Earth’s missing argon paradox resolved by recycling of oceanic crust
In: Nature Geoscience. Nature Publishing Group: London. ISSN 1752-0894; e-ISSN 1752-0908, meer
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| Auteurs | | Top |
- Tucker, J.M.
- van Keken, P.E.
- Ballentine, C.J.
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| Abstract |
The extent to which primordial mantle domains have survived billions of years of convective mixing is a fundamental question in mantle dynamics and geochemistry. The observation that around half of Earth’s 40Ar is missing from the atmosphere has been used to argue for a largely primordial, convectively isolated lower mantle. This hypothesis is apparently supported by lower 40Ar/36Ar ratios in the mantle source of ocean-island basalts compared with mid-ocean-ridge basalts. However, strongly layered convection is contradicted by seismic tomographic observations and geodynamic constraints. Using joint geodynamic–geochemical modelling of mantle convection, we show that high 40Ar concentrations associated with K-rich subducted oceanic crust plus unmelted material dispersed throughout the mantle can fully account for Earth’s 40Ar budget. This solution to the missing Ar paradox requires neither a substantial reduction in Earth’s assumed K concentration nor large isolated domains in the mantle. We additionally show that subducted atmosphere-derived Ar has little effect on the mantle 40Ar budget but can substantially reduce mantle 40Ar/36Ar ratios. Unlike He and Ne isotope systems, whose variations reflect primarily incorporation of primordial material, mantle 40Ar/36Ar ratios may instead result from subduction of atmosphere-derived Ar into the deep mantle. |
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