Geochemical and geophysical constraints on the causes of widespread active volcanism in the Galápagos Archipelago

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The proposed research focuses on one of the most volcanically active regions on Earth, the Galápagos Archipelago in the Pacific Ocean. The last major eruption was in April 2009 at Fernandina which is close to the postulated centre of a high-temperature anomaly (known also as a hot spot or mantle plume). Results of geophysical and geochemical studies have shown that the volcanism results from the juxtaposition of a mantle plume with a major oceanic spreading centre. The combination of thin lithosphere (~7 km) at the Galápagos Spreading Centre and high temperature (~1400oC) of the plume results in a large amount of mantle melting as the plume upwells. The Galápagos Archipelago is an ideal place to study the complex mass-transfer and chemical-mixing processes that occur in this dynamic tectonic setting because the mantle plume and oceanic spreading centre are sufficiently close to interact but still able to remain as distinct physical entities. The distribution and composition of volcanism in the Galápagos archipelago is different to that associated with conventional hotspots (such as Hawaii): there is no linear chain of ocean islands and active volcanism is widespread away from the zone of plume upwelling. A recent high-resolution regional geophysical study (published in 2007) has shown an anomalous zone of low seismic velocity, extending away from the hot spot towards the spreading centre. The anomaly is greatest at a depth of ~ 70 km beneath Santiago; an island located in the centre of the archipelago and on the periphery of the plume. The cause of the anomaly is unknown; it may represent a change in temperature and/or a zone of intense mantle upwelling and melting. Santiago is the fourth largest island in the archipelago but there are very few published geochemical analyses. The most recent volcanic eruptions on the island were in 1795, 1897 and 1904. The limited geochemical data indicates that lavas erupted from volcanoes in the west of the island are of different composition to those in the east, a distance of <20 km. Some of this variation may be a consequence of sub-volcanic magma chamber processes but large ranges in ratios of elements which readily partition in to magmas during mantle melting suggest that the island is located either above lithosphere that varies dramatically in thickness, from west to east, or that the underlying convecting mantle has a varied composition. This proposal seeks funding to undertake high-precision geochemical analyses on lavas from Santiago where systematic sample collection has already been done by the PI. Numerical modelling of the chemistry of these lavas together with published data for other Galápagos volcanoes will be used to constrain the composition, temperature and pressure of melting of the underlying mantle. The results will be combined with temperatures calculated from seismic velocity data, using newly published equations, to shed light on the physical processes that are operating between the axis of plume upwelling and the spreading centre, causing widespread volcanism. The presence of a large body of melt extending from the zone of mantle-plume upwelling towards the spreading ridge would place important constraints on the nature of mass transport that occurs when these two dynamic systems are juxtaposed. The proposed research will build on the PI's previous research on mantle melting. It is novel because it will produce the first integrated geochemical and geophysical model to account for the causes of widespread active volcanism in the Galápagos Archipelago. This will aid our understanding of the causes of volcanic activity in much less accessible settings, such as the Mid-Atlantic Ridge, and increase constraints on identifying where plume-ridge interactions have occurred in the geological past. This will in turn will increase our knowledge of global-scale deep-mantle and Earth surface processes.