Moving reactive interfaces and fractal carbonate replacement patterns in serpentinites: evidence from the southern Iberia Abyssal Plain

Serpentinized ultramafic rocks recovered from beneath the southern Iberia A byssal Plain (Ocean Drilling Programme Leg 173) provide the first record of fractal carbonate replacement patterns in a serpentinite. The patterns are expressed as microscopic branching aggregates (clusters) of aragonite diss eminated throughout the serpentinites. Aragonite growth was the final miner alization event. The aragonite diminishes rapidly in quantity from an essen tial to a trace component of the serpentinite over a distance of similar to 40 m from a normal fault. Decreasing abundance of aragonite away from the normal fault links the growth of the carbonate to the multistage hydrotherm al mineralization associated with the fault. Aragonite clusters are concentrated in picrolite, where they are interwoven with colloid-sized chrysotile, and show fractal growth habits. Areas adjac ent to the clusters are sites of Mg enrichment of the serpentine medium rel ative to aragonite-free picrolite. It is interpreted that the aragonite clu sters result from incursions of reactive seawater solutions through fine-sc ale pore structures in and around the fault in response to pressure gradien ts emanating from active tectonism. Cluster growth is interpreted to be a p ercolation phenomenon and provides a novel source of information on the nat ure of fine-scale reactive fluid flow, pore-space connectivity, and carbona te replacement processes in serpentinites.