Stratigraphy and Geochemistry of a Fond St. Jean Cinder Cone, Dominica
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Dominica is an independent island nation located in the central portion of the Lesser Antilles Island arc, a magmatic arc that results from subduction of the South American Plate beneath the Caribbean Plate (Smith et al. 2013). Subduction in this setting promotes partial melting of the mantle, producing magma that ultimately rises to the surface of the earth and erupts through variable styles of volcanism. Magmas may pond and differentiate on the way to the surface of the earth, resulting in deposition of chemically distinct rocks. Dominica is primarily composed of the intermediate rocks andesite and dacite, though an eruptive center in the southeast, Foundland Center, is composed predominantly of mafic basalts. Previous work (Smith et al., 2013; Wills, 1976) indicates that air fall samples collected at Fond St. Jean, a town within Foundland Center, are the most mafic samples collected on the island, and therefore potentially represent the islands least evolved rocks on record. This study analyses additional basaltic samples from a partially eroded cinder cone in Fond St. Jean. Techniques including inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS), as well as petrographic analyses and solid-melt modeling are used to analyze the mineralogy and textures of these samples and the styles of volcanism that erupted them, as well as determine a potential composition of the mantle from which these rocks were derived. Additionally, field observations, elevation measurements, and measurements of the thickness of units composing the cinder cone are used to construct a stratigraphic section through the volcano. Field relationships and alternating textures of volcanic material imply changes in volcanic styles through time. Petrographic textural analyses indicate that both explosive and effusive volcanism contributed to the cinder cones formation. Additional petrographic analyses indicate that while mineralogy is consistent throughout the cinder done, minor zoning implies slight crystal fractionation within a magma chamber. Discrimination diagrams show major and trace elements for these samples are consistent with the expected chemistry of island arc basalts, making them good candidates for modeling mantle melting. 5-15% melting of a garnet free lherzolite mantle fits the data for these samples best, and trace element ratios indicate that sediment incorporated into the subduction zone contributed to the chemistry of the mantle melt.