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Science and technology
Lavas of Bermuda island likely present the first sample of the melt from the Earth mantle transition zone.

The island of Bermuda is the surficial expression of a 1,500-km-long topographic swell, which rises 1 km above a 110–140-Myr-old oceanic crust of Atlantic Ocean. Like many ocean volcanic islands, Bermuda has been historically explained as being derived from a mantle plume- the jet of hot mantle material rising from the core-mantle boundary (ca 2900 km depth).

Using an 800 meters core sample – drilled in 1972, housed at Dalhousie University, Nova Scotia an international group of scientists from USA, Germany, France, and Russia discovers an alternative explanation for the origin of Bermuda

The major constraints came from petrology and geochemistry. High precision microanalysis of olivine phenocrysts from Bermuda lavas, made in Institute of Earth Sciences, University Grenoble Alpes (ISTerre, UGA) revealed crystallization temperatures much lower than expected for magmas derived from mantle plumes (e.g. Hawaii). Compositions of olivine also showed that the mantle source of lavas was a peridotite strongly reacted with volatile-rich carbonated melt. The comprehensive geochemical study of rocks in several universities of USA revealed unique enrichment of Bermuda lavas in elements typical for the most fusible components (Rb, Ba, Th, U, Nb, Ta, La, Ce and H2O) as well as by radiogenic isotope 206Pb, which is produced by the decay of radioactive isotope 238U.

Altogether petrological and geochemical data suggest that the source of Bermuda lavas was affected by melt coming from transition zone of the mantle between 410 and 660 km depth. The enrichment of this melt in particular elements was caused by oceanic crust subducted to the transition zone not earlier than 650 million years ago, which provided carbon and water-rich fluids that interacted with the ambient Atlantic upper mantle.

If true, this is the first direct sampling of melt from the mantle transition zone of the Earth.

Les Bermudes ont un passé volcanique unique. Bermuda has a unique volcanic past. About 30 million years ago, a disturbance in the mantle’s transition zone supplied the magma to form the now-dormant volcanic foundation on which the island sits. Wendy Kenigsberg/Clive Howard – Cornell University, modified from Mazza et al. (2019)

Publié le May 21, 2019
Mis à jour le May 21, 2019

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