Abyssal serpentinites, evidence of the geochemical variety of the Earth’s ocean floor

Near ridges, the thermal regime of the oceanic lithosphere is generally proportional to the abundance of magmatic rocks, playing a major role in the nature of geochemical exchanges during serpentinisation. At ultra-slow ridges and transform faults, low magmatic activity favours serpentinisation by slightly modified seawater, which is imprinted in the geochemical signature of the rocks. Conversely, at slow or fast ridges, high-temperature interaction between seawater and abundant magmatic rocks modifies the physico-chemical properties of the fluids, which are then more inclined to the mobility and storage of redox-sensitive elements (U, Eu, Ce, As, Sb), including metals (Zn, Cu). These differences lead to contrasting geochemical signatures of serpentinites around the world, reflecting the variability of chemical exchanges. On the other hand, the behaviour of elements that are not redox-sensitive but are mobile in fluids (Cs, Ba, Rb, B or Li) is little affected by these variations in context, leading to a homogeneous geochemical record of these elements in serpentinites. Serpentinisation of peridotites in forearc² contexts, where fluid chemistry is largely influenced by the underlying subduction, appears to be an exception in abyssal contexts.