Paleomagnetism and magnetic anomalies in Iceland

Kristjánsson, Leó; Jónsson, Geirfinnur

doi:10.1016/j.jog.2006.09.014

Cited by 25 publications

(32 citation statements)

References 74 publications

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“…2005). This value is similar to the mean value of 4 A m –1 reported by Kristjansson & Jonsson (2007) for Tertiary lava (1–15 Ma) sampled in Iceland. However, these authors also report values between 0.1 and about 20 A m –1 due to the inhomogeneous magnetization of the lava.…”

Section: Discussionsupporting

confidence: 89%

“…Bleil & Petersen 1983), and most NRM values reported for MORB older than 15 Ma are below 3 A m -1 (Bleil & Petersen 1983;Wang et al 2005). This value is similar to the mean value of 4 A m -1 reported by Kristjansson & Jonsson (2007) for Tertiary lava (1-15 Ma) sampled in Iceland. However, these authors also report values between 0.1 and about 20 A m -1 due to the inhomogeneous magnetization of the lava.…”

Section: Low Nrmsupporting

confidence: 84%

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Modification of the magnetic mineralogy in basalts due to fluid-rock interactions in a high-temperature geothermal system (Krafla, Iceland)

Oliva‐Urcia

Kontny²,

Vahle³

et al. 2011

Geophysical Journal International

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S U M M A R YActive high-temperature (>150 • C) geothermal areas like the Krafla caldera, NE-Iceland, often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity. The main alteration processes in such an environment are low-temperature oxidation (<350 • C, maghemitization) and fluid-rock interactions. We investigated the rock magnetic properties [natural remanent magnetization (NRM) magnetic susceptibility and their temperature and field variation] and the mineralogy, using X-ray diffraction, microscopic methods and electron microprobe analyses, of two drill cores (KH1 and KH3) from the rim of the Krafla caldera. The drill cores have distinctly lower NRM values (average <3 A m -1 ) compared to younger surface basalts (average 20 A m -1 ) along with a large variation in magnetic susceptibility (1.3 × 10 −7 -4.9 × 10 −5 m 3 kg -1 ). The secondary mineral assemblage (sulphides, sphene, rutile and chlorite) indicates an alteration within the chlorite-smectite zone for both cores without depth zoning. Optical miscroscopy in combination with the Bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite: (1) titanomaghemite with intermediate titanium concentration and probably high vacancy concentration, and (2) titanomaghemite with low titanium concentration and low vacancy concentration. The mineral assemblages, textures and magnetic properties deduced from the mentioned magnetic measurements indicate two-stage transformation mechanism:(1) Dissolution of titanium at low pH under oxidizing conditions. The ulvöspinel component of titanomagnetite and ilmenite forms rutile or sphene, and Fe 2 + migrates out of the spinel lattice forming titanomaghemite. (2) Formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions. The latter mechanism produces ghost textures (all titanomaghemite is transformed and only their former grain shapes are preserved), with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved. This mechanism could explain the significant magnetization loss, which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like Iceland or Hawaii. The production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of Krafla.

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Section: Discussionsupporting

confidence: 89%

Section: Low Nrmsupporting

confidence: 84%

Modification of the magnetic mineralogy in basalts due to fluid-rock interactions in a high-temperature geothermal system (Krafla, Iceland)

Oliva‐Urcia

Kontny²,

Vahle³

et al. 2011

Geophysical Journal International

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“…Our study will show that these rocks, which are best preserved in very young flows, can have distinctly higher J r values than the reported ones (e.g. Kristjánsson and Jonsson, 2007).…”

Section: Introductionmentioning

confidence: 48%

“…The basaltic lithologies of our study (picritic basalt of shield lava, olivine tholeiitic basalt of shield lava and fissure eruptions, and pillow basalts) are all younger (less than 20 ka) than the last reversal event (Laschamps event is about 43 ka; Kristjánsson and Jonsson, 2007). Therefore only normal polarity was considered for the magnetic modeling of our data.…”

Section: Discussionmentioning

confidence: 97%

Magnetic anomalies and rock magnetism of basalts from Reykjanes (SW-Iceland)

et al. 2011

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This study presents rock magnetic properties along with magnetic field measurements of different stratigraphic and lithologic basalt units from Reykjanes, the southwestern promontory of the Reykjanes peninsula, where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The basaltic fissure eruptions and shield lava of tholeiitic composition (less than 11500 a old) show a high natural remanent magnetization (NRM, J r ) up to 33.6 A/m and high Koenigsberger ratio (Q) up to 52.2 indicating a clear dominance of the NRM compared to the induced part of the magnetization. Pillow basalts and picritic shield lava show distinctly lower J r values below 10 A/m. Magnetic susceptibility () ranges for all lithologies from 2.5 to 26  10 3 SI. Heterogeneously distributed titanomagnetite with small grain sizes is the main carrier of magnetization. Magnetic susceptibility vs. temperature -T) curves reveal multiple Curie temperatures from 35 to 570C suggesting different Ti-concentrations in titanomagnetite. A minor oxidation to titanomaghemite is indicated by the irreversibility of some of the -T curves. Intra flow variation of the magnetic minerals is relatively high depending on crystallization history and resulting primary composition and amount of titanomagnetite as well as high-temperature oxidation.The total geomagnetic field was measured for regional field variations along three profiles normal to the spreading zone at Reykjanes. These measurements along with the rock magnetic data and field observations were used for modeling the geological subsurface. The models are in agreement with a feeder dyke system related to the youngest Stampahraun 4 fissure eruption in the western part and a hydrothermally active fault system in the eastern part of Reykjanes. Furthermore, topographic features like fault scarps, pillow basalt -hyaloclastite ridges and shield lava are considered.

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“…Such studies help unraveling the history and dynamics of magma generation, its temporal and spatial variation and the question of the nature of the heat source. Kristjansson and Jónsson (2007) give an overview of palaeomagnetic research in Iceland, based on one of the largest collections of lava samples existing for any region of the world. One consequence of enhanced volcanism and generation of thick oceanic crust, compared to the mid-ocean ridges, is a greater variety of volcanism in Iceland than in most other places.…”

Section: Crustal Structure Stratigraphy and Volcanismmentioning

confidence: 99%