210Pb–226Ra disequilibria in volcanic rocks

Isotopes of the U-decay series, such as 238U–230Th–226Ra, have been used successfully to study timescales of magmatic processes. This study provides an overview of a shorter lived isotope pair, 210Pb–226Ra, which allows study of processes occuring during a crucial time window for magma ascent and eruption. 210Pb is fractionated from its great-grandparent 226Ra by both Pb–Ra fractionation between crystal and melt and via its intermediate parent 222Rn during degassing. 210Pb–226Ra activity ratios have been measured in volcanic rocks of various settings for the last 40 years. Here we review published data and their implications for magmatic processes. In particular it is observed that 210Pb–226Ra fractionation is larger at subduction zone volcanoes than in Mid Ocean Ridge Basalts and Ocean Island Basalts. The larger fractionations, which include both 210Pb deficits and excesses, are the result of recent fractionation of 210Pb–226Ra during magma degassing and gas streaming. MORB and OIB suffer less from overprinting of prior 210Pb–226Ra signals by degassing. These magmas preserve 210Pb–226Ra fractionation induced by partial melting of the mantle placing tight constraints upon magma ascent rates.