ANALYSIS OF SILICA-UNDERSATURATED LAVA FROM BERMUDA: INSIGHTS INTO ERUPTIVE AND POST-ERUPTIVE HISTORY

Intraplate volcanoes are formed away from active plate boundaries and do not fit traditional models associated with tectonic processes. Located ~1,000 km off North America, Bermuda is one such example of intraplate magmatism. Recognized as having igneous origins in 1922 (Fisk, 1923), Bermuda was long thought to fall under the mantle plume model for intraplate volcanism. However, Bermuda lacks key characteristics of hotspot volcanism (Vogt & Jung, 2007). Recent geochemical research supports a different model with a source reservoir of geologically young material in the mantle transition zone. Isotopic signatures point to a source either from subduction-related crustal recycling processes associated with the amalgamation of Pangea or crustal delamination during the breakup of Pangea (Mazza et al., 2019). Under this model, melting and upwelling of volatile-rich material stored in the mantle transition zone are attributed to mantle convection.

We analyze a sample from the 1972 Deep Drill of Bermuda. Sample B357 lies within a silica-undersaturated unit close to the top of the core. Primarily we assess two different textural characteristics–one primary and one secondary–with geochemical data to better understand the eruptive and post-eruptive history and the magmatic development of Bermuda.

Radiating acicular clusters of clinopyroxene permeate this sample. These elongate grains show strong chemical zonation of Cr and Ti. Both of these diffuse slowly and thus offer records of crystal growth history (Cherniak & Liang, 2012). The zoning patterns combined with major and trace element compositions for the bulk rock and individual minerals can help interpret crystal growth.

Carbonate globules or “ocelli” are a common secondary structure in the core. We note at least two types in this segment; a globular structure of solely carbonate minerals, and a similar carbonate structure rimmed by silicate minerals. While the ocelli are almost certainly secondary, what was their formation mechanism? We investigate the textural and geochemical characteristics of each ocellus type to determine the nature of these apparently secondary structures.

The textural and chemical analysis of these primary and secondary structures combined with bulk rock geochemical data presents a compelling story of Bermuda’s igneous history.