79 research outputs found

    Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

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    ©2017. American Geophysical Union.The geological sources of large-scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure-dependent increase in the Curie temperature (initially in the 50-70°C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation

    Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

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    The geological sources of large-scale lithospheric magnetic field anomalies are poorly constrained. Understanding the magnetic behavior of rocks and minerals under the pressures and temperatures encountered at large crustal depths is particularly important in that task. The impact of lithospheric pressure is not well known and most of the time neglected in numerical models of the geological sources of magnetic anomalies. We present thermal remanent magnetization (TRM) acquisition and stepwise thermal demagnetization on synthetic titanomagnetite dispersed powder, within an amagnetic cell under hydrostatic pressure up to 1 GPa. TRM is measured after thermal cycling within a cryogenic magnetometer. Pressure-dependent increase in the Curie temperature (initially in the 50–70°C range) is observed, mostly between 0.3 and 0.6 GPa, on the order of 20 K/GPa. TRM intensity also increases with pressure up to 200% at 675 MPa, although the pressure variation with temperature inside the cell complicates the interpretation. ©2017. American Geophysical Union. All Rights Reserved

    The effect of irradiation on the magnetic properties of rock and synthetic samples: Implications to irradiation of extraterrestrial materials in space

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    © 2015, Pleiades Publishing, Ltd. We report here the results of laboratory analog experiments to consider the potential effects of solar energetic particles (SEP or solar-flare-associated particles) and galactic cosmic rays (GCR) on the magnetic properties of extraterrestrial materials. We carried out proton bombardment experiments (with irradiation energies E1=400, E2 =850 keV and three irradiation fluences in 1014–1016p/cm2 range) and lead-ion bombardment experiments (E =1 GeV) on (previously demagnetized by 120 mT alternating magnetic field) rock and synthetic samples with the following magnetic carriers: metallic iron and nickel iron, Ti-rich and Ti-free magnetite, pyrrhotite. Irradiation experiments resulted in either further demagnetization or magnetization of irradiated samples depending on the type of magnetic mineralogy and type of ionizing radiation involved. Apart for the formation of radiation-induced remanent magnetization (RIRM), we observed major changes in bulk magnetic properties, i.e., a moderate to dramatic decrease (up to 93%) in the coercivity of remanence Bcr for all iron-bearing phases (iron-in-epoxy and Bensour meteorite samples). Contrary to iron-bearing samples, several magnetite-bearing samples experienced a radiation-induced magnetic hardening (increase in Bcr). Magnetic hardening was also observed for Ar2+ ion-irradiated nickel iron-bearing HED meteorites, measured for comparison with the previously stated results. Therefore, the combined effect of SEP with GCR may magnetically soften iron-bearing materials and harden magnetite-bearing materials. In order to answer the question wether RIRM may account for natural remanent magnetization of meteorites and lunar samples, physical mechanism of RIRM formation and potential dependence of RIRM intensity on the background magnetic field present during irradiation event should be investigated

    The effect of hydrostatic pressure up to 1.61 GPa on the Morin transition of hematite-bearing rocks: Implications for planetary crustal magnetization

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    © 2015. American Geophysical Union. All Rights Reserved. We present new experimental data on the dependence of the Morin transition temperature (TM) on hydrostatic pressure up to 1.61 GPa, obtained on a well-characterized multidomain hematite-bearing sample from a banded iron formation. We used a nonmagnetic high-pressure cell for pressure application and a Superconducting Quantum Interference Device magnetometer to measure the isothermal remanent magnetization (IRM) under pressure on warming from 243 K to room temperature (T0). IRM imparted at T0 under pressure in 270 mT magnetic field (IRM270mT) is not recovered after a cooling-warming cycle. Memory effect under pressure was quantified as IRM recovery decrease of 10%/GPa. TM, determined on warming, reaches T0 under hydrostatic pressure 1.38-1.61 GPa. The pressure dependence of TM up to 1.61 GPa is positive and essentially linear with a slope dTM/dP = (25 ± 2) K/GPa. This estimate is more precise than previous ones and allows quantifying the effect of a pressure wave on the upper crust magnetization, with special emphasis on Mars

    Magnetic properties of tektites and other related impact glasses

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    © 2015 Elsevier B.V. We present a comprehensive overview of the magnetic properties of the four known tektite fields and related fully melted impact glasses (Aouelloul, Belize, Darwin, Libyan desert and Wabar glasses, irghizites, and atacamaites), namely magnetic susceptibility and hysteresis properties as well as properties dependent on magnetic grain-size. Tektites appear to be characterized by pure Fe2+ paramagnetism, with ferromagnetic traces below 1 ppm. The different tektite fields yield mostly non-overlapping narrow susceptibility ranges. Belize and Darwin glasses share similar characteristics. On the other hand the other studied glasses have wider susceptibility ranges, with median close to paramagnetism (Fe2+ and Fe3+) but with a high-susceptibility population bearing variable amounts of magnetite. This signs a fundamental difference between tektites (plus Belize and Darwin glasses) and other studied glasses in terms of oxygen fugacity and heterogeneity during formation, thus bringing new light to the formation processes of these materials. It also appears that selecting the most magnetic glass samples allows to find impactor-rich material, opening new perspectives to identify the type of impactor responsible for the glass generation

    Magnetic characterization of non-ideal single-domain monoclinic pyrrhotite and its demagnetization under hydrostatic pressure up to 2 GPa with implications for impact demagnetization

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    © 2016 Elsevier B.V..Here we present a comprehensive magnetic characterization of synthesized non-ideal single-domain (SD) monoclinic pyrrhotite (Fe7S8). The samples were in the form of a powder and a powder dispersed in epoxy. "Non-ideal" refers to a powder fraction of predominantly SD size with a minor contribution of small pseudo-single-domain grains; such non-ideal SD pyrrhotite was found to be a remanence carrier in several types of meteorites (carbonaceous chondrites, SNC. . .), which justifies the usage of synthetic compositions as analogous to natural samples. Data were collected from 5 to 633 K and include low-field magnetic susceptibility (χ0), thermomagnetic curves, major hysteresis loops, back-field remanence demagnetization curves, first-order reversal curves (FORCs), alternating field and pressure demagnetization of saturation isothermal remanent magnetization (SIRM), low temperature data (such as zero-field-cooled and field-cooled remanence datasets together with room temperature SIRM cooling-warming cycles) as well as XRD and Mössbauer spectra. The characteristic Besnus transition is observed at ∼33 K. FORC diagrams indicate interacting SD grains. The application of hydrostatic pressure up to 2 GPa using nonmagnetic high-pressure cells resulted in the demagnetization of the sample by 32-38%. Repeated cycling from 1.8 GPa to atmospheric pressure and back resulted in a total remanence decrease of 44% (after 3 cycles). Pressure demagnetization experiments have important implications for meteorite paleomagnetism and suggest that some published paleointensities of meteorites with non-ideal SD monoclinic pyrrhotite as remanence carrier may be lower limits because shock demagnetization was not accounted for

    Summanen, a new meteorite impact structure in Central Finland

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    The Summanen structure (62 degrees 39.0'N, 25 degrees 22.5'E) is located within the Paleoproterozoic Central Finland Granite Belt, Fennoscandian Shield. The structure is hidden under Lake Summanen and not directly observable. It owes its discovery to low-altitude airborne geophysical data, which revealed a circular, similar to 2.6 km wide electromagnetic in-phase, and resistivity, anomalies. Two field campaigns were conducted in 2017 to search for impact signatures. The fieldwork concentrated on the southeastern side of the lake following the ice flow direction of the latest (Weichselian) glaciation. In addition, the islands and the SE peninsulas of the mainland were investigated for outcrops and glacial erratics. A few tens of erratic boulders with shatter cones and striated features, and a few brecciated rocks were discovered. Lamposaari Island in the eastern part of the lake revealed one fractured outcrop containing in situ porphyritic granite with converging striated features. Microscopic shock metamorphic features in two shatter-cone-bearing samples of porphyritic granite were found. These are planar deformation features (PDFs; up to two sets) in quartz and kink bands in biotite. Based on these geological, geophysical, and petrographic results, we suggest that Lake Summanen hides a relatively small, probably simple, meteorite impact structure, the twelfth confirmed one in Finland, of so far unknown age.Peer reviewe

    The effects of 10 to >160 GPa shock on the magnetic properties of basalt and diabase

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    © 2016. American Geophysical Union. All Rights Reserved.Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First-order reversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters

    Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds

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    Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50\% over that of initial conditions by 2 GPa, and then decreases until only 33\% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth’s surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used
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