Origin of magnetic minerals and natural remanent magnetization of basaltic rocks from Lower Silesia, Poland

AbstractA single component, natural remanent magnetization (NRM) is carried largely by pseudosingle domain magnetite in the Cretaceous Lower Chalk and Red Chalk of eastern England. The Red Chalk also records the same direction in haematite. Most of the ferro-magnetic minerals occur as primary clastic or early diagenetic grains. A stable remanence component is resistant to demagnetization, and is carried by both magnetite and haematite. Nevertheless, it has a steep inclination close to the present Earth's field and it is too steep for the previously reported palaeolatitude of these rocks at the time of sedimentation. A postglacial slump breccia scatters the ChRM but also provides some evidence of viscous, partial magnetic overprinting during slumping. Despite its resistance to thermal and alternating field demagnetization the characteristic remanent magnetization (ChRM) is probably a young Bruhnes epoch viscous remanent remagnetization (VRM).

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Magnetic Mineralogy of Speleothems From Tropical-Subtropical Sites of South America

Frontiers in Earth Science ◽

10.3389/feart.2021.634482 ◽

2021 ◽

Vol 9 ◽

Author(s):

Plinio Jaqueto ◽

Ricardo I. F. Trindade ◽

Joshua M. Feinberg ◽

Janine Carmo ◽

Valdir F. Novello ◽

...

Keyword(s):

Magnetic Properties ◽

South America ◽

Remanent Magnetization ◽

Drainage Basin ◽

Environmental Magnetism ◽

Rank Correlation ◽

Natural Remanent Magnetization ◽

Magnetic Minerals ◽

Magnetic Mineralogy ◽

Diverse Range

Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.

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Under pressure: How pressure affects magnetic remanence

10.5194/egusphere-egu2020-915 ◽

2020 ◽

Author(s):

Michael Volk ◽

Roger Fu ◽

Josh Feinberg

Keyword(s):

Magnetic Field ◽

Spatial Resolution ◽

Remanent Magnetization ◽

Imaging Techniques ◽

Magnetic Domain ◽

Natural Remanent Magnetization ◽

Magnetic Minerals ◽

Magnetic Imaging ◽

Magnetic Remanence ◽

Series Of Experiments

Rocks have complicated histories and form under various conditions. However, all rocks, terrestrial and extraterrestrial, have been subjected to some form of pressure during their genesis. The effect of pressure (strain) on the magnetic remanence is a largely unexplored problem, with most of the work being focused on the study of meteorites.&#160;In the absence of a magnetic field, subjecting a rock to pressure can demagnetize the natural remanent magnetization (NRM). This loss of magnetic remanence can lead to an underestimation of paleointensities. On the other hand, in the presence of a magnetic field, magnetic minerals can record a pressure remanent magnetization (PRM). The superposition of the remaining NRM and a newly acquired PRM can influence the remanence direction as well as the paleointensity. Since the reconstruction of the temporal changes of Earths&#8217; magnetic relies on robust estimations of direction and intensity, the effects of pressure on the remanence should be taken into account.Here we present a series of experiments that aim to explore the acquisition process of PRMs and their net contribution with respect to the rock&#8217;s original magnetization. Stoichiometric magnetites of four different grain sizes (65 nm, 440 nm, 16.9 &#181;m, and 18.3 &#181;m) and magnetic domain states were subjected to crustal pressures (226, 301, and 376 MPa) in the presence of a magnetic field. Surprisingly, the PRM intensity showed no detectable dependence on grain size. However, because the acquisition of a thermal remanence (TRM) is strongly dependent on particle size,&#160; populations of large multidomain particles can acquire a PRM, which may represent up to 30% of a TRM acquired in the same field.Finally, we will show how the influence of pressure on the magnetic remanence can be visualized by modern magnetic imaging techniques like the quantum diamond microscope (QDM). The QDM has a&#160; ~1 &#181;m maximum spatial resolution that is able to resolve the magnetic fields of individual mineral assemblages with ~10 &#181;m diameter. The high spatial resolution and sensitivity enables us to visualize the changes in magnetic remanence due to pressure cycling and can help to better understand the possible implications for paleomagnetism.

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Inclinaison magnétique de roches basaltiques de la dorsale est-pacifique à 21°N et validité des critères de polarité

Canadian Journal of Earth Sciences ◽

10.1139/e83-029 ◽

1983 ◽

Vol 20 (2) ◽

pp. 322-333 ◽

Cited By ~ 1

Author(s):

Brigitte Smith

Keyword(s):

Remanent Magnetization ◽

Natural Remanent Magnetization ◽

Mean Value ◽

Magnetization Direction ◽

Basaltic Rocks ◽

Stable Direction ◽

Normal Polarity ◽

The Stability

Among the basaltic rocks sampled by submersible or dredged at the East-Pacific Rise axis around 21°N latitude (French–American–Mexican legs CYAMEX and RISE), 22 samples have been selected according to macroscopic, oriented criteria (elongation plane of feeder tubes in sheeted lavas, bubbles and gas cavities, stalactites, septa) from which the polarity of a sample, acquired in situ during its cooling, could be established.The natural remanent magnetization intensity of these generally voluminous samples (up to about 350 cm3) has a mean value of 11.35 ± 2.31 A∙m−1 (10−3 emu∙cm−3); the scatter of the values can be explained by the varying proportion of nonmagnetic material (glass, Mn) in the samples.The magnetic viscosity is always negligible and the stability of the remanent magnetization direction after alternating field demagnetization indicates that the stable direction determined is the direction of the original thermoremanence.Despite the relatively low latitude, which makes the occurrence of negative magnetic inclinations within a normal polarity period (Brunhes, in this case) more probable, no reverse polarity has been found. The mean inclination of the 22 samples is I = +41.8 ± 16.7° and becomes I = +42.2 ± 19.4° if the samples are grouped in 15 pin-point sampling sites; these values are not significantly different from those of the local dipole field, I = ±37.4°, or from those of the actual local field, I ~ +47°, which may be the result of effusions from the youngest lava flows.The scatter of the magnetic inclination values is more tightly bound to the quality of the polarity criteria than to the secular variation of the Earth ' s magnetic field, the latter interfering in only a few cases. It is obvious that the validity of the reorientation depends on (1) the morphological quality of the criteria used: those that are well developed, well represented in the sample, and apparently the most crystallized (glassy criterion has proved to be doubtful), permit a better reorientation of the sample; and (2) the principle of the criterion itself: criteria referring to vertical axes (stalactites) or vertical planes (septa) are more reliable than those based upon the determination of a horizontal plane (elongation plane of tubes in sheeted lavas).It appears from this study that macroscopic polarity criteria may be used in an almost systematic way to find the polarity of an in situ sample.

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Upper Devonian (Frasnian stage) of the north-western part of the Russian Platform: paleomagnetic data

Neftegazovaya Geologiya Teoriya I Praktika ◽

10.17353/2070-5379/42_2020 ◽

2020 ◽

Vol 15 (4) ◽

Author(s):

A.G. Iosifidi ◽

◽

V.V. Popov ◽

A.V. Zhuravlev ◽

◽

...

Keyword(s):

Remanent Magnetization ◽

Natural Remanent Magnetization ◽

Upper Devonian ◽

Paleomagnetic Data ◽

Magnetic Minerals ◽

The North ◽

Apparent Polar Wander Path ◽

Paleomagnetic Pole ◽

Frasnian Stage ◽

Main Devonian Field

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

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