| 1. |
- Lewerentz, Alexander, 1987-
(författare)
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Fluid-induced alteration of metasedimentary rocks in the Scottish Highlands
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fluids, mainly H2O and CO2, are released from H- and C-bearing phases during prograde metamorphism. Because of the buoyancy of these fluids, they rise within the crust towards the surface of the Earth. Metamorphic fluids take advantage of permeable horizons, shear zones, fold hinges, fractures, and are channelled into high-flux zones. Fluid fluxes for channelized fluid flow may exceed background pervasive fluxes by several orders of magnitude. Metamorphic fluids react with the surrounding rock during fluid flow, and altered zones are commonly observed adjacent to high-flux conduits. Fluid-altered rock is texturally, mineralogically, chemically, and isotopically different from rock unaffected by fluid flow. In this thesis, fluid-rock interaction is studied at two localities in the Scottish Highlands: Glen Esk and the Isle of Islay.Glen Esk is one of the type localities used by George Barrow (1853-1932) to propose the concept of metamorphic zones and metamorphic index minerals as an approximate determination of metamorphic grade. In several of the metamorphic zones in Glen Esk, index mineral distribution is highly dependent on proximity to veins. The occurrence of index minerals is therefore not only controlled by pressure and temperature, but also by the availability of metamorphic fluids. Evidence of a retrograde fluid flow event from the North Esk Fault is observed in Glen Esk, for which a time-averaged fluid flux of 0.0003 – 0.0126 m3∙m-2∙yr-1 is calculated. The duration of the fluid event is estimated to between 16 and 334 kyr.On the Isle of Islay, kyanite is observed in rocks of chlorite or lower-biotite metamorphic grade, i.e. much lower temperatures than usually associated with kyanite formation. The favoured explanation for this is retrograde infiltration of extremely high-CO2 fluids, at least locally XCO2 > 0.7, at ~340°C, which altered these rocks and stabilised kyanite in a carbonate-bearing assemblage. Oxygen and carbon stable isotope profiles across the Islay Anticline reveals highly channelized fluid flow along the axial region of this fold, with fluid:rock ratios at least four times higher than in rock farther away from the fold. Although carbon and oxygen isotope ratios of metacarbonate rocks were altered along the Islay Anticline, negative anomalies observed below and above the Port Askaig Tillite Formation cannot solely be attributed to metamorphic fluid flow, which implies that these rocks to varying degree retain their primary paleoclimatological isotopic signatures.
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| 2. |
- Tollefsen, Elin, 1960-
(författare)
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Experimental, petrological and geochemical investigations of ikaite (CaCO3·6H2O) formation in marine environments
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbonates are a group of minerals that play an essential role in several processes on planet Earth, for example in the global carbon cycle and as a product of biomineralisation. Calcite (CaCO3) is by far the most common mineral in the carbonate group, and the stable form of carbonate at Earth surface conditions. However, calcite growth is often kinetically limited and polymorphs of calcite or hydrous calcium carbonates will form instead under certain circumstances. In this thesis, I investigate a hydrous form of calcium carbonate, ikaite (CaCO3 · 6H2O), which occasionally forms under conditions where normally calcite formation would be expected.Ikaite is metastable at surface conditions and has only been observed in nature at temperatures below 7°C. In Ikka Fjord, southwest Greenland, several hundred ikaite columns occur at the bottom of the fjord. Previous studies in Ikka Fjord have shown that ikaite columns are forming above submarine springs that are extremely sodium carbonate rich (pH ~10.5). An association with the surrounding igneous rocks, which comprise nepheline syenite and carbonatite, has been suggested. In the first part of this thesis, I investigate this association. A petrographic study of rocks samples from the igneous complex showed that the combined alteration of the minerals siderite and nepheline could explain the composition of the submarine spring water, and thereby the unique formation of ikaite columns at this site.It is from the mixture of sodium carbonate spring water and seawater that ikaite precipitates in Ikka Fjord, despite the fact that all other calcium carbonates are supersaturated in this mixture. Why ikaite precipitates and not the other forms of calcium carbonate was investigated by a series of experiments in the second and third parts of this thesis. Previous studies have suggested that ikaite was favoured by the low temperature in the fjord (<7°C) and the presence of phosphate (95- 263 μmol/kg) in the submarine spring water, which is known to inhibit calcite growth even at only trace concentrations. In the second part of this thesis, we simulated Ikka Fjord conditions in laboratory and showed that ikaite precipitation is not controlled by the presence of phosphate in the mixture. Instead, after a second series of experiments I found that it is the presence Mg in seawater that inhibits calcite growth and therefore favour ikaite precipitation.Ikaite is metastable and at temperatures above 7°C the mineral will transform or decompose to calcite and water. The transformation can occur pseudomorphically and pseudomorphs after ikaite have been found worldwide in the sediment record. Pseudomorphs after authigenic ikaite in sediments are named glendonite, and because of the narrow temperature range of ikaite observations in nature, glendonite has been used as a paleotemperature indicator. In the fourth part of this thesis, I explore the temperature range of ikaite nucleation by a series of experiments and found that ikaite nucleation can occur up to at least 35°C. This challenges the use of glendonite as a paleotemperature indicator.
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| 3. |
- Nosenzo, Francesco, 1993-
(författare)
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Chemical and isotopic records of polycyclic histories in a subducted continental crust (Dora-Maira Massif, Western Alps)
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- At convergent plate margin, part of the continental crust can be subducted and exhumed. During continental subduction a pre-existing crust is reworked. Remnants of an older orogen are recycled and subjected to (ultra)-high-pressure metamorphism. During subduction, polycyclic rocks undertake a second metamorphic cycle, whereas monocyclic rocks are metamorphosed for the first time. In reworked rocks the pre-subduction record is overprinted and partially or completely lost. Despite this difficulty, reconstructing the pre-subduction history of the recycled crust is crucial, because pre-subduction characters (such as H2O content) can strongly influence how rocks respond to reworking during subduction.The Dora-Maira Massif is worldwide renowned as a (ultra)-high-pressure continental terrane. However, its northern part remained essentially unexplored in recent times. In this thesis work the northern Dora-Maira Massif is used as a case study to investigate recycling of continental crust. The aim is to constrain what type of crust is subducted and exhumed and to unravel the role of fluids during subduction of polycyclic material. Field work, petrology, thermodynamic modelling and geochronology are integrated.New field and geochronological evidence indicate that the northern Dora-Maira Massif displays an internal architecture more complex than what previously thought. It is subdivided in several tectonic units likewise the southern Dora-Maira Massif. Chemical and isotopic records of the reworked rocks reveal a pre-Alpine history spanning from the Lower Palaeozoic to the Mesozoic. A polycyclic basement preserves relicts of a pre-Alpine Barrovian metamorphism connected with the Variscan orogenesis. The absence of granulite-facies partially molten pre-Alpine rocks indicates that only the upper crust was reworked in the Dora-Maira Massif.Thermodynamic modelling indicates that polycyclic micaschists were rehydrated between the Variscan and the Alpine peak metamorphism. Polycyclic garnet texture and chemistry and metamorphic zircon record a main episode of fluid infiltration at the end of the Variscan cycle and not during the Alpine cycle. Pre-Alpine re-hydration of the upper crust allowed high-pressure re-equilibration during subduction.
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| 4. |
- Peillod, Alexandre, 1988-
(författare)
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The metamorphic history of Naxos (central Cyclades, Greece) : Deciphering the Oligocene and Miocene exhumation events
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence ofpast subduction zones. Understanding the tectonics processes that control the exhumation of HP-LT metamorphic rocks in these subduction zones requires full comprehension of the pressure-temperature-time (P–T–t) cycle that the rocks experienced. In the Cyclades, Greece, the Cycladic Blueschist Unit (CBU) hosts eclogite and blueschist facies rocks. However, the processes that exhumed them are debated. The overall aim of this thesis is to understand how the Eocene HP-LT rocks were exhumed in the central Cyclades based on a study of the metamorphic history of Naxos Island and nearby Syros Island. In this thesis, I carried out a systematic geothermobarometric and geochronological investigation on Naxos to better constrain the P–T–t paths that are recorded by the rocks. The data indicate that high-P metamorphism on Naxos occurred in the Eocene at c. 40 Ma and the HP-LT rocks were exhumed by two tectonic events. The first exhumation event occurred in the Oligocene. The HP-LT rocks were exhumed in a convergent setting by an extrusion wedge. The top of the sequence reached greenschist facies conditions at c.32 Ma, whereas the bottom of the sequence remained at greater depth (equating to pressures of 8–12 kbar). Additionally rocks from southeastern Syros recorded a similar Eocene/Oligocene P–T–t history to that recorded by the top of the sequence on Naxos, suggesting a common Eocene/Oligocene metamorphic history for the central Cyclades. The second exhumation event occurred in the Miocene. The rocks were further exhumed in an extensional setting from c. 20 to 8 Ma. The top of the sequence on Naxos was already in the brittle crust at that time and therefore did not record this Miocene metamorphism. The bottom of the sequence was first isothermally exhumed at high-T conditions and thereafter cooled rapidly.
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| 5. |
- Claesson, Lillemor, 1976-
(författare)
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Fluid-rock interaction in two seismically active areas : The Tjörnes Fracture Zone, northern Iceland and the Shillong Plateau, northeastern India
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrogeochemical monitoring can improve our understanding of fluid-rock interaction, and may detect anomalies that are precursory to- or caused by- earthquakes. In this study, hydrogeochemical monitoring was carried out in two distinct tectonic settings, to study coupling between hydrogeochemical changes and seismic activity. The Tjörnes Fracture Zone, north Iceland (HU-01), and the Shillong Plateau, northeastern India (Silver Drop), are two seismically active areas with differing geology. Hydrogeochemical monitoring included regular groundwater sampling with subsequent major and trace element analysis and supporting stable isotope analysis at HU-01 only. Transition metal concentrations anomalies were detected at HU-01 prior to a MW 5.8 earthquake. Concentration increases of many major elements were detected after this earthquake. Hydrogeochemical recovery took place during the subsequent two years. The preseismic anomalies are interpreted as the result of increased fluid-rock interaction due to preseismic fracturing whereas the postseismic changes are attributed to source switching and/or mixing of fluids enabled by fault unsealing. The two-year recovery is probably related to the gradual sealing off of a fluid source. Pre- and post-seismic hydrogeochemical shifts (e.g. Na/Si, Ba/Sr) were detected at Silver Drop coinciding with two MW > 5 earthquakes. These shifts are likely due to changes in the type of feldspar weathering. A basalt dissolution experiment was carried out in order to understand the preseismic changes at HU-01. Rates and dissolution mechanisms interpreted from this experiment confirm that the metal anomalies at HU-01 could be induced by transient exposure of weakly altered basalt to groundwater interaction caused by microfracturing. Finally, from this study it can be concluded that a seismic event need not cause a hydrogeochemical shift, but a hydrogeochemical shift was always associated with a seismic event.
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| 6. |
- Engström, Anna, 1975-
(författare)
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Deformation and fluid-flow in magma-poor margins : A study of the Tasna Ocean-Continent transition, SE Switzerland
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, interaction between fluids and deformation during the final stages of magma-poor rifting was investigated. The Tasna Ocean-Continent transition, located in the Swiss Alps, was studied and a large data set was obtained from profiles oriented perpendicular to two detachment faults. One of these juxtaposed serpentinized mantle against continental crust and the other exhumed both mantle and continental crust to the seafloor. Deformation associated with detachment faulting showed many common features but also some phenomena which were unique to each fault, confirming their sequential activity and differing roles in the exhumation process. Oxygen isotopes indicated the presence of both pervasive and channeled fluid phases, either accompanying or post-dating serpentinization. Deformation in the fault zone occurred in previously serpentinized mantle indicating that serpentinization pre-dated final exhumation. Most strain localization and displacement occurred in fault cores which are narrow zones beneath the detachments. These are underlain by wide zones characterized by more distributed strain. Transitional fabrics as well as reactivated and/or overprinted deformation structures indicated that the final phase of rifting is complex. Fault cores acted as fluid conduits or barriers. Thus, the most deformed zones may become the least permeable. Hence the coupling between deformation and fluid flow is complex in a study area subjected to several phases of deformation and fluid flow. Finally the importance of serpentinization in the evolution of magma-poor rifting was investigated. It was found that serpentinization is the consequence rather than the reason for strain localization at magma-poor margins. However, serpentinization may be an important process which can accelerate exhumation rates in the very latest stages of magma-poor rifting. The pre-existing deformation history of the crust may also be of importance for the development and location of margins.
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| 7. |
- Lewerentz, Alexander, 1987-
(författare)
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Vein controlled crystal size distributions of Barrovian index minerals
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The concept of index mineral based metamorphic zones was first introduced by George Barrow in 1912, and Barrovian metamorphism is still applied by metamorphic petrologists. Today the importance of metamorphic fluids for driving metamorphic reactions is widely recognised. Even so, the general view is that Barrovian metamorphism is solely controlled by pressure, temperature, and protolith composition. This thesis aims to establish if and how fluids control index mineral formation and distribution during Barrovian metamorphism. To do so, samples from Barrow’s own type locality in Glen Esk, Southeast Scottish Highlands, are used to investigate possible relationships between veining and index mineral distribution. This was done using a combination of petrographic and textural observations and analyses, whole rock chemistry, mineral chemistry, as well as oxygen isotope analyses. These data show a grade dependant association of index mineral distribution and veining: in the chlorite zone and most of the biotite zone, no correlation between veining and index mineral distribution is seen. For the garnet and staurolite zones, index mineral abundance is shown to decrease away from veins. The kyanite zone shows a fairly homogenous kyanite distribution, but also indications of extensive fluid-rock interaction. Based on these observations and the analysis datasets, it is concluded that fluid played a major role in the stabilisation and distribution of the Barrovian index minerals in Glen Esk, and that the fluid control was larger at metamorphic grade.
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| 8. |
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| 9. |
- Peillod, Alexandre, 1998-
(författare)
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How were high-pressure rocks exhumed in Naxos, Greece?
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Combining petrological and geochronological data we were able to show that rocks in south Naxos completed a full blueschist-/greenschist-facies metamorphic loop in about 10 Myr, distinctly faster than hitherto believed. We show that the high-pressure rocks reached peak pressure (16.3±0.9 kbar - 619 ±32°C) between 40.5±1.0 and 38.3±0.5 Ma and were re-equilibrated in the middle crust (3.8±1.1 kbar - 384±30 °C) under greenschist-facies metamorphism during in the Oligocene ~32 Ma. Our data indicate that this exhumation occurred at rates of 7.4±4.6 km Ma-1. Therefore, the Oligocene greenschist-facies overprint of the blueschist-facies rocks from south Naxos is unrelated to the Miocene amphibolite-/greenschist-facies metamorphism observed in Naxos.
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| 10. |
- Sundvall, Rickard, 1976-
(författare)
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Water as a trace component in mantle pyroxene: Quantifying diffusion, storage capacity and variation with geological environment
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, distribution and diffusion of water in pyroxene are examined in an effort to explain the water content variation observed in natural pyroxene. Water is a common trace component in many nominally anhydrous minerals (NAMs) from the Earth's crust and mantle and greatly impacts their physical properties. Therefore, it is crucial to constrain the processes that control water incorporation in these minerals. The pyroxene group has a fairly simple mineral chemistry, the highest amount of water and the greatest water content variation measured in mantle NAMs. Therefore, they are ideal for a case study such as the present. The redox reaction: OH- + Fe2+ ↔ O2- + Fe3+ + ½H2, is believed to control water diffusion in many NAMs having sufficiently high iron contents. Nevertheless, reactions involving vacancies and charge-deficient substitutions which are mainly controlled by cation diffusion are also present and have reaction kinetics that is significantly slower than the redox exchange. Therefore, diffusion and reaction kinetics were studied in (1) synthetic diopside with ~0.7 wt % FeO which allows the study of contributions from both types of reactions (i.e. Fe-redox and cation diffusion). These results were then compared (2) with reaction kinetics in pure synthetic diopside. The diffusion rates are faster in Fe-free diopside, as reaction kinetics is dominated by cation diffusion in samples with low Fe contents. Next (3), water content variation and zonation were investigated in natural pyroxene using high resolution FTIR imaging. The results show no water zonation and a correlation between mantle source and water content. Finally (4), thermal annealing experiments in H2 on natural pyroxene show little or no re-hydration capacity for mantle samples. Altogether, the results indicate that the water contents of most mantle pyroxenes do reflect mantle conditions and that many types of reactions controlling water uptake and release seem to be present in pyroxene.
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| 11. |
- Sundvall, Rickard, 1976-
(författare)
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Hydrogen diffusion and storage mechanisms in diopside
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hydrogen is a widespread trace element in many nominally anhydrous minerals (minerals without water or hydroxyl ions in their structural formula) from the Earth's crust and mantle. The hydrogen is normally incorporated in the form of hydroxyl ions and can be regarded as structurally bound water. The most important minerals of the upper mantle: olivine, orthopyroxene, clinopyroxene and garnet, all contain small but significant amounts of hydrogen. This means that the upper mantle has the capacity to store the equivalent of several world oceans. To know how much water there is in the Earth's interior is important knowledge, as small differences in mantle water content influences models of mantle dynamics. The mantle plays an important role in the hydrological cycle as water in oceanic crust and sediments is subducted at converging plate boundaries and again released through volcanic eruptions during millions of years. Detailed knowledge of the amount of water that is retained within the mantle for longer periods of time (~109 years) is still lacking. Investigating the water content of the mantle is a task shared between the fields of mineralogy, petrology, geophysics, and theoretical physics (i.e. atomistic models). My approach as a mineralogist has been to investigate in detail the mechanisms that are responsible for water incorporation in nominally anhydrous upper mantle minerals, with a special emphasis on the pyroxenes as they can carry substantial amounts of water, up to 1300 ppm H2O. The fundamental questions here are how much of the original xenolith water is lost during transport to the surface and if the spectroscopic features measured in the minerals are representative for mantle conditions. The redox reaction: Fe2+ + OH- ↔ Fe3+ + O2- + ½H2, which is relatively fast, is thought to be the dominant hydrogen exchange reaction in many minerals (Ingrin & Skogby, 2000). The reaction is fast enough to suggest that water in nominally anhydrous minerals equilibrates with the transporting magma and related fluids during ascent to the surface. Nevertheless, several studies show systematic variations in water content with geological environment (Bell & Rossman, 1992; Peslier et al., 2002), implying a complex relationship between host mineral, mantle source region, magma type and eruption style. This thesis is focused on the dehydration-hydration mechanisms in diopside, the most common variety of clinopyroxene in the upper mantle. The approach has been to study the kinetics and temperature dependence of the reactions controlling hydrogen diffusion in synthetic Fe-poor diopside. Other reactions are likely to be obscured by the iron redox reaction if measured in natural mantle diopside containing several wt% FeO. Therefore, synthetic diopside with very low amounts (0.7 wt% FeO) of iron had to be used in order to measure the influence and co-dependence of the iron redox reaction with other possible reactions. The experiments were carried out by stepwise heating of the samples in both air and hydrogen. After each step, OH-absorbance was measured using Fourier Transform Infrared spectroscopy, and the relative amounts of ferric and ferrous iron was monitored by Mössbauer spectroscopy. When comparing the amounts of ferric iron and hydrogen (in atoms per formula unit), there is considerable deviation from the ideal 1:1 relationship expected from the iron redox reaction. The dehydration process is reversible to a great extent and re-hydration continues even after all iron is reduced. The results of this study show that other reactions apart from the iron redox reaction are active, and that they are significantly slower. If these slower reactions are active in mantle diopsides, there is a possibility that they may preserve signatures from the mantle source region.
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