Formation of olivine-rich veins by reactive fluid flow in a dehydrating serpentinite

Abstract

Many exposed high-pressure meta-serpentinites comprise a channelized network of olivine-rich veins which formed during dehydration at depth and served as pathway for fluid escape. Previous studies showed that the formation of an olivine enriched vein-like interconnected porosity network on the µm-scale is controlled by chemical heterogeneities in the rock. However, the evolution towards larger scale and nearly pure olivine veins is not yet well understood. Here we study the effects of reactive fluid flow on a developing vein system during dehydration. We use thermodynamic equilibrium calculations to investigate the effects of bulk silica content variations in serpentinites on the dehydration reaction of antigorite + brucite = olivine + free fluid and silica content of this fluid phase. We develop a numerical model combining the effects of intrinsic chemical heterogeneities with reactive silica transport. Increasing temperatures lead to local fluid overpressure and the liberation of a silica-poor fluid in a subdomain with initially increased bulk iron and decreased silica content. The fluid overpressure drives fluid flow into other subdomains where the fluid enhances dehydration and leads to olivine enrichment in an iron-enriched vein. Our model shows how reactive silica transport can lead to vein widening and olivine enrichment within the veins as observed in the Erro Tobbio meta-serpentinites. Thus, reactive fluid flow is a critical step in the evolution towards a larger scale vein system and a dynamic porosity evolution by accounting for a chemical feedback between the dehydrating rock and the liberated fluid.