Effects of seawater alteration on the 234U/238U-ratios of mid-ocean ridge basalts

After having been formed at mid-ocean ridges, oceanic plates cool as they move away from the ridges and become older. Much of this cooling is related to hydrothermal alteration by seawater infiltrating the plates. During seawaterrock interaction, primary minerals are transformed into secondary minerals, influencing the bulk rock composition. Two processes mainly control element abundances in the oceans: continental weathering and hydrothermal activity in the oceanic plate. The hydrothermal activity is further divided into high-T (near ridge) and low-T (off the ridge) alteration. High-T alteration processes can be easily studied in hydrothermal vent regions. As low-T alteration processes in the oceanic crust are rather diffuse and slow, their limits and conditions (time, temperature, etc.) are largely unknown and thus investigation of off-ridge alteration is important. This study investigates the alteration processes of mid-ocean ridge basalts (MORB) in the low-T environments and constructs a time frame for seawater-rock interaction in these regions. Weathering of the continental plates keep the U-serie decay chain in the ocean in disequilibrium (a 234U/238U-ratio of ~1.14). On the otherhand, MORB formed at the ridge are in secular equilibrium. Thus any seawater alteration of MORB leeds to an enriched 234U/238U-ratio. After ~5 half-life times the MORB are again in secular equilibrium. Prelimiary 234U/238Udata on old oceanic crust from ODP Site 1179 (~129Ma) and ODP Site 843 (~94Ma) shows that altered MORB are in secular equilibrium. Nevertheless, higher U concentrations in the altered MORB compared to fresh MORB on both Sites suggest that seawater alteration occurred earlier, but outside the radiogenic detection window of ~1.25Ma for the 234U/238U decay chain. Additional U-series measurements on altered MORB from younger ODP Sites are currently in progress.