scholarly journals GEODYNAMICS

GEODYNAMICS ◽  
2020 ◽  
Vol 2(29)2020 (2(29)) ◽  
pp. 89-96
Author(s):  
M. I. Orlyuk ◽  
◽  
V. V. Drukarenko ◽  
O. Ye. Shestopalova ◽  
◽  
...  
Keyword(s):  
Magnetic Properties ◽  
Curie Temperature ◽  
Transition Zone ◽  
Upper Mantle ◽  
Subduction Zones ◽  
Magnetic Anomalies ◽  
Oceanic Lithosphere ◽  
Magnetic Minerals ◽  
Native Iron ◽  
Lithospheric Plates

The purpose of the study. It needs to substantiate that sources of magnetic anomalies with wavelengths of the first thousand kilometers detected at the present time might have a magneto-mineralogical origin due to the existence of magnetic minerals at the mantle depths, in particular magnetite, hematite, native iron, as well as iron alloys. It should be also shown that present temporal changes of long-wave magnetic anomalies should be induced by changes of the magnetic properties of these minerals due to thermodynamic and fluid modes. According to numerous authors, the transformations of magnetic minerals occur in special tectonic zones of the upper mantle of the Earth, in particular at junction zones of lithospheric plates of different types, rifts, plumes, tectonic-thermal activation, etc. Areas of the upper mantle with temperatures below the Curie temperature of magnetite can be magnetic, such as subduction zones, cratons, and regions with the old oceanic lithosphere. Iron oxides might be a potential source of magnetic anomalies of the upper mantle besides magnetite and native iron, in particular hematite (α-Fe2O3), which is the dominant oxide in subduction zones at depths of 300 to 600 km. It was proved experimentally by foreign researchers that in cold subduction slabs, hematite remains its magnetic properties up to the mantle transition zone (approximately 410-600 km). Conclusions. A review of previous studies of native and foreign authors has made it possible to substantiate the possibility of the existence of magnetized rocks at the mantle depths, including native iron at the magneto-mineralogical level, and their possible changes due to thermodynamic factors and fluid regime. It has been experimentally proven by foreign researchers that in subduction zones of the lithospheric slabs their magnetization might be preserved for a long time at the mantle depths, as well as increase of magnetic susceptibility may observed due to the Hopkinson effect near the Curie temperature of magnetic minerals. Practical value. Information about the ability of the mantle to contain magnetic minerals and to have a residual magnetization up to the depths of the transition zone was obtained. It should be used in the interpretation of both modern magnetic anomalies and paleomagnetic data.

scholarly journals A MODEL OF THE EVOLUTION OF THE LITHOSPHERE OF THE HIMALAYAN-TIBETAN OROGEN

2021 ◽  
pp. 89-107
Author(s):  
R.S. Alekseev, ◽  
◽  
Yu.L. Rebetsky ◽  
Keyword(s):  
Upper Mantle ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
Indian Plate ◽  
Tibet Plateau ◽  
Small Scale ◽  
Continental Lithosphere ◽  
Principal Stresses ◽  
Vertical Movements ◽  
Reference Parameters

The Himalayan-Tibetan orogen is one of the active orogens on Earth. The processes caused by the collision of two continents have attracted attention of many researchers, and over the past decades, a large amount of geological and geophysical data has accumulated, on which models of the evolution of the region are based. The paper presents a model of the evolution of the Tibet plateau and the adjacent mountain chains, which complies with the modern concepts of the structure of the crust. The reference parameters of this model are the data on the values of stresses and on the patterns of the spatial distribution of principal stresses obtained in our own tectonophysical studies in region, as well as in other intracontinental orogens and in subduction zones between lithospheric plates. The basic assumptions of the model are the factors of the long stage of the Indian plate underthrusting beneath the Eurasian continent, metamorphic processes in the submerged slab (oceanic lithosphere) and in the continental lithosphere above it, combination of absolute horizontal movements of the Eurasian and Indian plates, small-scale convection in the upper mantle and vertical movements of matter, both in the continental lithosphere itself and in the upper mantle.

Mantle heterogeneities and their significance: results from Lithoprobe seismic reflection and refraction – wide-angle reflection studiesThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.Lithoprobe Contribution 1486.

2010 ◽  
Vol 47 (4) ◽  
pp. 409-443 ◽  
Author(s):  
Ron M. Clowes ◽  
Don J. White ◽  
Zoltan Hajnal
Keyword(s):  
Upper Mantle ◽  
Lithospheric Mantle ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
P Wave ◽  
Wide Angle ◽  
Velocity Anisotropy ◽  
Tectonic Processes ◽  
Wide Range ◽  
Restricted Region

Within Lithoprobe’s 10 transects, data from more than 20 000 km of multichannel seismic (MCS) reflection profiling and 12 refraction – wide-angle reflection (R/WAR) surveys were acquired. While the main results related to crustal structure, the data also indicated substantial heterogeneity in the lithospheric mantle. Images of fossilized subduction zones from the Eocene to the Neoarchean demonstrate that current plate tectonic processes have been active for more than 2.6 Ga. The Canadian Cordillera has a thin (50–60 km) lithosphere that is likely receiving some dynamic support from the asthenosphere below. Vestiges of the last stage of accretionary tectonic processes that formed the Archean Superior craton are indicated by an unusual anisotropic high velocity layer that may represent relic oceanic lithosphere. Within the Paleoproterozoic Trans-Hudson Orogen, a restricted region of upper mantle P-wave velocity anisotropy is identified with the continental collision between the bounding Hearne and Superior cratons. In the Archean Hearne and Wyoming provinces, two dipping structures within the sub-crustal lithosphere are interpreted as subduction features related to the assembly of the two cratons. Finite-difference modeling of long-offset data (over 1300 km) reveals fine-scale heterogeneities within a layer between 90 and 150 km in the continental lithosphere, perhaps formed through lateral flow or deformation within the upper mantle. Based on Lithoprobe data, heterogeneities within the lithospheric mantle are reasonably common. They have a wide range of seismic signatures, include many different types and show differing scales. Nevertheless, their extent in the lithospheric mantle is considerably less than in the crust.

scholarly journals Depletion of the upper mantle by convergent tectonics in the Early Earth

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
A. L. Perchuk ◽  
T. V. Gerya ◽  
V. S. Zakharov ◽  
W. L. Griffin
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Upper Mantle ◽  
Subduction Zones ◽  
Convergence Rates ◽  
Oceanic Lithosphere ◽  
Early Earth ◽  
Plate Convergence ◽  
Depleted Mantle ◽  
Mantle Peridotites

AbstractPartial melting of mantle peridotites at spreading ridges is a continuous global process that forms the oceanic crust and refractory, positively buoyant residues (melt-depleted mantle peridotites). In the modern Earth, these rocks enter subduction zones as part of the oceanic lithosphere. However, in the early Earth, the melt-depleted peridotites were 2–3 times more voluminous and their role in controlling subduction regimes and the composition of the upper mantle remains poorly constrained. Here, we investigate styles of lithospheric tectonics, and related dynamics of the depleted mantle, using 2-D geodynamic models of converging oceanic plates over the range of mantle potential temperatures (Tp = 1300–1550 °C, ∆T = T − Tmodern = 0–250 °C) from the Archean to the present. Numerical modeling using prescribed plate convergence rates reveals that oceanic subduction can operate over this whole range of temperatures but changes from a two-sided regime at ∆T = 250 °C to one-sided at lower mantle temperatures. Two-sided subduction creates V-shaped accretionary terrains up to 180 km thick, composed mainly of highly hydrated metabasic rocks of the subducted oceanic crust, decoupled from the mantle. Partial melting of the metabasic rocks and related formation of sodic granitoids (Tonalite–Trondhjemite–Granodiorite suites, TTGs) does not occur until subduction ceases. In contrast, one sided-subduction leads to volcanic arcs with or without back-arc basins. Both subduction regimes produce over-thickened depleted upper mantle that cannot subduct and thus delaminates from the slab and accumulates under the oceanic lithosphere. The higher the mantle temperature, the larger the volume of depleted peridotites stored in the upper mantle. Extrapolation of the modeling results reveals that oceanic plate convergence at ∆T = 200–250 °C might create depleted peridotites (melt extraction of > 20%) constituting more than half of the upper mantle over relatively short geological times (~ 100–200 million years). This contrasts with the modeling results at modern mantle temperatures, where the amount of depleted peridotites in the upper mantle does not increase significantly with time. We therefore suggest that the bulk chemical composition of upper mantle in the Archean was much more depleted than the present mantle, which is consistent with the composition of the most ancient lithospheric mantle preserved in cratonic keels.

A mechanism for ocean-floor spreading

1971 ◽  
Vol 268 (1192) ◽  
pp. 731-731 ◽  
Keyword(s):  
Transition Zone ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Garnet Peridotite ◽  
Ocean Floor ◽  
Ocean Floor Spreading ◽  
Melted Layer ◽  
Lithospheric Plates ◽  
Melted Material

I wish to suggest a mechanism for ocean-floor spreading, other than deep mantle convection cells. If the geotherm intersects the mantle solidus at the base of the upper mantle, a partly melted layer will result. It will be less dense than the overlying unmelted garnet peridotite lithosphere and the situation will be gravitationally unstable. It will be more voluminous than the overlying unmelted mantle and will distend the lithosphere which will break up into plates. It will have low rigidity and decouple those lithospheric plates from the underlying transition zone and lower mantle. Melted material, perhaps with crystals, will escape in two ways.

Magnetic anomalies, chemical and magnetic properties at wide temperature range (15–1000 K) in LiSrxFe5-xO8 (x = 0, 0.025, 0.05)

2021 ◽  
Vol 859 ◽  
pp. 158290
Author(s):  
S. Udhayakumar ◽  
G. Jagadish Kumar ◽  
E. Senthil Kumar ◽  
M. Navaneethan ◽  
K. Kamala Bharathi
Keyword(s):  
Magnetic Properties ◽  
Wide Temperature Range ◽  
Magnetic Anomalies ◽  
Temperature Range

Suppression of Ferromagnetism in EuxCa1-xB6

2012 ◽  
Vol 190 ◽  
pp. 97-100 ◽  
Author(s):  
V.V. Glushkov ◽  
A.V. Kuznetsov ◽  
I. Sannikov ◽  
A.V. Bogach ◽  
S.V. Demishev ◽  
...  
Keyword(s):  
Magnetic Properties ◽  
Magnetic Susceptibility ◽  
Curie Temperature ◽  
Single Crystals ◽  
Effective Magnetic Moment ◽  
Paramagnetic State ◽  
Low Field ◽  
Wide Range ◽  
Free Ion ◽  
The Eu

We report the magnetic properties of EuxCa1-xB6 single crystals (0.756x1) studied in the wide range of temperatures (1.8-300 K) and magnetic fields (up to 50 kOe). It was found that low field magnetic susceptibility χ (T) follows the Curie-Weiss law χ~(T-Θp)-1 at high temperatures for all the concentrations studied. The effective magnetic moment of the Eu2+ ion estimated from the data diminishes from the free ion value μeff7.93μB (μB - Bohr magneton) for x=1 to μeff7.3μB for x=0.756. A universal behavior of magnetic susceptibility χ~(T-Θ)-α (α=1.5) is detected close to the Curie temperature TC in the paramagnetic state at both metallic (x>xC~0.8) and dielectric (xC.

Holocene climate recorded by magnetic properties of lake sediments in the Northern Rocky Mountains, USA

The Holocene ◽  
2019 ◽  
Vol 30 (3) ◽  
pp. 479-484
Author(s):  
Daniel P Maxbauer ◽  
Mark D Shapley ◽  
Christoph E Geiss ◽  
Emi Ito
Keyword(s):  
Grain Size ◽  
Magnetic Properties ◽  
Rocky Mountains ◽  
Future Research ◽  
Magnetic Minerals ◽  
Holocene Climate ◽  
Northern Rocky Mountains ◽  
The North ◽  
Lake Bottom ◽  
Northern Rockies

We present two hypotheses regarding the evolution of Holocene climate in the Northern Rocky Mountains that stem from a previously unpublished environmental magnetic record from Jones Lake, Montana. First, we link two distinct intervals of fining magnetic grain size (documented by an increasing ratio of anhysteretic to isothermal remanent magnetization) to the authigenic production of magnetic minerals in Jones Lake bottom waters. We propose that authigenesis in Jones Lake is limited by rates of groundwater recharge and ultimately regional hydroclimate. Second, at ~8.3 ka, magnetic grain size increases sharply, accompanied by a drop in concentration of magnetic minerals, suggesting a rapid termination of magnetic mineral authigenesis that is coeval with widespread effects of the 8.2 ka event in the North Atlantic. This association suggests a hydroclimatic response to the 8.2 ka event in the Northern Rockies that to our knowledge is not well documented. These preliminary hypotheses present compelling new ideas that we hope will both highlight the sensitivity of magnetic properties to record climate variability and attract more work by future research into aridity, hydrochemical response, and climate dynamics in the Northern Rockies.

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