VARIATIONS IN BRUCITE COMPOSITION AND ABUNDANCE IN COMPOSITIONAL DOMAINS IN A SERPENTINIZED HARZBURGITE

In a system closed except to water, stoichiometry of the olivine serpentinization reaction requires either the formation of solid brucite or of a dissolved Mg component. In the presence of excess silica, as, for example, when orthopyroxene (opx) is present in addition to olivine, the brucite component need never occur as long as there is chemical contact between the serpentinizing olivine and pyroxene. Here we report observations concerning brucite in a small region in a harzburgite at and near an orthopyroxene-olivine contact. As expected, brucite is entirely absent in serpentinite veins within the opx. More interesting is the observation that veins within the opx are not stoichiometrically balanced. In particular they are silica deficient. As one moves out of the opx into olivine domains, the olivine is completely replaced by serpentine – brucite is absent. We interpret this as reflecting a transfer via the fluid of excess silica from the opx into the region of serpentinizing olivine. As the distance from the opx boundary increases, brucite (identified both chemically and by Raman spectroscopy) appears in serpentinite veins within the olivine The veins are of limited length and both intragranular and anastomosing/intergranular. This brucite is enriched in Fe relative to coexisting serpentine minerals (Mg# of ~80 vs. ~93), as is typical of brucite in serpentinized olivine. Note that magnetite is not found in any of the domains as yet described. Finally, brucite also occurs in late stage, throughgoing, magnetite-cored veins. The brucite in these veins is highly magnesian (Mg#>90). We interpret this brucite as having supplied iron, in part via a ferric iron-H2O redox reaction, to magnetite and consider this observation to be strong evidence that brucite is a major source of the iron that produces the magnetite found in virtually all serpentinites.