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  • 1. Ranta, E.
    et al.
    Stockmann, G.
    Wagner, T.
    Fusswinkel, T.
    Sturkell, E.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fluid-rock reactions in the 1.3Ga siderite carbonatite of the GrOnnedal-ika alkaline complex, Southwest Greenland2018In: Contributions to Mineralogy and Petrology, ISSN 0010-7999, E-ISSN 1432-0967, Vol. 173, no 10, article id 78Article in journal (Refereed)
    Abstract [en]

    Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure-temperature conditions during crystallization and fluid-rock interaction caused by magmatic-hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid-rock interaction history of carbonatites. We present new LA-ICP-MS trace-element data for magnetite, calcite, siderite, and ankerite-dolomite-kutnohorite from the iron-rich carbonatites of the 1.3Ga GrOnnedal-ika alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.

  • 2.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl-Claesson, L.
    Wästeby, Niklas
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, G.
    Sturkell, E.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stefansson, A.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Siegmund, Heike
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Keller, N.
    Kjartansdóttir, R.
    Hjartarson, H.
    Kockum, I.
    Hydrochemical Changes Before and After Earthquakes Based on Long-Term Measurements of Multiple Parameters at Two Sites in Northern IcelandA Review2019In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 124, no 3, p. 2702-2720Article, review/survey (Refereed)
    Abstract [en]

    Hydrochemical changes before and after earthquakes have been reported for over 50years. However, few reports provide sufficient data for an association to be verified statistically. Also, no mechanism has been proposed to explain why hydrochemical changes are observed far from earthquake foci where associated strains are small (<10(-8)). Here we address these challenges based on time series of multiple hydrochemical parameters from two sites in northern Iceland. We report hydrochemical changes before and after M >5 earthquakes in 2002, 2012, and 2013. The longevity of the time series (10 and 16years) permits statistical verification of coupling between hydrochemical changes and earthquakes. We used a Student t test to find significant hydrochemical changes and a binomial test to confirm association with earthquakes. Probable association was confirmed for preseismic changes based on five parameters (Na, Si, K, O-18, and H-2) and postseismic changes based on eight parameters (Ca, Na, Si, Cl, F, SO4, O-18, and H-2). Using concentration ratios and stable isotope values, we showed that (1) gradual preseismic changes were caused by source mixing, which resulted in a shift from equilibrium and triggered water-rock interaction; (2) postseismic changes were caused by rapid source mixing; and (3) longer-term hydrochemical changes were caused by source mixing and mineral growth. Because hydrochemical changes occur at small earthquake-related strains, we attribute source mixing and water-rock interaction to microscale fracturing. Because fracture density and size scale inversely, we infer that mixing of nearby sources and water-rock interaction are feasible responses to small earthquake-related strains. Plain Language Summary Changes in groundwater chemistry before and after earthquakes have been reported for over 50years. However, few studies have been able to prove that the earthquakes caused these changes. Also, no study has explained why these changes are often reported far from where the earthquake occurred. Here we address these challenges based on measurements of groundwater chemistry made at two sites in northern Iceland over time periods of 10 and 16years. We used statistical methods to prove that the earthquakes caused changes of ground water chemistry both before and after the earthquakes. We showed that changes of groundwater chemistry before earthquakes were caused by slow mixing between different groundwaters, which triggered reactions with the wall rock that changed groundwater chemistry, and that changes of groundwater chemistry after earthquakes were causes by rapid mixing between different groundwaters. That these changes were detected far from where the earthquakes occurred suggests that cracking of the wall rock at a very small scale was all that was needed for mixing of different groundwaters and reactions with the wall rock to occur.

  • 3.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Einarsson, Draupnir
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Orr, Tim
    Dimmuborgir: a rootless shield complex in northern Iceland2016In: Bulletin of Volcanology, ISSN 0258-8900, E-ISSN 1432-0819, Vol. 78, no 5, article id 40Article in journal (Refereed)
    Abstract [en]

    The origin of Dimmuborgir, a shield-like volcanic structure within the Younger Laxa lava flow field near Lake Myvatn, in northern Iceland, has long been questioned. New airborne laser mapping (light detection and ranging (LiDAR)), combined with ground-penetrating radar results and a detailed field study, suggests that Dimmuborgir is a complex of at least two overlapping rootless shields fed by lava erupting from the nearby Ludentarborgir crater row. This model builds upon previous explanations for the formation of Dimmuborgir and is consistent with observations of rootless shield development at Kilauea Volcano, Hawaii. The larger rootless shields at Dimmuborgir, 1-1.5 km in diameter, elliptical in plan view, similar to 30 m in height, and each with a 500-m-wide summit depression, were capable of storing as much as 2-3x10(6) m(3) of lava. They were fed by lava which descended 30-60 min lava tubes along a distance of 3 km from the crater row. The height difference generated pressure sufficient to build rootless shields at Dimmuborgir in a timescale of weeks. The main summit depressions, inferred to be drained lava ponds, could have emptied via a 30-m-wide x 5-m-deep channel, with estimated effusion rates of 0.7-7 m(3) s(-1) and minimum flow durations of 5-50 days. We argue that the pillars for which Dimmuborgir is famed are remnants of lava pond rims, at various stages of disintegration that formed during pond drainage.

  • 4.
    Stockmann, Gabrielle
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Brüchert, Volker
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Langhof, Jörgen
    Skogby, Henrik
    Karlsson, Andreas
    Control of a calcite inhibitor (phosphate) and temperature on ikaite precipitation in Ikka Fjord, southwest Greenland2018In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 89, p. 11-22Article in journal (Refereed)
    Abstract [en]

    Ikaite (CaCO3 center dot 6H(2)O) forms submarine tufa columns in Ikka Fjord, SW Greenland. This unique occurrence is thought to relate to aqueous phosphate concentration and low water temperatures (< 6 degrees C). Phosphate ions are well-known inhibitors of calcite precipitation and Ikka Fjord has a naturally high-phosphate groundwater system that when mixing with seawater leads to the precipitation of ikaite. In the study presented here, experiments simulating conditions of Ikka Fjord show that a) the formation of ikaite is unrelated to the aqueous phosphate concentration (0-263 mu mol/ kg PO43-) in 0.1 M NaHCO3/0.1 M Na2CO3 solutions mixing with seawater at 5 degrees C and pH 9.6-10.6, and b) ikaite forms at temperatures up to 15 degrees C without phosphate and in open beakers exposed to air. Instead, supersaturation of ikaite and the seawater composition are the likely factors causing ikaite to precipitate in Ikka Fjord. This study shows that adding Mg2+ to a NaHCO3/Na2CO3 - CaCl2 mixed solution leads to the formation of ikaite along with hydrated Mg carbonates, which points to the high Mg2+ concentration of seawater, another known inhibitor of calcite, as a key factor promoting ikaite formation. In experiments at 10 and 15 degrees C, increasing amounts of either nesquehonite (Mg(HCO3)(OH)center dot 2H(2)O) or an amorphous phase co-precipitate with ikaite. At 20 degrees C, only the amorphous phase is formed. In warming Arctic seawater, this suggests Mg carbonate precipitation could become dominant over ikaite in the future.

  • 5.
    Tollefsen, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Balic-Zunic, Tonci
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bruchert, Volker
    Lee, Cheng Choo
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ikaite nucleation at 35°C challenges the use of glendonite as a paleotemperature indicatorManuscript (preprint) (Other academic)
  • 6.
    Tollefsen, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lundqvist, Lena
    Sturkell, Erik
    Secondary alteration of the Gronnedal-Ika igneous complex and the genesis of ikaite, CaCO3 center dot 6H(2)O, SW Greenland2019In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 510, p. 18-30Article in journal (Refereed)
    Abstract [en]

    The mineral ikaite (CaCO3 center dot 6H(2)O) precipitates from a mixture of spring water and seawater as tufa columns which grow at a rate of up to 50 cm per year reaching heights of up to 18 m in Ikka Fjord, SW Greenland. In the fjord, column formation occurs only at the base of a nepheline syenite-carbonatite complex that flanks the fjord and an association has therefore been proposed. The spring water that seeps up at the bottom of the fjord is oversaturated in Na+ and HCO3-. In this study, we show that these ions were acquired by alteration reactions in the syenite-carbonatite complex: Na+ is released during replacement of nepheline by illite and analcime in nepheline-syenite rocks and HCO3- is released by oxidation of siderite to goethite in carbonatite rocks. The chemically charged groundwater mixes with seawater and gives rise to the formation of the tufa columns. We performed a mass balance to show that the mass of the carbonatite in the complex is more than sufficient to provide the CO2 needed to produce the observed mass of tufa columns. We estimated a time frame of similar to 600 years to produce the necessary CO2 to form the 700 ikaite columns in the fjord.

  • 7.
    Tollefsen, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dupraz, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sturkell, Erik
    Chemical controls on ikaite formation2018In: Mineralogical magazine, ISSN 0026-461X, E-ISSN 1471-8022, Vol. 82, no 5, p. 1119-1129Article in journal (Refereed)
    Abstract [en]

    The hydrated carbonate mineral ikaite (CaCO3 center dot 6H(2)O) is thermodynamically unstable at all known conditions on Earth. Regardless, ikaite has been found in marine sediments, as tufa columns and in sea ice. The reason for these occurrences remains unknown. However, cold temperatures (<6 degrees C), high pH and the presence of Mg2+ and SO42 in these settings have been suggested as factors that promote ikaite formation. Here we show that Mg concentration and pH are primary controls of ikaite precipitation at 5 degrees C. In our experiments a sodium carbonate solution was mixed with seawater at a temperature of 5 degrees C and at a constant rate. To test the effect of Mg2+ and SO42 we used synthetic seawater which allowed us to remove these elements from the seawater. The pH was controlled by different ratios of Na2CO3 and NaHCO3 in the carbonate solution. We found that ikaite precipitated when both seawater and synthetic seawater from which SO4 had been removed were used in the experiments. However, ikaite did not precipitate in experiments conducted with synthetic seawater from which Mg had been removed. In these experiments, calcite precipitated instead of ikaite. By varying the Mg concentration of the synthetic seawater and the pH of the sodium carbonate solution, we constructed a kinetic stability diagram for ikaite and calcite as a function of Mg concentration and pH. One possible explanation of our finding is that Mg2+ inhibits calcite nucleation and thereby allows metastable ikaite to form instead.

  • 8.
    Wästeby, Niklas
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tollefsen, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andrén, Margareta
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stockmann, Gabrielle
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Liljedahl, Lillemor Claesson
    Sturkell, Erik
    Mörth, Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hydrochemical monitoring, petrological observation, and geochemical modeling of fault healing after an earthquake2014In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 119, no 7, p. 5727-5740Article in journal (Refereed)
    Abstract [en]

    Based on hydrochemical monitoring, petrological observations, and geochemical modeling, we identify a mechanism and estimate a time scale for fault healing after an earthquake. Hydrochemical monitoring of groundwater samples from an aquifer, which is at an approximate depth of 1200 m, was conducted over a period of 10 years. Groundwater samples have been taken from a borehole (HU-01) that crosses the Husavik-Flatey Fault (HFF) near Husavik town, northern Iceland. After 10 weeks of sampling, on 16 September 2002, an M 5.8 earthquake occurred on the Grimsey Lineament, which is approximately parallel to the HFF. This earthquake caused rupturing of a hydrological barrier resulting in an influx of groundwater from a second aquifer, which was recorded by 15-20% concentration increases for some cations and anions. This was followed by hydrochemical recovery. Based on petrological observations of tectonically exhumed fault rocks, we conclude that hydrochemical recovery recorded fault healing by precipitation of secondary minerals along fractures. Because hydrochemical recovery accelerated with time, we conclude that the growth rate of these minerals was controlled by reaction rates at mineral-water interfaces. Geochemical modeling confirmed that the secondary minerals which formed along fractures were saturated in the sampled groundwater. Fault healing and therefore hydrochemical recovery was periodically interrupted by refracturing events. Supported by field and petrographic evidence, we conclude that these events were caused by changes of fluid pressure probably coupled with earthquakes. These events became successively smaller as groundwater flux decreased with time. Despite refracturing, hydrochemical recovery reached completion 8-10 years after the earthquake.

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