Melt generation, mixing and degassing at El Hierro, Canary Islands

Abstract

The formation of ocean islands and the cause of melt generation in the mantle beneath them is of great interest to geoscientist. Much of our current understanding of deep Earth processes and phenomena, such as mantle plumes and long term cycling elements, including volatiles (H, C, S, halogens), between the surface and the mantle derive from the study of ocean island basalts (OIBs). The Canary Islands is one of the most studied ocean island chain on Earth, yet processes driving melt generation underneath it are still debated. The main focus of this work is to investigate the driving forces of generation beneath El Hierro, located in the Western Canary Islands, using a source to surface approach. In my thesis I present major, trace, volatile element, and sulfur isotope data from samples representing the past ~20000 years of volcanic activity on the island of El Hierro and use these to further increase our understanding of melt formation and evolution of OIBs erupting at the Western Canary Islands and beyond. Major, trace and volatile element data from El Hierro melt inclusions indicate prevalent carbon and sulfur enrichment in young El Hierro magmas. Primary mantle-derived melt CO2 contents may be as high as 4 wt%. Trace and volatile element ratios indicate C and S enrichment at El Hierro is either caused by extremely low melting degrees or by addition of S and C to the mantle source from recycled lithosphere. F/Nd ratios of melt inclusions point to the presence of recycled F in the mantle source, indicating volatiles can survive subduction and be effectively recycled into the mantle source of OIBs. Sulfur isotope ratios measured in melt inclusions using a novel microanalytical technique reveal that primary El Hierro magmas are enriched in heavy 34S compared to normal upper mantle. Modelling the composition of the mantle source using primary magma sulfur content, sulfur isotope ratios, and trace element contents indicate that the El Hierro mantle source is up to three times more enriched in sulfur than the primitive upper mantle. The presence of such a large amount of excess S in the mantle source can only be explained if the original asthenospheric mantle reacted with volatile-rich metasomatic melts originating from a recycled lithosphere. Magma temperature, magma storage pressure, and melt oxygen fugacity estimates from El Hierro point to a long and complex pre-eruptive history. Magma residence times of ~400 years calculated by diffusion modelling enabled extensive chemical re-equilibration of minerals and melt inclusions, obscuring primary magma temperature and oxygen fugacity conditions. Olivine-spinel pairs reveal that El Hierro magmas are among the most oxidised OIBs on Earth. Oxidising conditions combined with near-ambient mantle potential temperature estimates from the Canary Islands contradict the hypothesis of a deep, thermal mantle plume origin for the Canary Islands. These observations, together with extreme carbon and sulfur enrichment observed in El Hierro magmas, point to a melt generation process where volatile-enrichment of the mantle is the main driving force of melt generation at the Canary Islands, caused by the addition of metasomatic melt/fluid to the asthenospheric mantle originating from previously subducted material. The transfer of oxidised volatiles, sulfur in particular, could be a major pathway for oxidised surface material to be introduced into the mantle. Results presented in the thesis have implications for mantle dynamics, as volatile-rich plumes may originate from shallower depth than suggested for deep thermal plumes, and also point to the importance of previously subducted material in the generation of OIBs. Both hypotheses of volatile and temperature induced melting could be correct at different ocean island locations, with volatiles likely playing a primary role in melt formation at ocean islands with low melt fluxes, such as the Canary Islands and Cape Verde.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Raymond Burgess (Supervisor) & Margaret Hartley (Supervisor)