53 D3. Structural Geology, Tectonics, Petrology, and Planetary Geology

Sunday, 19 March 2023
1:30 PM - 5:30 PM
Lake Fairfax

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

Paper number: 53-5
Program: Technical Sessions
Time: 2:50 PM – 3:10 PM

Authors

Abstract

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

Geological Society of America Abstracts with Programs. Vol. 55, No. 2

doi: 10.1130/abs/2023SE-385764

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