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 40years. 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.