Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria

Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges(1) - where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere - the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay- chain nuclides, because chemical disruption to secular equilibrium systems initiates parent - daughter disequilibria, which re- equilibrate by the shorter half- life in a pair. Here we use (210) Pb - (226) Ra - (230) Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earth's mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. (210) Pb deficits of up to 15 per cent relative to (226) Ra occur in normal mid- ocean ridge basalts, with the largest deficits in the most magnesium- rich lavas. The 22- year half- life of (210) Pb requires very recent fractionation of these two uranium- series nuclides. Relationships between (210) Pb- deficits, ( (226) Ra/ (230) Th) activity ratios and compatible trace- element ratios preclude crustal- magma differentiation or daughter- isotope degassing as the main causes for the signal. A mantle- melting model(2) can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.