Academic Interests
- Rock microstructures
- Lower crustal earthquakes
- Coupling of deformation and reaction
- Alteration of ultramafic rocks
Teaching
Background
since 2021: Associate Professor, Dept of Geosciences, University of Oslo
2017 - 2021: Postdoctoral Fellow, GEO-PGP, University of Oslo
2013 - 2017: PhD student, GEO-PGP, University of Oslo
2011 - 2013: MSc of Geosciences, University of Münster, Germany
2008 - 2011: BSc of Geosciences, University of Münster, Germany
Publications
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Dunkel, Kristina G.; Zertani, Sascha; Roddatis, Vladimir & Jamtveit, Bjørn
(2026).
Zoned feldspar records hydration-dehydration cycles in lower-crustal shear zones.
Contributions to Mineralogy and Petrology.
ISSN 0010-7999.
181(5).
doi:
10.1007/s00410-026-02317-w.
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Metamorphic reactions provide crucial insights into the conditions that rocks experienced and significantly influence their rheological behaviour. However, some reactions are challenging to identify in the rock record. This study focuses on zonation and microstructures in plagioclase as indicators of metamorphic history, employing a case study from Lofoten, northern Norway, where various types of plagioclase microstructures are preserved together. We use a combination of electron microscopy techniques (electron backscatter diffraction, transmission electron microscopy, and electron microprobe analyses) complemented with local equilibrium thermodynamic modelling to examine the relationships between composition, microstructures, and underlying metamorphic processes, and put that in context with previous observations of metamorphically zoned plagioclase, often referred to as “complex feldspar”. Needle-shaped Ca-richer zones in plagioclase represent pseudomorphs after zoisite/clinozoisite, indicative of plagioclase breakdown following fluid infiltration at high pressure and subsequent reversal of the reaction. Polygonal plagioclase with complex zonations also originates from the hydration/dehydration cycle but has additionally undergone deformation during hydration. Minor changes in pressure and temperature at upper amphibolite-facies conditions can significantly affect the stability and microstructures of plagioclase and thereby the rheological behaviour of the lower crust.
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Dunkel, Kristina G.; Rødsrud, Christian Løchsen & Fredriksen, Per Ditlef
(2026).
Mineralogy of ceramic wares from the Augland workshop (c. 200–450 CE) in southernmost Norway: New insights from petrography, mineral chemistry and clay mineral X-ray diffraction.
Journal of Archaeological Science: Reports.
ISSN 2352-409X.
74.
doi:
10.1016/j.jasrep.2026.105892.
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This pilot study of ten samples from the Early Iron Age ceramic workshop at Augland (c. 200–450 CE) in southernmost Norway identifies the microstructural and geochemical signatures of the main types of wares and relates these to the overall chronology of the site. Having the advantage of an already established chronology and
related range of pastes, this study improves our understanding of key factors of the workshop’s technological development. A more detailed geological data set offers new insights into the dynamics between the initial main recipe, a remarkably stable feature throughout the workshop’s lifetime of more than 250 years, and later paste
compositions, and into the provenance of imported tempering minerals. A novel mode of production entering Augland in the mid-4th century brought innovative ceramic paste recipes that were used alongside the traditional main recipe in the workshop’s final century. This pilot study adds important insights to the signatures of
the two latest recipes during the first part of the 5th century. These recipes have been related to two successive technological pulses entering Augland, both of which were identified in a previous comprehensive statistical cluster analysis. However, our results reveal that these two recipes are both microstructurally and geochemically
distinct, indicating the two technological pulses may have different geographical origins. By linking this new geological dataset to the established chronology of the Augland site, we present a framework for future provenance studies and regional database of ceramic material found at the Augland site.
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Lu, Yang; Sednev-Lugovets, Anton; Carvalho, Patricia Almeida; Guzik, Matylda N.; Dayaghi, Amir Masoud & Dunkel, Kristina G
[Show all 9 contributors for this article]
(2025).
Dypingite as a hydration series with reversible change of H2O content.
American Mineralogist.
ISSN 0003-004X.
111(5),
p. 746–757.
doi:
10.2138/am-2024-9603.
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Dypingite has recently been found to have promising applications in material and environmental sciences. However, its exploitation is limited due to its ambiguous identification via powder X-ray diffraction (PXRD), which is caused by the occurrence of several phases with similar structures and PXRD patterns to that of dypingite. Multiple dypingite-like phases with partially overlapping X-ray diffraction traces appear to exist. To better understand dypingite, we studied samples from Dypingdal and Feragen Ultramafic Body, Norway. Dypingite sensu stricto (s.s.), several dypingite-like phases, and hydromagnesite were identified by PXRD. Three regions of 2θ (CuKα) were recognized where diffraction peaks are readily observed to transition between dypingite s.s., dypingite-like phases, and hydromagnesite. The position of the peak at around 8.5° 2θ varies with the H2O contents measured by the thermogravimetric and Fourier transform infrared analyses, suggesting that structural and compositional characteristics of dypingite-like phases form a continuum between dypingite s.s. and hydromagnesite. Humidity incubation experiments at room temperature indicated a reversible change of H2O content between dypingite s.s. and dypingite-like phases. High humidity conditions increased H2O contents of dypingite-like phases to dypingite s.s., whereas low humidity conditions reversed the direction towards hydromagnesite, but without complete reaction. Such transition behavior suggests that the structures of dypingite s.s. and the dypingite-like phases are very similar, probably identical in building unit topology but with different degrees of hydration, whereas the structure of hydromagnesite is less closely related. This study draws attention to the sample processing on dypingite hydration series, contributes to the applications of this series in CO2 sequestration, and gives suggestions to the refinement of structures of dypingite and dypingite-like phases.
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Austrheim, Håkon Olaf; Engvik, Ane; Ganerød, Morgan; Dunkel, Kristina G & Velo, Mari Roen
(2022).
Low-grade prehnite-pumpellyite facies metamorphism and metasomatism in basement rocks adjacent to the Permian Oslo rift: The importance of displacive reactions.
Journal of Metamorphic Geology.
ISSN 0263-4929.
40(9),
p. 1467–1492.
doi:
10.1111/jmg.12682.
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The Kongsberg and Bamble lithotectonic domains of SE-Norway are known as classical Precambrian high-grade metamorphic terrains. The area has undergone extensive metasomatism with formation of albitites and scapolite-rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low-grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite-pumpellyite and zeolite facies. Structurally, the low-grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low-grade minerals are part of larger fracture systems. The low-grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K-feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low-grade minerals occur. As albite, K-feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low-grade minerals, they probably belong to the low-grade assemblage and some of the albitization in the region presumably occurred at low-grade conditions. Alteration of olivine (Fo69) at low-grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low-grade assemblages in south-western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar-Ar dating of K-feldspar from the low-grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low-grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro-fractures and porosity in the adjacent minerals that enhance fluid flow and low-grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the opening of the larger, horizontally oriented fracture systems needed to transport the fluid over a distance of several kilometres was also the results of displacive reactions remains unknown. The low-grade metamorphism and metasomatism formed in the shoulder of the Oslo rift and may have contributed to its uplift.
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Jamtveit, Bjørn; Dunkel, Kristina G; Petley-Ragan, Arianne Juliette; Austrheim, Håkon Olaf; Corfu, Fernando & Schmid, Daniel Walter
(2021).
Rapid fluid-driven transformation of lower continental crust associated with thrust-induced shear heating.
Lithos.
ISSN 0024-4937.
396-397.
doi:
10.1016/j.lithos.2021.106216.
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Dunkel, Kristina G; Morales, Luiz F.G. & Jamtveit, Bjørn
(2021).
Pristine microstructures in pseudotachylytes formed in dry lower crust, Lofoten, Norway.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.
ISSN 1364-503X.
379(2193).
doi:
10.1098/rsta.2019.0423.
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Feldspar-rich pseudotachylytes from the island of Moskenesøya, Lofoten, formed in dry granulites under lower crustal conditions during the Caledonian orogeny. The central parts of the pseudotachylytes, where the cooling rates were slowest, are characterized by microlites and spherulites of plagioclase and K-feldspar. K-feldspar surrounding plagioclase is consistent with crystallization from a melt during cooling instead of devitrification as the origin of the spherulites. Very thin (a few micrometres wide) injection veins, which experienced very rapid quenching, contain amorphous or cryptocrystalline material. The preservation of this material and of the fine-grained microstructures shows that, under fluid-absent conditions, recrystallization and reactions are slow and the original microstructures of the pseudotachylytes can be preserved. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.
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Dunkel, Kristina G; Zhong, Xin; Arnestad, Paal Ferdinand; Valen, Lars Vesterager & Jamtveit, Bjørn
(2020).
High transient stress in the lower crust: Evidence from dry pseudotachylytes in granulites, Lofoten Archipelago, northern Norway.
Geology.
ISSN 0091-7613.
49(2),
p. 135–139.
doi:
10.1130/G48002.1.
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Seismic activity below the standard seismogenic zone is difficult to investigate because the geological records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here, we describe unusually pristine pseudotachylytes in lower-crustal granulites from the Lofoten Archipelago, northern Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (mainly plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling, i.e., feldspar microlites and spherulites and “cauliflower” garnets. Mylonites are absent, both in the wall rocks and among the pseudotachylyte clasts. The absence of features recording precursory ductile deformation rules out several commonly invoked mechanisms for triggering earthquakes in the lower crust, including thermal runaway, plastic instabilities, and downward propagation of seismic slip from the brittle to the ductile part of a fault. The anhydrous mineralogy of host and pseudotachylytes excludes dehydration-induced embrittlement. In the absence of such weakening mechanisms, stress levels in the lower crust must have been transiently high.
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Jamtveit, Bjørn; Petley-Ragan, Arianne Juliette; Incel, Sarah; Dunkel, Kristina G; Aupart, Claire Olga Maryse & Austrheim, Håkon Olaf
[Show all 9 contributors for this article]
(2019).
The effects of earthquakes and fluids on the metamorphism of the lower continental crust.
Journal of Geophysical Research (JGR): Solid Earth.
ISSN 2169-9313.
124(8),
p. 7725–7755.
doi:
10.1029/2018JB016461.
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Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.
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Dunkel, Kristina G; Jamtveit, Bjørn & Austrheim, Håkon Olaf
(2019).
Ophicarbonates of the Feragen Ultramafic Body, central Norway.
Norwegian Journal of Geology.
ISSN 2387-5844.
99(3),
p. 1–18.
doi:
10.17850/njg99-3-3.
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The carbonation of ultramafic rocks is a common alteration process in ophiolites and can occur in various settings. We provide the first detailed description of the carbonated peridotites (ophicarbonates) of the Feragen Ultramafic Body, central Norway, which have unusually variable compositions and microstructures. Lithologies range from pervasively carbonated serpentinites through carbonated serpentinite breccias to carbonated ultramafic conglomerates. Carbonate phases are Ca-carbonate, magnesite and dolomite. Some breccias are also cemented by coarsegrained brucite. This variability records strong variations in fluid chemistry and/or pressure and temperature conditions, both spatially and temporally. By analysing these altered ultramafic rocks using field relationships, optical microscopy, electron microprobe analysis and oxygen and carbon isotope compositions, we elucidate the history of the Feragen Ultramafic Body in more detail and emphasise the importance of deformation for the extent and type of alteration.
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Aupart, Claire Olga Maryse; Dunkel, Kristina G; Angheluta, Luiza; Austrheim, Håkon Olaf; Ildefonse, Beniot & Malthe-Sørenssen, Anders
[Show all 7 contributors for this article]
(2018).
Olivine Grain Size Distributions in Faults and Shear Zones: Evidence for Nonsteady State Deformation.
Journal of Geophysical Research (JGR): Solid Earth.
ISSN 2169-9313.
123(9),
p. 7421–7443.
doi:
10.1029/2018JB015836.
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The grain size distribution of deformed rocks may provide valuable information about their deformation history and the associated mechanisms. Here we present a unique set of olivine grain size distributions from ultramafic rocks deformed under a wide range of stress and strain rate conditions. Both experimentally deformed and naturally deformed samples are included. We observe a surprisingly uniform behavior, and most samples show power law grain size distributions. Convincing lognormal distributions across all scales were only observed for samples experimentally deformed at high temperature (1200 °C) and for some mantle‐deformed natural samples. Single power law distributions were observed for natural samples deformed by brittle mechanisms and by samples deformed experimentally in the regime of low‐temperature plasticity. Most natural samples show a crossover in power law scaling behavior near the median grain size from a steep slope for the larger grain fraction to a more gentle slope for the smaller grains. The small grain fraction shows a good data collapse when normalized to the crossover length scale. The associated power law slope indicates a common grain size controlling process. We propose a model that explains how such a scaling behavior may arise in the dislocation creep regime from the competition between the rate involved in the dislocation dynamics and the imposed strain rate. The common departure from lognormal distributions suggests that naturally deformed samples often have a deformation history that is far from a steady state scenario and probably reflects deformation under highly variable stress and strain rates. ©2018. American Geophysical Union. All Rights Reserved.
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Petley-Ragan, Arianne Juliette; Dunkel, Kristina G; Austrheim, Håkon Olaf; Ildefonse, Benoit & Jamtveit, Bjørn
(2018).
Microstructural Records of Earthquakes in the Lower Crust and Associated Fluid-Driven Metamorphism in Plagioclase-Rich Granulites.
Journal of Geophysical Research (JGR): Solid Earth.
ISSN 2169-9313.
123(5),
p. 3729–3746.
doi:
10.1029/2017JB015348.
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Coseismic damage associated with earthquakes in the lower continental crust is accompanied by postseismic annealing and fluid‐mediated metamorphism that influence the physical and chemical development of the continental crust on regional scales. A transition from brittle deformation to crystal‐plastic recrystallization is a recurring characteristic of rocks affected by lower crustal earthquakes and is observed in plagioclase adjacent to pseudotachylytes in granulite facies anorthosites from the Bergen Arcs, western Norway. The microstructural and petrological records of this transition were investigated using electron microscopy, electron microprobe analysis, and electron backscatter diffraction analysis. Microfractures associated with mechanical twins are abundant within plagioclase and contain fine‐grained aggregates that formed by fragmentation with minor shear deformation. The presence of feather features, which are described for the first time in feldspar, suggests that fractures propagate at near the shear wave velocity into the wall rock of earthquake slip planes. Grain size insensitive recrystallization took place within the time frame of pseudotachylyte formation, forming high‐angle grain boundaries required for shear zone initiation. Fluid infiltration synfracture to postfracture facilitated the epitactic replacement of plagioclase by alkali feldspar and the nucleation of clinozoisite, kyanite, and quartz. The grain size reduction and crystallization associated with the microfractures create rheologically weak areas that have the potential to localize strain within the plagioclase‐rich lower crust. ©2018. American Geophysical Union. All Rights Reserved.
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Austrheim, Håkon Olaf; Dunkel, Kristina G; Plümper, Oliver; Ildefonse, Benoit; Liu, Yang & Jamtveit, Bjørn
(2017).
Fragmentation of wall rock garnets during deep crustal earthquakes.
Science Advances.
3(2).
doi:
10.1126/sciadv.1602067.
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Fractures and faults riddle the Earth’s crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale.
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Dunkel, Kristina G; Austrheim, Håkon Olav; Renard, Francois; Cordonnier, Benoit & Jamtveit, Bjørn
(2017).
Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway.
Earth and Planetary Science Letters.
ISSN 0012-821X.
463,
p. 277–285.
doi:
10.1016/j.epsl.2017.01.047.
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Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values ‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.
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Dunkel, Kristina G; Austrheim, Håkon Olaf; Ildefonse, Benoit & Jamtveit, Bjørn
(2017).
Transfer of olivine crystallographic orientation through a cycle of serpentinisation and dehydration.
Contributions to Mineralogy and Petrology.
ISSN 0010-7999.
172(8).
doi:
10.1007/s00410-017-1378-5.
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Our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from one stage to another. Within the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures: The outer part of the veins comprises coarse-grained olivine that forms an unusual, brick-like microstructure. The inner part of the veins, surrounding a central fault, is composed of fine-grained olivine. Where the fault movement included a dilational component, optically clear, equant olivine occurs in the centre. Electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation (CPO) from the olivine in the porphyroclastic host rock; however, misorientation is weaker and associated to different rotation axes. We propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its CPO. Growth towards pre-existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick-like microstructure in the veins. When dehydration embrittlement induced slip, a strong strain localisation on discrete fault planes prevented distortion of the CPO due to cataclastic deformation; grain size reduction did not significantly modify the olivine CPO. This illustrates how a CPO can be preserved though an entire metamorphic cycle, including hydration, dehydration, and deformation processes, and that the CPO and the microstructures (e.g. grain shape) of one phase do not necessarily record the same event. © 2017 Springer Verlag
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Dziadkowiec, Joanna; Linga, Gaute; Dunkel, Kristina G.; Valtiner, Markus & Renard, Francois
(2026).
In-situ, real-time replacement of calcite under geometrical confinement.
doi:
10.5194/egusphere-egu26-3925.
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Mineral replacement by dissolution-precipitation reactions that occur under confinement critically influences subsurface deformation by modifying rock porosity, permeability, and cohesion, and by inducing fracturing. Yet real-time, experimental observations of these phenomena at the nano- to microscale remain insufficient. In this work, we follow the in-situ replacement of confined calcite crystal using a surface force apparatus (SFA) technique. Calcite undergoes dissolution under low pH conditions, followed by replacement into three various Ca-minerals: calcium oxalate, calcium sulfate (gypsum), or calcium phosphate (brushite), depending on the initial composition of the solution. We monitor these reactions in real time, map the spatial distribution of precipitates as a function of confinement gap size, and evaluate how epitaxy between the secondary phases and the parent calcite governs preferred nucleation and growth sites. In addition, we measure forces that act on the confining pore wall during the replacement and estimate the associated crystallization pressures. This work contributes to the understanding of the mineral growth under confinement and its consequences for porous rock integrity, with immediate relevance to subsurface fluid and gas storage operations, where rapid mineralization is common.
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Rosenqvist, Marija Plahter; Dunkel, Kristina; Planke, Sverre; Nogueira, Liebert Parreiras; Polteau, Stephane & Menegon, Luca
(2025).
Porosity structures and local compositional variations associated with natural CO2 sequestration in basalts offshore Norway.
doi:
10.5194/egusphere-egu25-11073.
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Permanent storage of CO2 as carbonates in basalt deposits utilises natural weathering reactions between silicate minerals and carbonic water. By studying how CO2-enriched water has altered basalt in 43 samples from the Vøring Margin, offshore Norway, we demonstrate the pathways and reactions of CO2 within a basalt reservoir.The pore spaces occupied by carbonates in the samples were studied through μ-CT (computer tomography) studies on three minicores and detailed microanalyses on 32 thin sections via scanning electron microscopy (SEM), electron microprobe analysis, and electron backscatter diffraction (EBSD). Single crystal X-ray diffraction on 15 samples determined the type of carbonate minerals and precipitation ages were determined for two samples through U/Pb-radiomeric dating. δ18O and δ13C-isotope analyses were used to determine the fluid origin in 11 samples.Three types of carbonate precipitation were characterised based on the pore structures they fill and the associated mineralogy. Type I carbonates, seen in 18 thin sections, fills vesicles, mainly along lava flow margins. SEM analyses showed that all vesicles are initially coated with a smectite layer which incorporates most of the Mg2+ and Fe2+ from dissolving silicate minerals. As pH and Ca concentrations increase, calcite precipitates in some vesicles. The secondary mineral assemblage and precipitation order fit with low-temperature alteration of basalt with pore water and atmospherically balanced CO2 levels (pH≥8). Type II carbonates appear in seven thin sections as
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Solheim, Mari; Svensen, Henrik Hovland & Dunkel, Kristina G
(2025).
Evolution of Mafic Dykes in a Major Fault Zone, the Oslo rift Petrology and geochemistry of fluid-rock interaction along the Ekeberg Fault.
Universitetet i Oslo.
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This thesis investigates how magmatic intrusions in the Oslo Rift were modified by hydrothermal fluid circulation, with a focus on dykes intersected in the Ekeberg Fault zone. A continues drill core from Alna was studied using petrography, bulk-rock geochemistry, and stable isotopes to trace evolution of Permian dykes and their surrounding rocks. The dykes, originally composed of feldspar, pyroxene, and oxides, are now strongly altered. Away from the fault, alteration is dominated by albite, chlorite, and epidote, reflecting higher temperature fluids. Within the fault zone, the rocks are overprinted by dolomite, clays, barite and sulfides, consistent with lower temperature CO2- and sulfur-rich fluids linked to organic-rich alum shale and basement-derived fluids. Textures such as porous albite and zoned dolomite show that alteration progressed through fluid driven replacement reactions, which also created porosity and sustained fluid flow. Geochemical and isotopic data highlight the contribution of multiple fluid sources: magmatically heated seawater, shale derived volatiles, and basement fluids. The Alna dykes therefore express a complex history of overprinting by successive hydrothermal events.
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Published
Oct. 16, 2014 11:23 AM
- Last modified
Feb. 18, 2026 5:08 PM
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