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  • 11
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    Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forets granite from the EPS-1 borehole, Upper Rhine Graben (2007)
    In:  Geological Society Special Publication 238: 509-526.
    Details
    Publication Date: 2007-10-08
    Description: The Variscan, magnetite-bearing Soultz-sous-Forets granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility ({kappa}) of the granite is between 10 and 80 x 10-3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced {kappa} (< 1 x 10-3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.
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  • 12
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    Untersuchungen zur elektrischen Leitfähigkeit in der Kontinentalen Tiefbohrung und ihrem Umfeld - was bringen sie uns Neues? (1994)
    In:  DGG Mittlg., Washington D.C., Bundesanstalt für Geowissenschaften und Rohstoffe, vol. 108, no. 4, pp. 2-40, pp. 2122
    Details
    Publication Date: 1994
    Keywords: Geoelectrics ; magneto-tellurics ; KTB ; Review article ; Project report/description ; Kueck ; Kuck ; Joedicke ; Jodicke ; tensor
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    BIBLIOGRAPHY SEISMOLOGY
  • 13
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    Petrophysical properties of the 9 km deep crustal section at KTB (1997)
    In:  J. Geophys. Res., San Francisco, Pergamon, vol. 102, no. B8, pp. 18,337-18,361, pp. 1246
    Details
    Publication Date: 1997
    Keywords: Mineralogy ; Physical properties of rocks ; scientific drilling ; Rock mechanics ; JGR
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    BIBLIOGRAPHY SEISMOLOGY
  • 14
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    GADD45 induction of a G2/M cell cycle checkpoint (1999)
    National Academy of Sciences
    Details
    Publication Date: 1999-03-30
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 15
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    Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India (2015)
    Geological Society of America (GSA)
    Details
    Publication Date: 2015-12-30
    Description: The study of shock pressure indicators can provide important clues for understanding the cratering process, though the estimation of shock pressures in weakly shocked rocks is commonly difficult. In this study, we selected a very young and well-preserved impact structure, the Lonar crater in India. The crater, devoid of any tectonic overprint, can be assumed as pristine. We used a combination of rock magnetic and microfracture studies to estimate shock pressure in the crater rim. On the basis of present results, the magnetic fabrics are interpreted to be of magmatic origin related to the Deccan basalt emplacement. The high-coercivity component of the natural remnant magnetization in the crater rim basalt is similar to that in the unshocked basalt. The lack of any shock-related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater. In contrast, radial and concentric microfractures observed in basalts at the crater rim and farther away show symmetric distribution with respect to the crater. The concentric microfractures consistently overprint the radial microfractures. We infer that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional shock wave components, respectively. The results of our rock magnetic and microfracture studies, when interpreted in light of published experimental and numerical simulation studies on the Lonar basalt, reveal that the shock pressure in the Lonar crater rim was less than 0.5 GPa but greater than 0.2 GPa. This shock pressure was high enough to produce fractures but too low to affect the magnetic fabrics. These results give new information on the relationship between shock pressure and resulting microfractures.
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
    Published by Geological Society of America (GSA)
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  • 16
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    Shock-induced deformation phenomena in magnetite and their consequences on magnetic properties (2016)
    Wiley
    Details
    Publication Date: 2016-05-29
    Description: This study investigates the effects of shock waves on magnetic and microstructural behavior of multidomain magnetite from a magnetite-bearing ore, experimentally shocked to pressures of 5, 10, 20 and 30 GPa. Changes in apparent crystallite size and lattice parameter were determined by X-ray diffraction, and grain fragmentation and defect accumulation were studied by scanning and transmission electron microscopy. Magnetic properties were characterized by low-temperature saturation isothermal remanent magnetization (SIRM), susceptibility measurements around the Verwey transition as well as by hysteresis parameters at room temperature. It is established, that the shock-induced refinement of magnetic domains from MD to SD-PSD range is a result of cooperative processes including brittle fragmentation of magnetite grains, plastic deformation with shear bands and twins as well as structural disordering in form of molten grains and amorphous nano-clusters. Up to 10 GPa, a decrease of coherent crystallite size, lattice parameter, saturation magnetization and magnetic susceptibility, and an increase in coercivity, SIRM, width of Verwey transition is mostly associated with brittle grain fragmentation. Starting from 20 GPa, a slight recovery is documented in all magnetic and non-magnetic parameters. In particular, the recovery in SIRM is correlated with an increase of the lattice constant. The recovery effect is associated with the increasing influence of shock heating/annealing at high shock pressures. The strong decrease of magnetic susceptibility at 30 GPa is interpreted as a result of strong lattice damage and distortion. Our results unravel the microstructural mechanisms behind the loss of magnetization and the modification of magnetic properties of magnetite and contribute to our understanding of shock-induced magnetic phenomena in impacted rocks on earth and in meteorites. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 17
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    Modification of the magnetic mineralogy in basalts due to fluid–rock interactions in a high-temperature geothermal system (Krafla, Iceland) (2011)
    Oxford University Press
    Details
    Publication Date: 2011-05-06
    Description: SUMMARY Active 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.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 18
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    Rapid surficial oxidation of synthetic Fe-Ti oxides at high temperature: Observations and consequences for magnetic measurements (2012)
    Wiley
    Details
    Publication Date: 2012-08-03
    Description: Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmtss; ilmenite-hematitess, Ilmss; pseudobrookitess, Psbss) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300°C of a Tmtss-Ilmss sample produces surficial oxidation down to a depth of some 100 μm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis “oxyexsolution” textures, i.e., fine lamellae of Ilmss within the Tmtss crystals and lamellae of Psbss within the Ilmss crystals. In this oxidation zone the newly grown Ilmss lamellae and the surrounding Tmtss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmtss and Ilmss, show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmtss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (MS-T) curves, however, the contribution of more Fe-rich Tmtss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmtss does occur during MS-T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 19
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    Shock experiments up to 30 GPa and their consequences on microstructures and magnetic properties in pyrrhotite (2013)
    Wiley
    Details
    Publication Date: 2013-02-01
    Description: [1]  Shock experiments with pressures ranging from 3 to 30 GPa have been conducted on a mixed assemblage of hexagonal and monoclinic pyrrhotite. All samples were studied with respect to their particular shock-induced microstructures and magnetic properties at high and low temperatures. Up to 8 GPa, microstructures in shocked pyrrhotite are characterized by mechanical deformation producing a damage of the crystal structure. At pressures of 20 GPa and upward, amorphization and mechanical twinning are the dominant structural features induced by shock. Within the lower-pressure range coercivity, saturation isothermal remanent magnetization and coercivity of remanence increase with shock pressures, in agreement with more single-domain (SD)-like behavior. Simultaneously, the λ-peak of hexagonal pyrrhotite decreases and the 34 K transition of monoclinic pyrrhotite broadens and is depressed. Magnetic hardening is triggered by grain-size reduction, but also by the formation of SD within discrete multidomain grains. Planar deformation features subdivide such multidomain grains into lath-shaped domains with average sizes lying in the SD range. The planar deformation features disappear at 20 GPa and irregular, nanometer-sized “amorphous domains” occur instead. Pressure release from 30 GPa finally triggers partial melting of pyrrhotite. The sharp interfaces between molten and crystalline pyrrhotite document a rapid change of thermal conditions. Within molten pyrrhotite, quenched iron crystals occur. The presence of native iron strongly influences the magnetic properties, depending on the particular amount in the studied sample and likely affects the magnetic properties of impact lithologies on Earth and extraterrestrial material.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 20
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    Variation in magnetic fabrics at low shock pressure due to experimental impact cratering (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Magnetic fabrics provide important clues for understanding impact cratering processes. However, only a few magnetic fabric studies for experimentally shocked material have been reported so far. In the framework of MEMIN (Multidisciplinary Experimental and Modeling Impact Research Network), we conducted two impact experiments on blocks of Maggia Gneiss with the foliation oriented perpendicular (A37) and parallel (A38) to the target surface. Maggia gneiss has plenty of biotite bands forming a strong rock foliation. The bulk magnetic susceptibility varies from 0.376 x 10‐3 to 1.298 x 10‐3 SI in unshocked, and from 0.443 x 10‐3 to 3.940 x 10‐3 SI in shocked gneiss. The thermomagnetic curves reveal a Verwey transition at ‐147 °C and a Curie temperature between 576° ‐ 579° C in unshocked and shocked samples, indicating nearly pure magnetite, which carries the magnetic fabrics. In A37 and A38 kinking is prominent from the point source down to a depth of 2 and 4.2 dp (projectile diameter) or 1 and 2.1 cm, respectively. Kinking, folding and fracturing changed the position of magnetite grains with respect to each other to re‐orient the magnetic fabrics. Re‐orientation of magnetic fabrics is conspicuous down to 20 dp (10 cm) in A38, where no other impact related deformation is visible. The re‐orientation of magnetic fabrics may, therefore, aid in identifying impact processes at very low pressures, starting at 0.1 GPa, when other common indicators are absent.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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