Vein Formation and Element Mobility During Serpentinization of Peridotites: Mineralogy and Thermodynamic Modeling

Abstract:

Serpentinization is a common mineralogical process that occurs in ultramafic rocks when water interacts with the primary minerals olivine and pyroxene to form a rock dominated by serpentine. Temperature of hydration and the primary mineralogy of the peridotite strongly influence the alteration products. We studied peridotites with low to extremely low degrees of serpentinization from two settings, the Santa Elena ophiolite in Costa Rica and the Gakkel Ridge, to test how temperature and primary mineralogy control reactions, reaction sequences, and subsequent vein textures during peridotite hydration. Serpentinization of a harzburgite from Costa Rica occurred at around 250°C, resulting in olivine-hosted veins that are a mixture of brucite and serpentine – with a bulk composition approximately equivalent to hydrated host olivine – and later stage veins that also contain magnetite. Serpentinite that formed by hydration of orthopyroxene is accompanied by talc, with a bulk composition (serp + tlc) that is depleted in silica relative to the orthopyroxene host. In contrast, veins in orthopyroxene in an almost un-serpentinized peridotite (degree of serpentinization ~ 5 %) from the ultra-slow spreading Gakkel Ridge have an average composition very similar to the host orthopyroxene, whilst veins in olivine suggest slight enrichment in silica compared to the host olivine. Replacement of orthopyroxene here results in the formation of serpentine, talc and amphibole. The presence of amphibole in orthopyroxene and the higher degree of orthopyroxene replacement compared to olivine suggests that serpentinization occurred at higher temperatures than in the Santa Elena peridotites, resulting in the mobilization of more silica through the system.

Thermodynamic models of the hydration of olivine and orthopyroxene agree with the mineralogical observations and show that hydration of olivine first results in the simple formation of serpentine and brucite. Increasing water-rock ratios then increase fluid availability, permitting magnetite formation in olivine-hosted veins. At increased silica activity brucite becomes instable so that hydration of orthopyroxene results in the formation of serpentine and talc, with locally forming amphibole.