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

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.

The Mid-Atlantic ridge near 45° N. VII. Magnetic properties and opaque mineralogy of dredged samples

1970 ◽  
Vol 7 (2) ◽  
pp. 239-256 ◽  
Author(s):  
C. M. Carmichael
Keyword(s):  
Magnetic Properties ◽  
Remanent Magnetization ◽  
Complex Structure ◽  
Magnetic Anomalies ◽  
Natural Remanent Magnetization ◽  
Coarse Grained ◽  
X Ray Diffraction ◽  
Field Polarity ◽  
Fine Grained ◽  
Mid Atlantic Ridge

Measurements of the magnetic properties, paleomagnetic field intensity, and the inferred paleomagnetic field polarity have been made using fine grained basalt and coarser grained rock samples dredged from the mid-Atlantic ridge near 45° N and supplied by the Geological Survey of Canada. The opaque mineralogy of the samples was studied by microscope, Curie point, and X-ray diffraction techniques. The natural remanent magnetization of the basalt is of the order of 5 to 10 × 10−3 e.m.u./cm3 with some values from the center of the median valley reaching 10−1 e.m.u./cm3. Magnetic anomalies over the ridge can be accounted for by the remanent magnetization of a few hundred meters of this basalt. The coarse grained rocks were relatively weakly magnetized, and while they contribute little to the magnetic anomalies, their diverse character suggests that the major portion of the oceanic crust, below a thin veneer of fine grained basalt, has differentiated into a complex structure.

Magnetic remanence in the Chalk of eastern England: an unusually resistant VRM?

1994 ◽  
Vol 131 (5) ◽  
pp. 593-608 ◽  
Author(s):  
Graham J. Borradaile
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Single Component ◽  
Magnetic Minerals ◽  
Magnetic Remanence ◽  
Ferro Magnetic

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).

Some magnetic properties of Athabasca Oil Sand samples, Alberta, Canada

1984 ◽  
Vol 21 (3) ◽  
pp. 278-283 ◽  
Author(s):  
Brooks B. Ellwood ◽  
S. George Pemberton
Keyword(s):  
Magnetic Properties ◽  
Remanent Magnetization ◽  
Magnetic Measurements ◽  
Magnetic Fabric ◽  
Oil Sand ◽  
Magnetic Minerals ◽  
Remanent Magnetism ◽  
Sand Core ◽  
Bitumen Content ◽  
High Concentrations

A number of induced and remanent magnetic measurements have been performed on 55 cubic samples, each 8 cm3, from a single segment of Athabasca Oil Sand core. Samples are representative of four major lithofacies types, which have been classified in the following manner: facies A—laminated, bitumen-free sediments; facies B—bioturbated (laminae are biologically disrupted to some degree), bitumen-free sediments; facies C—bioturbated sediments with moderate bitumen content; and facies D—massive sand, bitumen-rich sediments. Samples of facies D are divided into subfacies D1 and D2, based upon extremely high percentage values of anisotropy of magnetic susceptibility (AMS) observed for D2 samples. Magnetic fabric ranges progressively from undisturbed in samples of facies A to anomalous, considered highly disturbed, for samples of facies D2. Only samples of facies D record a stable remanent magnetization and, since the magnetic fabric is anomalous, it is inferred that the remanent magnetism may also be anomalous. It is concluded that zone D2 may result from locally high concentrations of authigenic magnetic minerals such as maghemite or siderite. Such D2 zones may prove to be useful stratigraphic markers in bitumen-saturated samples in which sedimentary structures are usually obscure.

Under pressure: How pressure affects magnetic remanence

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

<p>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. </p><p>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’ magnetic relies on robust estimations of direction and intensity, the effects of pressure on the remanence should be taken into account.</p><p>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’s original magnetization. Stoichiometric magnetites of four different grain sizes (65 nm, 440 nm, 16.9 µm, and 18.3 µm) 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,  populations of large multidomain particles can acquire a PRM, which may represent up to 30% of a TRM acquired in the same field.</p><p>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  ~1 µm maximum spatial resolution that is able to resolve the magnetic fields of individual mineral assemblages with ~10 µm 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.</p>

The Mid-Atlantic Ridge near 45° N. XIII. Magnetic properties of basalt bore-core

1970 ◽  
Vol 7 (6) ◽  
pp. 1515-1527 ◽  
Author(s):  
J. Brooke ◽  
E. Irving ◽  
J. K. Park
Keyword(s):  
Magnetic Properties ◽  
Coercive Force ◽  
Physical Properties ◽  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
The Core ◽  
Mid Atlantic Ridge ◽  
Technical Interest

Three bore-cores containing basalts have been obtained from the Mid-Atlantic Ridge at 45° N. The material is fresh and ideal for the study of physical properties. The drilling record and the nature of the core itself suggest that much of the basalt is from detached boulders, although one core may be in situ. In one core, variations in coercivity by a factor 2 occur within a distance of 5 cm. The coercive force spectra of anhysteretic and natural remanent magnetization are similar, but there are small differences due to secondary components which are used to predict correctly the polarity of 11 out of 12 specimens studied. This result may be of technical interest only since there is no guarantee that the material is in situ.

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