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Explosive to effusive transition in intermediate volcanism : changing magma system conditions in Dominica
Jessica J. Bersson
May 9, 2018
The oscillation between explosive and effusive intermediate (59-62 wt% SiO2) volcanism in the Roseau Valley on Dominica, an island in the Lesser Antilles Arc, provides an opportunity to investigate temporal changes in the magmatic system. In order to determine possible explanations for the shifting eruptive style a detailed petrologic analysis of phenocrysts is conducted to determine commonalities or changes in pre-eruptive conditions (i.e., intensive variables) for the Roseau Ignimbrite Group (~1-65 ka) (i.e., the explosive end-member) and the Micotrin Lava Dome (~1.1-26 ka) (i.e., the effusive end-member), from the Wotten Waven magma system (Frey, 2016; Howe et al., 2014; Lindsay et al., 2005). The ignimbrites are saturated in five phenocrysts (plagioclase + orthopyroxene + clinopyroxene + ilmenite + magnetite ± amphibole ± quartz), and the lava dome contains the same assemblage, but with notable differences: Micotin Lava Dome deposits contain more quartz and relict amphiboles. Plagioclase in the ignimbrites ranges in composition from An22-93, and those in the dome range from An45-85. Fe-Ti oxide geothermometry (Ghiorso and Evans, 2008) reveals pre-eruptive temperatures of 831 ± 15˚C for the Roseau Ignimbrite Group, whereas the pre-eruptive temperature for the dome is slightly hotter (851 ± 24°C). Oxygen fugacity (relative to NNO) of the Roseau Ignimbrite Group is 0.40 ± 0.12 Δ NNO, which overlaps with those from the dome (0.30 ± 0.15 Δ NNO). Pre-eruptive temperatures, plagioclase compositions, whole rock and glass compositions are incorporated into a plagioclase-liquid hygrometer to determine pre-eruptive melt H2O contents for each sample. Melt water contents for ignimbrites range from 4.6 - 10.1 wt % H2O, and those from the lava dome range from 2.8 - 7.1 wt % H2O. Application of a H2O solubility model shows that water contents for the Roseau magmas correspond to pressures of ~2 - 4 kbar. The most notable difference between the explosive and effusive deposits is that the lava dome has a higher pre-eruptive temperature than the ignimbrites. However, the results collectively suggest that more recent volcanism in the Roseau Valley (ka) has hotter pre-eruptive temperatures, regardless of eruptive style, and that these intermediate magmas originate from sources in the deep crust. Constraining the conditions of caldera evolution at Micotrin informs the volcanic hazards Dominica is subject to as one of the most volcanically active islands in the Caribbean.