EVIDENCE FOR PRIMARY CARBONATE IN BERMUDA SI-UNDERSATURATED MAGMAS

The Bermuda Rise, located east of the North American continent, contains ~30 Myr old silica-undersaturated lavas. They have some of the most radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios found in ocean island basalts (Mazza et al., 2019), equalling that of HIMU end-members. Bermuda is unique among HIMU localities for having low ²⁰⁷Pb/²⁰⁴Pb ratios. The difference in ²⁰⁷Pb/²⁰⁴Pb ratios between Bermuda and HIMU ‘sensu-stricto’ shows the mantle reservoirs have different ages, with Bermuda tapping into a young (~650 Ma) mantle reservoir (Mazza et al., 2019) that may be unusually volatile-rich.

Carbonate veins, assumed to be secondary, are common throughout samples from the Bermuda Deep Drill of 1972. Additionally, other carbonate textures are present in some core sections, including occasional carbonate-silicate ocelli and what appears to be primary groundmass carbonate. We conducted leaching studies and Pb isotopic analysis to see to what extent the carbonate is a primary carbonate related to the mantle source or a secondary carbonate.

Two leaching methods were used to remove carbonates from the samples. The first was a “countertop” leaching method where the coarsely crushed sample was left in 10% HCl for 10 minutes before being rinsed, dried, and powdered. The second was a more intense method where powdered samples were left in 6 M HCl for two hours in a 100˚C oven before the leachate was removed and the sample was digested. The leachate from the second method was then saved for analysis. The Pb isotopic composition of the leached and unleached samples are not very different and plot on the extant ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb trend, which rules out the possibility of substantial seawater Pb in the carbonate that was removed by leaching. When combined with petrographic and textural analysis, the negligible difference in ratios between leaching techniques and leached and unleached samples have the potential to determine if the carbonate is magmatic in origin and a petrogenic feature of the Bermuda silica-undersaturated lavas.