Oxygen isotopic fluxes associated with high-temperature processes in the rift zones of Iceland

High-temperature reactions in the rift zones in Iceland, involving magma, crust and hydrothermal fluids, cause the lowering of the 18O/16O ratio of the crust relative to the mantle. A significant flux of 18O to the hydrosphere results from these reactions. We use the available oxygen isotope data to estimate the isotopic composition of the crust produced in this environment. This enables us to place limits on the flux of 18O from the mantle to the hydrosphere. The rift zones in Iceland are efficient in transferring 18O to the hydrosphere; we find that they release up to five times the amount of 18O as does an equivalent length of mid-ocean ridge. In spite of this large flux the net depletion of the Icelandic crust is relatively small, the data show that the δ18O of the crust in the rift zone, has been lowered by about 1 to 3‰. Our findings have implications for the 18O-history of seawater. We argue that the rift zones in Iceland are a model analog for a global rift system above sea level, a proposed tectonic configuration that perhaps was prevalent in the geological past. Contrary to the proposed termination of the 18O-flux from the mantle to the hydrosphere, we demonstrate that a significant 18O-flux still prevails. This particular tectonic configuration, therefore, does not allow depleted seawater to persist.