EVIDENCE OF AN ERODED VOLCANO: ORIGIN OF MAFIC GRAINS IN WHALEBONE BAY, BERMUDA

The Bermuda seamount, located offshore of the Carolinas, is an extinct intraplate volcano that last erupted around 30 million years ago. The truncated volcanic pedestal is now mostly covered by quaternary carbonate sediments. Mafic mineral grains are found in Pleistocene-age carbonate beach deposits from Whalebone Bay (WB) on the northern shore of St. George’s Island. We present new geochemical data to determine the possible volcanic origin of these surficial sediments and compare that data with the mineral chemistry of the volcanic basement.

The bulk composition of the black sand layers within the carbonate beach deposit is calcite, with predominantly euhedral grains of perovskite, iron oxide, and andradite garnet making up the majority of the mafic mineral assemblage. Subhedral to rounded almandine garnet, grossular garnet, clinopyroxene, titanite, rutile, apatite, magnesiochromite, and zircon grains are also present. A potential source of perovskite, titanite, andradite garnet, and pyroxenes is the volcanic pedestal (Prognon et al., 2011). However, garnets have not yet been identified in any cores from the volcanic basement (Mazza et al., 2019). Results of SEM-EDS analysis shows systematic differences between the composition of clinopyroxene and perovskite in the volcanic rocks preserved in the core (Mazza et al., 2019) and those found at WB. A high percentage of the clinopyroxenes from WB have no TiO2, while clinopyroxenes from the volcanic basement average 3 wt% TiO2. The diopside described in Prognon et al. (2011) is aluminum rich, as are the clinopyroxene observed here. However, the garnet described in Prognon et al. (2011) is TiO2 rich, and there are andradite, grossular, and almandine garnet grains that do not contain TiO2.

The presence of multiple species of garnets and the discrepancy in compositions of the clinopyroxenes suggests the source of some of the mafic sediment is eroded Bermudan lavas and an as-yet undiscovered source. The mafic sediment from WB was deposited about 30 million years after the last known eruption on Bermuda, further complicating the provenance of the sediments. We present new chemical composition data of the andradite garnets to determine if they are a) igneous in origin or b) associated with metamorphic skarns that have since been eroded away from Bermuda.