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


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.

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Figure 1: U-series data for young lava flows.
Figure 2: 210 Pb– 226 Ra radioactive disequilibria versus other chemical parameters.
Figure 3: Instantaneous melting 2 and 210 Pb– 226 Ra– 230 Th disequilibria.

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We thank M. Perfit and J. Sinton for providing most of the samples, associated geological insight and unpublished trace element data used in this research, L. Sacks for performing some of the chemical analyses, D. Pyle and K. Spencer for effective management of the SOEST Isotope lab, K. Sims and T. Elliott for thorough reviews, and J. D. Macdougall on the occasion of his retirement for earlier work inspiring this effort. The US National Science Foundation supported this work. Author Contributions K.H.R. designed the experiment; all authors participated in data acquisition and analysis.

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Correspondence to K. H. Rubin.

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Supplementary information

Supplementary Table S1

U-series data and metadata for young submarine lava flows in this study. This file contains information about samples, supplementary analytical data, and additional notes on the "Aldo-Kihi" S-EPR eruption, with additional references. (PDF 11 kb)

Supplementary Table S2

Major and trace element compositions of young submarine lava flows in this study. This file contains XRF, EMP, and ICP-MS data on samples and rock standards. (PDF 15 kb)

Supplementary Table S3

U-series replicates data on samples and synthetic standards. This file also contains data comparisons to literature data (including Th and U isotope ratios and 238U-230Th-226Ra data). (PDF 20 kb)

Supplementary Table S4

This file contains results of accuracy and external reproducibility tests of U-series replicates data by repeat analyses of rock standard K1919. (PDF 11 kb)

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Rubin, K., van der Zander, I., Smith, M. et al. Minimum speed limit for ocean ridge magmatism from 210Pb–226Ra–230Th disequilibria. Nature 437, 534–538 (2005).

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