| 1. |
- Boschetti, Tiziano, et al.
(författare)
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Geothermometry and water-rock interaction modelling at Hafralækur : Possible implications of temperature and CO2 on hydrogeochemical changes previously linked to earthquakes in northern Iceland
- 2022
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Ingår i: Geothermics. - : Elsevier BV. - 0375-6505 .- 1879-3576. ; 105
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Tidskriftsartikel (refereegranskat)abstract
- The low enthalpy (T < 150 °C) groundwater in the HA01 borehole at Hafralækur has a long time series (2008–2018) of chemical and isotopic data. In the previous studies, the variations in chemical and isotope parameters were statistically related to seismic activity. However, the possible effect of temperature has not yet been evaluated. To fill this gap, the results obtained from the classical geothermometric equations (silica solid phases, Na/K, Na-K-Ca) were compared. However, considering that the use of classical geothermometry using the Na/K ratio or silica solid phases solubility is limited by the presence of clay minerals and alkaline conditions (i.e., the presence of pH-dependant silicate anions), new equilibria reactions between labradorite, zeolites (analcime, stilbite) and the activity of the dissolved species in the fluid are presented to overcome this problem. In addition, kinetic reaction path models are presented to trace the possible role of both temperature and CO2 during the most evident chemical variations during earthquakes.
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| 2. |
- Fru, Ernest Chi, et al.
(författare)
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Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals
- 2023
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Ingår i: Nature Communications. - 2041-1723. ; 14:1
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Tidskriftsartikel (refereegranskat)abstract
- Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, similar to 720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval.
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| 3. |
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| 4. |
- 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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| 5. |
- Skelton, Alasdair, et al.
(författare)
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Seismic-hydrogeochemical coupling in north-eastern India
- 2008
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Ingår i: 33rd International Geological Congress Oslo August 6 -14th 2008 General Proceedings. - : EurekaMag.com. ; , s. CD-ROM
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We report the result of a hydrogeochemical monitoring program, which has been operational north of the Shillong Plateau, Assam, India from December 2003. The aim of this ongoing study is to test for coupling between the groundwater chemistry collected from a granite-hosted aquifer, located at a depth of 110m, and seismic activity. Based on molar Na+/Ca2+ and molar HCO3-/SiO2 ratios after Garrels (1967), we interpret that groundwater chemistry is normally buffered by the alteration of feldspar (plagioclase) to kaolinite.During the study, we monitored transient chemical changes which coincided temporally with a period of increased seismic activity. This included (1) MW = 5.3 and MW = 5.0 earthquakes which occurred on December 9, 2004 and February 15, 2005, south of the Shillong Plateau and 206 and 213 km from the sampling station, respectively, and (2) the Great Sumatra – Andaman Islands Earthquake of December 26, 2004. These are the only three MW > 5 earthquakes which have occurred during our study and for which our monitoring site is within their respective strain radii as given by Dobrovolsky et al. (1979).The most dramatic chemical change was a coincident and approximately 2-fold increase of the ratios [Na+K]/Si, Na/K and [Na+K]/Ca. This was accompanied by significant increases of conductivity, alkalinity and chloride concentration. The onset of this chemical shift occurred 3-5 weeks before the first (MW = 5.3) earthquake. We interpret a transient switchover between source aquifers, which induced an influx of groundwater from a second and probably deeper aquifer, where groundwater chemistry was dominantly buffered by the alteration of feldspar to smectite. This could have occurred in response to fracturing of a hydrological barrier. We also recorded a rapid drop in the ratio Ba/Sr, which occurred 3-6 days before the final (MW = 5.0) earthquake. We interpret a transient switchover to anorthite dissolution caused by exposure of fresh plagioclase to groundwater interaction. This could have been induced by microfracturing, locally within the main aquifer. Both of these changes were transient and “recovery” occurred over periods of 2-4 weeks. By comparison with experimental studies of feldspar dissolution, we suggest that hydrogeochemical recovery was facilitated by groundwater interaction and clay mineralization, which could have been coupled with fracture sealing.The main argument in support of seismic-hydrogeochemical coupling is the coincidence in timing of two hydrogeochemical events with two MW 5 earthquakes. Reasons for ambiguity include the lack of similar hydrogeochemical anomalies temporally coupled with smaller seismic events which occurred much closer to the monitoring station, the >200 km length scale of inferred seismic-hydrogeochemical coupling, and the potential for far-field effects related to the Great Sumatra – Andaman Islands Earthquake of December 26, 2004. The hydrogeochemical anomalies reported in this study meet some of the validation criteria of the IASPEI (International Association of Seismology and Physics of the Earth’s Interior) sub-commission on earthquake prediction (Wyss, 1991; 1997) in that a relation to pre-seismic stress and that some dependence on distance from the earthquake foci is inferred. However, hydrogeochemical data was collected from only one site, and even although the hydrogeochemical anomalies are recorded using several instrumental methods the reported anomalies are not truly independent of one another.
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| 6. |
- Villa, Igor M., et al.
(författare)
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Petrochronology of polygenetic white micas (Naxos, Greece)
- 2023
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Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 41:3, s. 401-423
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Tidskriftsartikel (refereegranskat)abstract
- Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with 39Ar-40Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by 39Ar-40Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at <20 Ma). A lower-grade sample from southern Naxos was less pervasively recrystallized, provides older ages and preserves at least three WM generations, including a relict WM with a pre-Palaeocene K-Ar age, consistent with the high Ar retentivity of WM in the absence of complete recrystallization. The age of the Cl-poor end-member WM approximates the age of the HP event, 38 Ma. Ar inheritance in Cretaceous mica relicts is heterogeneous at the single-grain scale. Comparing the degassing rates of the WM fractions rules out ‘multidomain’ diffusion. As no sample is monomineralic, the degassing rate of each polygenetic mica is instead controlled by the mass balanced sum of the unrelated rate constants of its constituent minerals. Given the commonness of zoned and composite micas, the approach detailed here is potentially useful for reconstructing polyphase metamorphic histories worldwide.
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