SOLVING THE MYSTERY OF BERMUDA: USING ZN ISOTOPES TO TRACE THE SOURCE OF DEEP CARBON

Bermuda is an enigmatic intraplate volcano found along the Eastern North American Passive Margin, erupting ~80 million years after the formation of regional oceanic crust. Recent geochemical data from Bermuda suggests that silica undersaturated (SU) melts sampled a previously unknown mantle reservoir characterized by significant enrichments in volatiles (e.g. CO₂ and H₂O) and incompatible elements, and a unique isotopic signature [1]. 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 the mantle source for Bermuda must be <650 Ma. These SU melts are interpreted to be sourced in the transition zone, tapping a young mantle reservoir that resulted from recycling and storage of incompatible elements and volatile-rich material. However, the source of this recycled volatile-rich lithology remains unknown.

Stable Zn isotopes (δ⁶⁶Zn as the ‰ deviation of ⁶⁶Zn/⁶⁴Zn from the JMC-Lyon standard) have been used to differentiate various recycled carbon-bearing lithologies in mantle-derived melts [e.g., 2, 3]. Partially melting mantle peridotites (δ⁶⁶Zn = 0.18 ± 0.06 ‰) can generate slight fractionation in δ⁶⁶Zn (δ⁶⁶Zn = 0.28 ± 0.04 ‰), whereas marine carbonates are isotopically heavy (δ⁶⁶Zn = 0.91 ± 0.24 ‰), making Zn isotope systematics well-suited to trace recycling of carbonates in the mantle.

We present new δ⁶⁶Zn data to help improve our understanding of the source of the Bermuda SU lavas. Bermuda SU lavas are on the high-end of Zn concentration averages reported for other OIBs ([Zn] = 122.3 ± 18.3 ppm, Zn/Fe⋅10⁴ = 13.4 ± 2.4, n = 23), and while such elevated [Zn] could suggest incorporation of carbonated eclogite in the mantle source, olivine trace element data indicate a carbonated peridotite source instead [1]. Preliminary δ⁶⁶Zn from Bermuda yields an average of 0.23 ± 0.07‰ (n=6), which implies that Bermuda is the result of partially melting peridotite without significant input from recycled marine carbonates.

[1] Mazza et al. (2019) Nature [2] Beunon et al. (2020) Earth-Science Reviews [3] Zhu et al. (2021) Chemical Geology