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

Abstract

Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges¹—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 ²¹⁰Pb–²²⁶Ra–²³⁰Th 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. ²¹⁰Pb deficits of up to 15 per cent relative to ²²⁶Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of ²¹⁰Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between ²¹⁰Pb-deficits, (²²⁶Ra/²³⁰Th) 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 model² 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.