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Browsing by Subject "diffusional modeling"

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    Item type:Thesis, Access status: Restricted ,
    Petrography, mineral chemistry and modeling of time scales of magmatic processes at the Nola Seamount, Cape Verde Islands
    (Data obrony: 2012-07-16) Kohut, Marzena
    Wydział Geologii, Geofizyki i Ochrony Środowiska
    Cape Verde Islands are well known example of hot spot activity, but still not fully understood. The archipelago is located in the Central – Atlantic. The Cape Verde submarine plateau was sampled during the Meteor M80/3 research cruise in January 2010. An extensive suite of submarine lavas were collected by dredging (DR) and remotely operated vehicle (ROV). Detailed petrography and mineral chemistry of the Nola Seamount, located in the northwest of the Cape Verde archipelago is presented with the aim to unravel the pre-eruptive processes, and hence contribute to our understanding of magma ascent and evolution in the Cape Verde. The sampled volcanics are basanites and ankaramites. All samples comprise a glassy to microcrystalline groundmass that hosts phenocrysts and microphenocrysts of mainly euhedral and subhedral clinopyroxenes and euhedral olivines. Clinopyroxene crystals are the dominant phenocryst phase and they exhibit strong zonation with optically distinct cores of variable composition with increasing FeO, TiO2 and Al2O3 and decreasing MgO content from core to rim of the crystals. Moreover, three geochemically different groups of clinopyroxenes can be distinguish, the most primitive with high Mg low Al2O3 contents, a group with intermediate composition, and the most evolved group, with low Mg and high Al2O3. The high Al contents of the rims suggest formation during decompression. Based on high resolution BSE images and geothermobarometry data, diffusional smoothing of Fe–Mg compositional gradients in clinopyroxene crystals is used to model the pre-eruption residence times of crystals at magmatic temperatures. We obtained three timescales: 800 – 1000 years, 200 – 400 years, <100 years, which correlate with the compositional variations of Mg and Al contents, and thus allow to trace magma ascent in considerable detail.