Shock-diminished paleomagnetic remanence at the Charlevoix impact structure, Quebec

1979 ◽  
Vol 16 (9) ◽  
pp. 1842-1856 ◽  
Author(s):  
P. Blyth Robertson ◽  
J. L. Roy
Keyword(s):  
Remanent Magnetization ◽  
Marginal Zone ◽  
Natural Remanent Magnetization ◽  
Significant Fraction ◽  
Magnetic Characteristics ◽  
Thermal Treatments ◽  
Magnetic Carrier ◽  
Impact Structure ◽  
Late Precambrian ◽  
Dual Polarity

The magnetic characteristics of shocked and unshocked anorthosites of the Charlevoix impact structure have been reexamined to assess the effects of high thermal and alternating field (af) cleaning treatments, and to evaluate any shock-induced features of the remanence in terms of mineralogy of the shocked rocks.From 16 sites, 197 specimens were subjected to stepwise thermal treatments to 710 °C, and a further 46 to incremental af cleaning to 290 mT. The 10 sites from the unshocked St-Urbain anorthosite carry a magnetization with unblocking temperatures (TUB) generally between 600–625 °C, with a significant fraction of remanence with TUB > 670 °C. Similarly, most of the remanence has a resistive coercive force (rcf) of 100–175 mT, with a significant fraction exceeding 290 mT. The remanence is carried by two exsolution phases of titano-hematite, with the later and more hematite-rich lamellae having the higher TUB and rcf. Although only one direction exists, at two sites where intensities are somewhat lower and some natural remanent magnetization (NRM) directions scattered, treatment reveals a dual polarity remanence. The pole (154°E, 02°S) obtained from these unshocked St-Urbain anorthosite sites falls on a well established segment of the late Precambrian apparent polar path of suitable age (≈950 Ma). Significant results could not be obtained from three additional sites in the marginal zone due to their ready acquisition of a viscous magnetization following thermal treatments.Data from two sites which display no shock effects and lie near the margin of the central uplift, and from one highly shocked sample from the crater centre show some possible effects of the shock event. All three have intensities substantially lower than in the St-Urbain sites, which is interpreted in the case of at least one of these sites as a shock-diminished remanence. A puzzling feature of the highly shocked samples is that the phase with a texture reminiscent of the ilmenite–hematite exsolution in the unshocked anorthosites, contains no iron. The remaining remanence, whose direction is parallel with that of the unshocked sites, is interpreted as pre-crater rather than shock-produced. Although many instances have been reported of a new shock-induced remanence imparted to rocks of equivalent and even lesser shock grade than those examined at Charlevoix, in all cases the magnetic carrier seems to have been magnetite. The lack of a shock-induced remanence at Charlevoix is attributed to the high TUB and rcf of hematite in these rocks.

Magnetic characteristics of Luna 16 and 20 samples

1977 ◽  
Vol 284 (1319) ◽  
pp. 151-156 ◽  
Keyword(s):  
Grain Size ◽  
Iron Content ◽  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Magnetic Behaviour ◽  
Heavy Fraction ◽  
Magnetic Characteristics ◽  
Iron Particles ◽  
Isothermal Remanent Magnetization ◽  
Rock Fragments

The natural remanent magnetization of rock fragments L2015,3,1 and L2015,3,11 was found to be < 3.5 x 10 -7 and < 40 x 10 -6 G cm 3 g -1 respectively. The former sample, from isothermal remanent magnetization (i.r.m.) measurements, contained very little iron, while the latter sample had a much higher iron content and exhibited i.r.m. characteristics similar to breccia samples from Apollo missions. Susceptibility and i.r.m. measurements have shown that Luna 16 fines contain about four times as much iron as Luna 20 samples and that the light fractions from the density separations contain about twice as much iron as the heavy fraction. Like the Apollo fines, the magnetic behaviour of Luna 16 and 20 fines is dominated by small iron particles, most of which are superparamagnetic and of grain size less than about 13 nm.

Review of effects of shock (<60 kbar; <6x109 Pa) on magnetism of lunar samples

1977 ◽  
Vol 285 (1327) ◽  
pp. 409-416 ◽  
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Shock Level ◽  
Magnetic Characteristics ◽  
Thermoremanent Magnetization ◽  
Experimental Shock ◽  
Lunar Samples

Experimental shock studies of highland and mare soils in the range of a few to 50 kbar (5 x 109 Pa) have given the following results: (1) Shock, if less than 20 kbar, does not change the magnetic characteristics of the soil substantially and only weak and unstable shock remanence is generated in a field of 0.5 Oe. (2) Shock of between 20 and 50 kbar lithifies the soil and gives rise to stable shock remanence. Acquisition is approximately linear in field for a given shock level. At 30 kbar the acquisition parameter for the highland soil was 10-5 G cm3 g-1 Oe-1. In this range of 20-50 kbar the products of shock are petrologically and magnetically similar to certain regolith breccias. (3) Shock demagnetization preferentially demagnetizes the softer part of thermoremanent magnetization (t.r.m.) and hence makes it relatively harder. The significance of these results is that shock remanence is likely to be the cause of the natural remanent magnetization (n.r.m.) of certain regolith breccias and shock may modify the primary remanence of other samples.

scholarly journals A Feasibility Study of Microbialites as Paleomagnetic Recorders

2021 ◽  
Vol 9 ◽  
Author(s):  
Ji Jung ◽  
Julie A. Bowles
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Inner Core ◽  
Salt Lake ◽  
Natural Remanent Magnetization ◽  
High Coercivity ◽  
Magnetic Carrier ◽  
Deep Time ◽  
Geologic Record ◽  
Magnetic Field Variations

Microbialites–layered, organosedimentary deposits–exist in the geologic record and extend back in deep time, including all estimated times of inner core nucleation. Microbialites may preserve magnetic field variations at high-resolution based on their estimated growth rates. Previous studies have shown that microbialites can have a stable magnetization. However, the timing and origin of microbialite magnetization were not well determined, and no study has attempted to evaluate whether actively growing microbialites record the geomagnetic field. Here, we present centimeter-scale magnetization and magnetic property variations within the structure of modern microbialites from Great Salt Lake (GSL), United States, and Laguna Bacalar, Mexico, Pleistocene microbialites from GSL, and a Cambrian microbialite from Mongolia. All samples record field directions close to the expected value. The dominant magnetic carrier has a coercivity of 35–50 mT and unblocking temperatures are consistent with magnetite. A small proportion of additional high coercivity minerals such as hematite are also present, but do not appear to appreciably contribute to the natural remanent magnetization (NRM). Magnetization is broadly consistent along microbialite layers, and directional variations correlate with the internal slope of the layers. These observations suggest that the documented NRM may be primarily detrital in origin and that the timing of magnetization acquisition can be close to that of sediment deposition.

scholarly journals Natural and Induced Magnetization in the Bobolink, Dolichonyx Oryzivorus (Aves: Icteridae)

1986 ◽  
Vol 125 (1) ◽  
pp. 49-56 ◽  
Author(s):  
Robert C. Beason ◽  
William J. Brennan
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Single Domain ◽  
7 Tesla ◽  
Magnetic Characteristics ◽  
Passerculus Sandwichensis ◽  
Dolichonyx Oryzivorus ◽  
Savannah Sparrow ◽  
Histochemical Tests ◽  
The Relationship

The magnetic characteristics of the heads from 28 bobolinks [Icteridae: Dolichonyx oryzivorus (L.)] were analysed using remanence magnetometers. The natural remanent magnetization of 12 freshly preserved heads averaged 3.20xl0−7 electromagnetic units (e.m.u.) (l e.m.u. = 103Am−1) and was oriented horizontally from side to side. An electromagnet was used to induce magnetization with external field strengths of up to O.7 Tesla (T). The average saturation isothermal remanent magnetization (sIRM) for 23 bobolink heads was 2.49×10−5e.m.u. Because the IRM was attained partially below 0.1 T and totally below 0.3 T, we believe that the magnetic material in the bobolink is at least partially single domain magnetite. The relationship between the IRM acquisition and alternating field (a.f.) demagnetization curves indicates that the single or pseudo-single domain grains of magnetite interact. Similar results were obtained on one savannah sparrow (Emberizidae: Passerculus sandwichensis) head and one indigo bunting (Emberizidae: Passerine cyanea) head. When some bobolink heads were subdivided, most or all of the magnetization was concentrated in the area from the nasal cavity to the orbit. Light microscopy and histochemical tests confirm the localization of inorganic iron in this region.

scholarly journals Large field impressed anisotropy of magnetic susceptibility (AMS) in metamorphic volcanoclastic rocks from the western Central Pamir with ilmeno-hematite as the main magnetic carrier.

2020 ◽  
Author(s):  
Pierrick Roperch ◽  
Jovid Aminov ◽  
Guillaume Dupont-Nivet ◽  
Stéphane Guillot ◽  
France Lagroix
Keyword(s):  
Magnetic Susceptibility ◽  
Remanent Magnetization ◽  
Strong Field ◽  
Large Field ◽  
Magnetic Characteristics ◽  
Magnetic Carrier ◽  
Isothermal Remanent Magnetization ◽  
Clastic Rocks ◽  
Magnetic Fabrics ◽  
Degree Of Anisotropy

&lt;p&gt;Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM).&amp;#160;Forty four paleomagnetic sites have been sampled in a &gt;2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan).&amp;#160;These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite.&amp;#160;The magnetic properties are very homogeneous across the sequence. Bulk magnetic susceptibilities vary between 150-250 &amp;#956; SI. The AMS magnetic fabrics correspond to triaxial tensors with a well defined foliation plane and a steeply dipping magnetic lineation. The degree of anisotropy varies between 1.03 and 1.2. This fabric was likely acquired during the deformation associated with the emplacement of Middle Miocene gneiss domes.&amp;#160;SEM/EDS data indicate that the main iron oxide mineral is hematite with up to 15% of ilmenite in solid solutions. This is in agreement with unblocking temperatures of SIRM around 630 &amp;#176;C, lower than the one of pure hematite.&amp;#160;One of the most surprising magnetic characteristics of these rocks is the effect of strong-field remanent magnetizations upon the AMS. During the acquisition of an Isothermal Remanent Magnetization (IRM), the initial AMS is progressively obliterated by a new AMS fabric. The field-impressed AMS is characterized by a decrease of the magnetic susceptibility along the direction of the IRM and an increase in magnetic susceptibility in the orthogonal plane. The field-impressed AMS is thus mainly oblate with a degree of anisotropy usually between 1.2 and 1.4. As far as we know, such a strong effect has never been reported. In sandstone with detrital hematite as the main carrier, the degree of the induced AMS fabric is less than 1.02 suggesting that the ilmenite content in the metamorphic hematite is the main cause of the large observed field induced fabric in these rocks.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Physical methods of research and monitoring

2019 ◽  
Vol 85 (1(I)) ◽  
pp. 35-44
Author(s):  
S. G. Sandomirski
Keyword(s):  
Heat Treatment ◽  
Coercive Force ◽  
Remanent Magnetization ◽  
Structural Changes ◽  
Mechanical Load ◽  
Magnetic Hysteresis ◽  
Steel Structures ◽  
Magnetic Characteristics ◽  
Magnetic Parameters ◽  
Saturation Intensity

The main magnetic parameters sensitive to the structure of steels are the parameters of their saturation loop of magnetic hysteresis: the coercive force Hcs and remanent magnetization Mrs. The saturation magnetization or saturation intensity Mr is most sensitive to the phase composition of steels. The variety of steel grades and modes of technological treatment (e.g., heat treatment, mechanical load) determined the use of magnetic structurescopy and magnetic characteristics — the coercive force Hc, remanent magnetization Mr , and specific hysteresis losses Wh on the subloops of the magnetic hysteresis of steels — as control parameters in diagnostics of the stressed and structural states of steel structures and pipelines. It has been shown that changes in Hc, Mr , and Wh are more sensitive to structural stresses and structures of steels than the parameters of the saturation hysteresis loop of magnetic hysteresis (Hcs, Mrs, and Mrs). The formulas for calculating Hc, Mr and Wh are presented to be used for estimation of changes in the parameters upon heat treatment of steels. Features of the structural sensitivity of the subloop characteristics and expediency of their use for magnetic structural and phase analyzes are determined. Thus, the range of changes in Ìr attributed to the structural changes in steels upon gradual Hm decrease is many times wider compared to the range of possible changes in Mrs under the same conditions. Conditions (relations between the magnetic parameters) and recommendations regarding the choice of the field strength Hm are given which provide the justified use of Hc, Mr and Wh parameters in magnetic structurescopy

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