SOLVING THE MYSTERY OF BERMUDA: VOLATILE RICH MELTS FROM THE MANTLE TRANSITION ZONE

Intraplate magmatic provinces found away from 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 subduction, 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, not all intraplate volcanoes are deep rooted and reservoirs in other shallower boundary layers likely participate in magma generation.

Bermuda is an intraplate volcano found off the coast of the Carolinas in the Atlantic Ocean. New geochemical data suggest that Bermuda sampled a previously unknown mantle domain, characterized by silica undersaturated melts that have significant enrichments in incompatible elements and volatiles, and a unique, extreme isotopic signature (Mazza et al., 2019). 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. These silica undersaturated melts are interpreted to be sourced in the transition zone, tapping a young mantle reservoir that resulted from recycling and storage of incompatible element and volatile rich material.

Layered between the silica undersaturated lavas from Bermuda are silica normative basalts and diabases. The basalts are interpreted as upper mantle melts with HFSE enrichments likely resulting from interaction with the silica undersaturated magmas. Here we present the first major and trace element data for the diabases, which share a similar shallow, upper mantle source as the basalts. We also present new geochemical data for porphyritic silica undersaturated lavas, with the goal of improving our understanding of the relationship between these two geochemically distinct melts from Bermuda.