The long-term evolution of Montserrat volcanism

This thesis investigates the long-term geochemical trends of volcanism on Montserrat and Guadeloupe, islands in the northern Lesser Antilles arc, through both their terrestrial and marine (from International Ocean Discovery Program core U1396C) records, to examine the processes driving geochemical evolution in island arcs.Detailed mapping of the Silver Hills, the least studied volcanic centre on Montserrat, reveals activity was dominated by andesite dome growth and collapse. New 40Ar/39Ar ages from terrestrial samples, combined with new palaeomagnetic and biostratigraphic ages of tephra layers from marine sediment core U1396C, reveal the previously unrecognised overlap in volcanic activity between the Silver and Centre Hills, and Centre and Soufrière Hills.Pb-Nd-Sr isotope and major and trace element data for the Silver Hills show that lavas fall into two compositional groups; ‘normal’ lavas and ‘low Al2O3-high Fe2O3’ lavas. The latter group formed from parental magmas which experienced up to 25% greater plagioclase fractionation compared to the parent magmas of the normal lavas, likely as a result of lower magmatic H2O content. There is no systematic geochemical variation between lava domes of the Silver Hills, but there is an observed systematic shift in 143Nd/144Nd to progressively less radiogenic values with time, which is inferred to result from mantle heterogeneity.Tephra layers from core U1396C have been analysed for Pb-Nd-Sr isotope and major and trace element data, which provide a record of the geochemical evolution of volcanism on Montserrat and Guadeloupe over the past ~4.5 Ma; 2.3 and 1.7 Ma longer than their respective terrestrial records. The main geochemical change for the Montserrat source region occurred ~4.5–3.9 Ma, resulting from an increase in slab sediment contribution to the mantle wedge via an aqueous fluid. Tephra layers from Guadeloupe display greater temporal geochemical variation, driven by crustal assimilation of mid-ocean ridge basalt.Componentry, grain size and grain morphology analyses of the thickest tephra layer in core U1396C derived from Guadeloupe, T2.36, reveal it was formed by deposition from fallout from an eruption column, followed by a co-ignimbrite ash cloud from the same eruption. Eruption reconstructions suggest that this deposit likely formed from a VEI 6 eruption