Magma formation and evolution in the Tonga intra-oceanic island arc

• Applicant: Professor Dr. Karsten Matthias Haase
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554738935

Magmatic activity occurs along the convergent plate boundaries of the Earth with highly variable magma compositions forming as a result of variations of composition and temperature of both the mantle wedge and the subducting slab, and of the thickness and composition of the upper plate. Intra-oceanic island arc volcanoes erupt variable lava series (basaltic to andesitic, or basaltic to rhyolitic) and show different structures (e.g. calderas, rejuvenated volcanism) that may indicate a systematic evolution of the magma system. Here we suggest to study the mineralogy as well as the composition of glass rims and melt inclusions of lava series from selected Tonga island arc volcanoes showing differences in volcanic structure and magma evolution. The compositions range from tholeiitic basalts to boninites/high-Mg andesites that will be studied in order to define differences in mantle depletion, depth and degree of partial melting, and slab contributions relevant for the formation of primary magmas along the island arc. Additionally, we will analyse the O isotope compositions of primitive olivine and clinopyroxene to define a possible change in slab input along the Tonga island arc. Mineral, glass rim, and melt inclusion analyses in the more evolved rocks will help to test whether these magmas formed by crystal fractionation or by re-melting of crustal rocks. Age determinations of selected lavas will allow to establish the time frame of magma evolution of individual arc volcanoes. Samples from four volcanoes are from different stratigraphic formations and volcanic structures, and Ar/Ar geochronology will be used to determine the evolution of the respective magmatic systems. Thus, we will use existing samples from our collection in order to study lavas (1) with variable primary magmas (basaltic to high magnesian andesite), (2) different tectonic situation and crustal structure, and (3) variable evolutionary stage of the magma system. On the basis of the results we will determine 1) the formation of primary melts, 2) the formation of the intermediate to felsic magmas, and 3) the volcano and magma system evolution in the Tonga island arc. This project will lead to a better understanding of the magma generation processes and evolution at arcs with relatively thin crust, which will have implications for our global models of subduction.

• DFG Programme: Research Grants