Sampling the volatile-rich transition zone beneath Bermuda

Abstract

Intraplate magmatic provinces found away from active plate boundaries, provide direct sampling of the Earth’s mantle composition and heterogeneity. Observed chemical heterogeneities in the mantle are commonly attributed to recycling during subduction1-3, 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 boundary4,5, not all intraplate volcanoes are deep rooted6 and reservoirs in other shallower boundary layers likely participate in magma generation. Here we present new evidence that suggests that Bermuda sampled a previously unknown mantle domain, characterized by silica under-saturated melts that have significant enrichments in incompatible elements and volatiles, and a unique, extreme isotopic signature. Bermuda records the most radiogenic 206Pb/204Pb isotopes ever documented in an ocean basin (19.9-21.7), coupled with low 207Pb/204Pb (15.5-15.6) and relatively invariant Sr, Nd, and Hf isotopes, suggesting that this source must be <650 Ma. We interpret the Bermuda source as a new, transient mantle reservoir that resulted from recycling and storage of incompatible elements and volatiles7-10 in the transition zone, aided by the fractionation of Pb by minerals that are only stable in this boundary layer such as K-Hollandite11-12. Recent recycling and storage of material into the transition zone suggests that this reservoir can only be found in the Atlantic Ocean. Our geodynamic models suggest that this layer was sampled by disturbances related to mantle flow. Seismic studies have shown that recycled materials can be stored in the transition zone13. For the first time we show geochemical evidence that this storage is key in the generation of extreme isotopic domains previously thought to be related only to deep recycling.