Hydration of Archean komatiites on the seafloor: Evidence from trace elements and oxygen isotopes

In the modern Earth, mantle rocks hydrated by seafloor serpentinisation play an important role in the geological (deep) water cycle. In the Archean, however, the geological water cycle may have been different. Ultramafic rocks were present as komatiites, high-MgO plume derived lavas which erupted onto oceanic plateaux or the margins of continents. Previous work has shown that these komatiites dominantly erupted in submarine environments and contain significant amounts of water bound in antigorite, chlorite and tremolite (∼6 wt%). We present in situ trace element and oxygen isotopic data of hydrated komatiites from the Kaapvaal, Superior, Singhbhum, Yilgarn and Pilbara Cratons, which further constrain thermal conditions and element uptake during seafloor hydration. Silicate phases are enriched in fluid mobile elements such as B, W, As, Sb and Pb, compared to likely komatiitic melt compositions. These fluid mobile elements were derived from seawater, or possibly seawater-sediment equilibrated fluids. The δ¹⁸O composition of olivine is mantle-like (∼5.5 ‰), and the δ¹⁸O compositions of antigorite are homogenous within sample, with most falling in a range between +1.5–4.1 ‰. Oxygen isotope modelling of serpentine in equilibrium with Archean seawater with a δ¹⁸O composition of −1 ‰ suggests that these samples were hydrated at temperatures of ∼150–210 °C, after komatiite crystallisation. The pervasive hydration of komatiite lavas was likely due to their high surface area, as they erupted as extensive but thin flows. Temperature-driven fluid circulation in the komatiites may have been promoted by the cooling of underlying and overlying lava flows, magmatic injections or proximity to the mantle plume. Despite their relatively low abundance in the geological record, komatiite lava flows in the Archean may have impacted the geochemistry of the oceans and atmosphere. Additionally, deep burial of these hydrated komatiites would release water at high temperatures, promoting the fluid-fluxed melting of basalts and in turn contributing to the formation of the early tonalite-trondhjemite-granodiorite (TTG) continental crust.