Source of Abundant Volcanism on Dominica: An Evaluation of Potential Mantle Components

Dominica has the most volcanic centers (9) in the Lesser Antilles arc. The abundance of volcanism has been attributed to Dominica's position in the middle of the arc, where a change in slab dip angle and rate suggests there is a kink in a single slab or that subduction in the region involves both the North American and South American plates subducting beneath the Caribbean plate. It has been proposed in the literature that Dominica may overlie the site of potential slab tear, which permits the hypothesis that there could be some aesthenospheric upwelling in the underlying mantle. Further evidence for potential involvement of an aesthenospheric component is found in whole rock compositions (wt% FeO and wt% MgO) of all Dominica lavas, which follow a tholeiitic liquid line of descent rather than the expected calc-alkaline liquid line typical of an island arc. To test if asthenospheric upwelling is contributing to the abundant volcanism on Dominica, we conducted a detailed petrographic analysis of the most primitive basalts on the island. The basalts in this study were sampled from Foundland, Fond St. Jean, and Springfield. Basalts are saturated in plagioclase + olivine + orthopyroxene + clinopyroxene + magnetite + chromite ± ilmenite. Mineral compositions were collected using quantitative SEM methodology. The Springfield basalt was found to be the most mafic sample on the island, containing the most magnesian olivine (Fo64-88) and highest whole rock concentration of MgO. Olivine, Cr-spinel, and the Springfield whole rock composition were incorporated into a Cr-spinel oxygen barometer to calculate average pre-eruptive temperatures and oxygen fugacities of 1142°C and logfO2 values of -8.95, respectively. The high oxygen fugacity calculated for the Springfield basalt is more consistent with subduction zone basalts, rather than those derived from aesthenospheric mantle. While the inferred mantle redox state cannot account for the observed "tholeiitic-like" liquid line of descent observed on Dominica, an outstanding question is to what extent subducted sediments impact the observed trend in major elements. Although patterns in trace elements suggest sedimentary contribution, the extent of such contribution is no different than other islands in the Lesser Antilles.