesian serpentine. However, the brucite-out reaction alone cannot account for the composition of the serpentine in the type 2 veins; desilication of ferrous serpentine must also occur.

In the core 227 samples olivine in contact with seawater initially reacts to form the low aSiO2 assemblage Fe-rich brucite plus serpentine. Unless isolated from seawater (as in type 1 veins), the brucite then reacts to form magnetite and a magnesian serpentine. Hence, magnetite and olivine are never in contact in these samples. The desilication of serpentine in the type 2 veins is a reflection of the instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine coexisting with magnetite in serpentinites is a function of serpentine-magnetite and not serpentine-olivine equilbria.