Sampling the volatile-rich transition zone beneath Bermuda

• Published in: Nature (London) 2019-05, Vol.569 (7756), p.398-403
• Main Authors: Mazza, Sarah E., Gazel, Esteban, Bizimis, Michael, Moucha, Robert, Béguelin, Paul, Johnson, Elizabeth A., McAleer, Ryan J., Sobolev, Alexander V.
• Format: Article
• Language: English
• Subjects: 140/125; 704/2151/209; 704/2151/210; 704/2151/431; Analysis; Boundary layer transition; Boundary layers; Boundary layers (Fluid dynamics); Chemical properties; Core-mantle boundary; Diamonds; Domains; Earth; Earth mantle; Fractionation; Heterogeneity; Humanities and Social Sciences; Inclusions; Isotopes; Lead isotopes; Letter; Lithosphere; Lower mantle; Magma; Mantle (Geology); Melts; multidisciplinary; Ocean basins; Ocean models; Oceans; Organic chemistry; Pangea; Plate boundaries; Recycled materials; Recycling; Reservoirs; Sampling; Science; Science (multidisciplinary); Seismology; Silica; Silicon dioxide; Stratigraphy; Subduction (geology); Transition zone; Volatile compounds; Volatiles; Volatility; Volcanoes
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1183-6

Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth’s mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction 1 – 3 , which allows for the addition of volatiles and incompatible elements into the mantle. Although many intraplate volcanoes sample deep-mantle reservoirs—possibly at the core–mantle boundary 4 —not all intraplate volcanoes are deep-rooted 5 , and reservoirs in other, shallower boundary layers are likely to participate in magma generation. Here we present evidence that suggests Bermuda sampled a previously unknown mantle domain, characterized by silica-undersaturated melts that are substantially enriched in incompatible elements and volatiles, and a unique, extreme isotopic signature. To our knowledge, Bermuda records the most radiogenic 206 Pb/ 204 Pb isotopes that have been documented in an ocean basin (with 206 Pb/ 204 Pb ratios of 19.9–21.7) using high-precision methods. Together with low 207 Pb/ 204 Pb ratios (15.5–15.6) and relatively invariant Sr, Nd, and Hf isotopes, the data suggest that this source must be less than 650 million years old. We therefore interpret the Bermuda source as a previously unknown, transient mantle reservoir that resulted from the recycling and storage of incompatible elements and volatiles 6 – 8 in the transition zone (between the upper and lower mantle), aided by the fractionation of lead in a mineral that is stable only in this boundary layer, such as K-hollandite 9 , 10 . We suggest that recent recycling into the transition zone, related to subduction events during the formation of Pangea, is the reason why this reservoir has only been found in the Atlantic Ocean. Our geodynamic models suggest that this boundary layer was sampled by disturbances related to mantle flow. Seismic studies and diamond inclusions 6 , 7 have shown that recycled materials can be stored in the transition zone 11 . For the first time, to our knowledge, we show geochemical evidence that this storage is key to the generation of extreme isotopic domains that were previously thought to be related only to deep recycling. The formation of Bermuda sampled a previously unknown mantle reservoir that is characterized by silica-undersaturated melts enriched in volatiles and by a unique lead isotopic signature, which suggests that the source is young.