HR: 0800h
AN: GP21A-0100    [Abstracts]
TI: Effect of hydrothermal alteration on rock magnetic properties from basalts in the Krafla geothermal field, Iceland
AU: * Oliva-Urcia, B
EM: de83@agk.uni-karlsruhe.de
AF: Geologisches Institut, Strukturgeologie und Tektonophysik, Univ.Karlsruhe, Hertzstrasse 16/ 6.36, Karlsruhe, 76187, Germany
AU: Kontny, A
EM: de80@agk.uni-karlsruhe.de
AF: Geologisches Institut, Strukturgeologie und Tektonophysik, Univ.Karlsruhe, Hertzstrasse 16/ 6.36, Karlsruhe, 76187, Germany
AU: Vahle, C
AF: Geologisches Institut, Strukturgeologie und Tektonophysik, Univ.Karlsruhe, Hertzstrasse 16/ 6.36, Karlsruhe, 76187, Germany
AU: Schleicher, A M
EM: aschleic@umich.edu
AF: Geologisches Institut, Univ. Erlangen-Nuernberg, Schlossgarten, 5, Erlangen, 91054, Germany
AB: The high-temperature Krafla geothermal field is situated within the caldera of the Krafla central volcano in NE Iceland. The last fissure eruptions (Krafla fires) occurred between 1975 and 1984. Aeromagnetic surveys from this area indicate a magnetic high corresponding to Mt. Krafla, whereas the magnetic low coincides with the caldera bottom where the Krafla geothermal field is located. The geothermal fluids are meteoric in origin and the Sudurhlídar field is boiling from depth until the surface. The permeability is higher in vertical than in horizontal profiles and the production of secondary minerals suggests a depth zonal distribution related to the temperature. The study of the magnetic properties of volcanic rocks affected by hydrothermal alteration is significant to understand magnetic anomalies related to MORB and its tectonic implications. Our study focuses in an area where the hydrothermal alteration diminishes the Ti-magnetite content of fissure subaerial lavas. The samples were taken from KH1 (200 m depth) and KH3 (400 m depth) drill cores, from the rim of the caldera. In our study we aim to correlate both, c-T curves and textural observations from the magnetic phases with the degree of hydrothermal alteration. NRM, field dependence of susceptibility (Fd) and Koenigsberg ratios (Q) from the samples are very low: NRM is < 3.1 A/m, Fd values range between 0.2 and 7.9, and Q between 0 and 6. Magnetic susceptibility varies with the magnetic mineral content. Typical textural features are shrinkage cracks from maghemitization together with exsolved textures in Ti-magnetite from high temperature oxidation. This texture is present in the deeper part of both cores (177 m in KH1 and 380 m in KH3), but KH1 samples show abundant ghost structures of Ti-magnetite, altered to a network formed by clays and Ti-oxide. A high quantity of sulphide precipitation accompanies the ghost structures. The magnetic phases strongly alter depending on the porosity of the rocks, but the alteration profile seems not to be dependent with depth. A higher degree of dissolved textures of Ti-magnetite is seen in KH1, which seems to be strongly affected by the recent activity in the geothermal field.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting