Háskóli Íslands


Snorri Guðbrandsson will be defending his doctoral thesis for a joint degree in geochemistry from University of Iceland and Paul Sabatier University, Toulouse, France, on Wednesday, October 30th at 15:00 in Hátíðarsalur Háskóla Íslands, Main Building.
The title of the thesis is: Experimental weathering rates of aluminium-silicates.

Opponents are dr. Marguerite Godard, Research director at Geosciences Montpellier, CNRS, Montpellier, France, professor Per Aagaard, Department of Geosciences, University of Oslo, and professor Kevin Burton from the Paul Sabatier University, Toulouse, France. The instructors have been Eric Oelkers, Sigurður Reynir Gíslason and Domenik Wolff-Boenisch.

The Ph.D. project was supported by the Environmental Fund of Reykjavik Energy, European Marie Curie network Delta-Min, and the EU funded Collaborative Project Carbfix.

The defence will be chaired by Magnús Tumi Guðmundsson, Head of the Geology Department.


Abstract

The chemical weathering of primary rocks and minerals in natural systems has a major impact on soil development and its composition. Chemical weathering is driven to a large extent by mineral dissolution.
This thesis focuses on the dissolution behaviour of Stapafell crystalline basalt, which consists of three major phases (plagioclase, pyroxene, and olivine). Steady-state element release rates from crystalline basalt at far-from-equilibrium conditions were measured at pH from 2 to 11 and temperatures from 5° to 75° C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH, where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Plagioclase is the most abundant phase present in crystalline basalts and thus influences strongly its reactivity. Plagioclase dissolution rates, based on Si release, show a common U-shaped behaviour as a function of pH, where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Constant pH plagioclase dissolution rates increase with increasing anorthite content at acid conditions, in agreement with literature findings. Gibbsite is commonly the first mineral formed as the result of the low temperature dissolution of plagioclase.
Gibbsite precipitation rates were measured in closed system reactors at alkaline condition, both at 25 °C and 80 °C as a function of fluid saturation state. Analyses of the solids demonstrate that gibbsite precipitation occurred in all experiments. The comparison of gibbsite precipitation to the dissolution rates of plagioclase at pH 11 shows that the rates are close to equal. Kaolinite precipitation rates were measured in mixed flow reactors as a function of fluid saturation state at pH=4 and 25 °C. Measured kaolinite precipitation rates are relatively slow compared with plagioclase dissolution rates. Results indicate that although gibbsite precipitation rates are relatively rapid, the relatively slow precipitation rates of kaolinite may be the process controlling the formation of this mineral at the Earth’s surface.

The doctoral candidate


Snorri Guðbrandsson was born in 1976 in Reykjavík, Iceland. He started geology studies at the University of Iceland in 2003 and received a B.Sc. in 2007. During his undergraduate studies he worked at the Icelandic Institute of Natural History and later at the Iceland Geosurvey. The PhD research has been carried out at the Institute of Earth Sciences, University of Iceland, GET-CNRS, Midi-Pyrenees laboratory in Toulouse, France, along with a short stint at the NanoGeoScience institute at the University of Copenhagen.

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