Formation of Olivine Veins by Reactive Fluid Flow in a Dehydrating Serpentinite

Abstract

Many exposed high-pressure meta-serpentinites comprise a channelized network of olivine-rich veins that formed during dehydration at depth and allowed the fluid to escape from the dehydrating rock. While previous studies have shown that chemical heterogeneities in rocks can control the formation of olivine-enriched vein-like interconnected porosity networks on the sub-millimeter scale, it is still unclear how these networks evolve toward larger scales and develop nearly pure olivine veins. To explore this, we study the effect of reactive fluid flow on a dehydrating serpentinite. We use thermodynamic equilibrium calculations to investigate the effect of variations in the bulk silica content in serpentinites on the dehydration reaction of antigorite + brucite = olivine + fluid and the silica content of this fluid phase. Further, we develop a numerical model that combines the effects of intrinsic chemical heterogeneities with reactive transport with dissolved silica as metasomatic agent. Our model shows how reactive transport can lead to vein widening and olivine enrichment within a vein in an antigorite-rich matrix, such as observed in the veins of the Erro Tobbio meta-serpentinites. This is a critical step in the evolution toward larger-scale vein systems and in the evolution of dynamic porosity, as this step helps account for the chemical feedback between the dehydrating rock and the liberated fluid.