DATING OF NEOVOLCANIC MORB USING (RA-226 TH-230) DISEQUILIBRIUM

A method for determining ages of Mid Ocean Ridge Basalts (MORB) erupted within the last 8000-10,000 years is presented and assessed. This technique employs radioactive disequilibrium between the U-series parent/daughter isotopes Th-230 and Ra-226. Ba is used as a surrogate stable Ra isotope in order to constrain the amount of Ra-Th fractionation during generation of the basalts. Ages are determined by comparing the present day (Ra-226/Th-230) activity ratio to an inferred initial value. Measurements on a number of MORB from the East Pacific Rise (EPR) show that there is a strong correlation between Ba/Th and (Ra-226/Th-230) for the youngest basalts, giving confidence in this approach. The ages derived for dredged and submersible-collected basalts from the axial zone span a range from 0 to about 8000 years, and provide direct information about the time scale and possible episodicity of eruptions in this environment.