| Popis: |
This dissertation consists of three projects that involve isotope and trace element investigations of magmatic processes and timescales in the Azores, Portugal. Two projects focus on the evolution of historical eruptions from Fogo and Furnas volcanoes on the island of São Miguel, addressing two fundamental questions: 1) What are the processes that generated the evolved (explosive) magma and chemical zonation in the magma chambers?; and 2) What are the timescales over which these processes occur? The Fogo 1563AD deposit (0.14 km3 d.r.e.) shows monotonic chemical zonation that can be explained by closed-system fractional crystallization. In contrast, the Furnas 1630AD deposit (0.65 km3 d.r.e.) shows more complex chemical zonation with variable Sr isotope signatures that require assimilation combined with fractional crystallization and a complex magma chamber geometry. The magma residence time estimated from U-series disequilibria for the Fogo 1563AD deposit is the shortest ever determined for silicic magmas, and reveals that alkaline silicic magmas can evolve extremely rapidly, with pre-eruptive magma residence timescale on the order of decades. The larger Furnas deposit has a substantially longer fractionation timescale (~500 years). The results of these studies are consistent with results from other silicic deposits, which display a global negative correlation between fractionation rate and eruptive volume.The third project constitutes the first platinum group element (PGE) abundance and 186Os/188Os isotope study of Azores basalts, aimed to address the origin of the Azores mantle plume and whether or not mantle plumes carry an outer core signature. The results of this study reveal that radiogenic 186Os/188Os ratios in some Azores basalts are consistent with a minor outer core contribution to the Azores mantle plume, which would suggest a deep (core-mantle boundary) origin of this plume. However, the lack of elevated PGE abundances in the Azores basalts suggest that alternative origins for the radiogenic 186Os/188Os also need to be considered, including a heterogeneous mantle with pyroxenite veins or metasomatic sulfides. |
