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 rocks^1–7 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 processes^3,6. 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.