226Ra excesses in mid-ocean-ridge basalts and mantle melting

Abstract

The discovery of radioactive disequilibrium among isotopes of the 238U decay chain in recent volcanic rocks1–7 has enhanced our understanding of both the timescale and processes of melting in the mantle. Excesses of 230Th (half life 75,200 yr) relative to its parent 234U in some of these rocks indicate that thorium is preferentially partitioned into the melt phase, and constrain the combined duration of melting and magma transport to less than a few hundred thousand years. Recent observations of 226Ra and 228Ra excesses over their respective parents 230Th and 228Th in individual subaerial volcanoes have led to even more precise determinations of the timescale of magmatic processes3,6. Here we report the observation and implications of 226Ra (half life 1,620 yr) excesses in mid-ocean-ridge basalts. These excesses are probably produced during melting in the mantle, and the timescale they impose suggests that they are controlled by kinetic processes, rather than chemical equilibrium. Regardless of the process, the 226Ra excesses constrain the length of the magma transfer times from melting through eruption (and sample collection) to less than about a thousand years.