scholarly journals TO THE 90TH ANNIVERSARY OF IVAR MURDMAA

2021 ◽  
Vol 49 (4) ◽  
pp. 136-161
Author(s):  
E. V. Ivanova ◽  
O. V. Levchenko ◽  
E. A. Ovsepyan ◽  
D. G. Borisov ◽  
T. F. Zinger
Keyword(s):  
Theoretical Concept ◽  
Formation Processes ◽  
Island Arcs ◽  
Ocean Ridges ◽  
Oceanic Sediments ◽  
Sediment Formation ◽  
90Th Anniversary ◽  
Rift Zones ◽  
Deposition Processes ◽  
Iron Manganese

On August 6, 2021, the chief researcher of the IO RAS, Doctor of Geological and Mineralogical Sciences, Professor Ivar Oskarovich Murdmaa turned 90 years old. The main focus of I.O. Murdmaa is the study of bottom sediments of seas and oceans, their lithology, mineralogy, deposition processes, facies and formations, the theory of oceanic sedimentogenesis. He first distinguished marine volcanoterrigenous sediments and described the facies variability of modern sediments of island arcs. Ivar Murdmaa is known for his studies in mineralogy of oceanic sediments, processes of pelagic sedimentogenesis and associated iron-manganese nodules formation. Studying sediment formation in rift zones of mid-ocean ridges, he identified a new genetic type of sediments named edaphogeonus sediments, elaborated mineralogical criteria for their recognition and formation processes. In recent years I.O. Murdmaa is actively developing the theoretical concept of "sedimentosphere", paying special attention to a new direction – the study of the erosion-accumulative activity of bottom currents and the formation of contourites.

The effect of geometry on the gravitational instability of a buoyant region of viscous fluid

1989 ◽  
Vol 202 ◽  
pp. 577-594 ◽  
Author(s):  
John R. Lister ◽  
Ross C. Kerr
Keyword(s):  
Growth Rate ◽  
Stokes Equations ◽  
Gravitational Instability ◽  
Cube Root ◽  
Scaling Analysis ◽  
Island Arcs ◽  
Layered Systems ◽  
Ocean Ridges ◽  
The Difference ◽  
The Stability

The low-Reynolds-number stability of a region of buoyant fluid surrounded by denser fluid is analysed in two situations. In the first study, the buoyant fluid lies in a thin layer sandwiched between two denser and much deeper layers. The growth rate and wavelength of the most unstable sinusoidal perturbation are calculated and the effects of the viscosity ratios and density differences between the fluids are investigated. It is found that if the buoyant fluid is much less viscous than the overlying fluid then, in quite general circumstances, both the most unstable wavelength and the corresponding growth rate are inversely proportional to the cube root of the viscosity of the buoyant fluid. A physical explanation of this result is given by scaling analysis of the total dissipation. In the second study, the buoyant fluid takes the form of a cylinder rising through a uniform environment. The eigenmodes of small perturbation about this state of motion are found for each axial wavenumber in terms of Fourier series of separable solutions to the Stokes equations. In contrast to the first study, it is found that the most unstable wavelength and growth rate are asymptotically independent of the viscosity of the buoyant fluid when this viscosity is small.The difference between the results of the two studies is of importance, particularly for geophysical applications in which viscosity ratios are very large. Previous models of linear regions of volcanism at mid-ocean ridges and at island arcs have assumed that results obtained in simple two-layered systems can be generalized to other geometries. The conclusions of these models are discussed in the light of the stability results for a cylindrical (and hence linea.

Petrochemistry of metamorphosed pillows, and the geochemical status of the amphibolites (Proterozoic) from the Sirohi district, Rajasthan, India

1984 ◽  
Vol 121 (5) ◽  
pp. 465-473 ◽  
Author(s):  
P. K. Bhattacharyya ◽  
A. D. Mukherjee
Keyword(s):  
Regional Metamorphism ◽  
Element Distribution ◽  
Island Arcs ◽  
Oceanic Sediments ◽  
Crystalline Core ◽  
Mineral Assemblages ◽  
Trace Element Contents ◽  
Closed Systems ◽  
Low K ◽  
Middle Proterozoic

AbstractRelic pillows in the middle Proterozoic amphibolites, occurring in the Sirohi Road–Abu Road tract of Rajasthan, India exhibit contrasted mineral assemblages from core to rim – mimetic after the crystalline core, the zone of incipient crystallization, and the rim of the original pillows. The major element distribution pattern across the pillows indicates exchange of Na–Al for Ca (Mg, Fe) in an inner reaction zone, surrounding the core and in the inner margin of the rim, and Fe–Al exchange for Ca–Si at the outer margin of the rim.Despite such exchanges around the rims, these pillows have retained their initial geochemical characteristics internally and thus have largely acted as closed systems during post-emplacement metamorphism. Mineral parageneses indicate that the contrasted mineral assemblages could evolve from domainal characters of the co-existing fluids, the compositions of which were only buffered by the reacting minerals during regional metamorphism.The major, minor and trace element contents of the pillows and of amphibolites of diverse petrographic character in the region further establish that the pillow interiors and the massive amphibolites were least modified during metamorphism(s), and represent oceanic tholeiites. Their average 2300 ppm K, 4.5 ppm Rb, 150 ppm Sr, along with the K/Rb and K/Sr ratios of 510 and 15 respectively resemble that of the low K-tholeiites, occurring nearest to the trenches in modern island arcs. On the other hand, the higher values of 17300 ppm K, 4.9 ppm Rb, and 210 ppm Sr of the banded and the schistose amphibolites indicate that they were contaminated in various magnitudes by oceanic sediments.

Plate Tectonics Now and in the Past

2004 ◽  
Author(s):  
John J. W. Rogers ◽  
M. Santosh
Keyword(s):  
Plate Tectonics ◽  
Oceanic Lithosphere ◽  
Continental Margins ◽  
Island Arcs ◽  
Low Velocity ◽  
S Waves ◽  
Ocean Ridges ◽  
1960S And 1970S ◽  
Tectonic Contact ◽  
The 1960S

The concepts known as plate tectonics that began to develop in the 1960s built on a foundation of information that included: • The earth’s mantle is rigid enough to transmit seismic P and S waves, but it is mobile to long-term stresses. • The earth’s temperature gradient is so high that convective overturn must occur in the mantle. • The top of the mobile part of the mantle is a zone of relatively low velocity at depths of about 100 to 200 km. This zone separates an underlying asthenosphere from a rigid lithosphere, which includes rigid upper mantle and crust. • Seismic activity, commonly accompanied by volcanism, occurs along narrow, relatively linear, zones in oceans and along some continental margins. • The zones of instability surround large areas of comparative stability. • Ocean lithosphere is continually generated along mid-ocean ridges and destroyed where it descends under the margins of continents and island arcs. This causes oceans to become larger, but shrinkage of oceans can occur where lithosphere is destroyed around ocean margins faster than it is formed within the basin. • Some of the belts of instability are faults with lateral offsets of hundreds of kilometers. • Some continental margins are unstable (Pacific type), but others are attached to oceanic lithosphere without any apparent tectonic contact (Atlantic type). • Different areas containing continents and attached oceanic lithosphere move around the earth independently of each other. Most of this chapter consists of a summary of plate tectonics in the present earth, including processes along plate margins and the types of rocks formed there (readers who want more detailed information are referred to Rogers, 1993a; Kearey, 1996; and Condie, 1999). We also briefly discuss plumes and then finish with a word of caution about interpreting the history of the ancient and hotter earth with the principles of modern plate tectonics. Starting from the body of continually expanding information summarized above, numerous earth scientists in the 1960s and 1970s began to establish a conceptual framework that would organize scientific thinking about the earth’s tectonic processes. This required a new terminology, and it arrived rapidly (Oreskes, 2002). Geologists decided to call the stable areas “plates” and the unstable zones around them “plate margins.” Thus, the concept became known as “plate tectonics.” Plates are essentially broad regions of lithosphere, although the failure to detect low-velocity zones under many continents leaves unresolved questions.

Systematic variations in magmatic sulphide chemistry from mid-ocean ridges, back-arc basins and island arcs

2017 ◽  
Vol 451 ◽  
pp. 67-77 ◽  
Author(s):  
Manuel Keith ◽  
Karsten M. Haase ◽  
Reiner Klemd ◽  
Ulrich Schwarz-Schampera ◽  
Henrike Franke
Keyword(s):  
Island Arcs ◽  
Ocean Ridges ◽  
Back Arc

Plate Tectonics

2021 ◽  
pp. 114-136
Author(s):  
Elisabeth Ervin-Blankenheim
Keyword(s):  
Plate Tectonics ◽  
Plastic Material ◽  
Convergent Margins ◽  
Island Arcs ◽  
Mountain Ranges ◽  
Ocean Ridges ◽  
Rock Types ◽  
The Pacific ◽  
The Pacific Ocean ◽  
The Many

Plate tectonics, the grand unifying theory of geology, and its relation to the Earth is explained in this chapter. The planet transforms through time by means of the movement of rigid plates carrying the continents riding on the plastic material in the Earth’s upper mantle. Three major plate boundaries are divergent margins, where new ocean floor is being created along mid-ocean ridges and plates separate from one another; convergent margins, where the material is subducted and consumed as different types of plates collide, creating trenches, island arcs or mountain ranges, and transform boundaries; and where plates slide past one another. Besides the three predominant boundaries, hot spots caused by mantle plumes and diffuse boundaries make up additional dynamic forces in tectonics. Beyond these categories, geologists still are learning about tectonics; some boundaries are unknown or speculative. Plate tectonics explains why many of the Earth’s hazards are found where there are. Earthquakes trace many plate margins, as do volcanoes. The area around the Pacific Ocean is called the “Ring of Fire” because of the many volcanoes related to subducting plates. Tectonics accounts for why certain rocks are located where they are; for example, all rock types are found at convergent margins. The theory also predicts where valuable mineral and economic deposits are located.

Origin and alteration of platinum group minerals in chromite deposits of the Ulan-Sardag ophiolite

2020 ◽  
Author(s):  
Olga Kiseleva ◽  
Eugenia Airiyants ◽  
Dmitry Belyanin ◽  
Sergey Zhmodik
Keyword(s):  
Solid Solutions ◽  
Stoichiometric Composition ◽  
Fluid Phase ◽  
Mineral Chemistry ◽  
Ministry Of Education ◽  
Island Arcs ◽  
Ocean Ridges ◽  
Magmatic Stage ◽  
Eastern Sayan ◽  
Individual Grains

<p>Ultrabasic Ulan-Saridag massif is part of the Eastern Sayan ophiolite belts, lying between the ophiolites of the southern and northern branches. It was suggested that ophiolites of the southern branch were created in mid-oceanic ridges, and southern one – in island arcs environment. Recent data indicate the formation of Ulan- Saridag ophiolites in supra-subduction conditions of ensimatic island arcs.</p><p>Ore podiform chromitites consist of alumochromite, chromite, and chrompicotite (first finding for this region). Cr-spinelides are divided into three groups according to geochemistry. They refer to the MORB-peridotite, supra-subduction peridotites to the complexes of Ural-Alaska type.</p><p>PGE mineralization in this massif is represented by Os-Ir-Ru solid solutions, native Os, Ru, laurite-erlichmanite (Ru, Os)S<sub>2</sub>, laurite (RuS<sub>2</sub>), irarsite (IrAsS), zaccarinite (RhNiAs).</p><p>Solid solutions of Os-Ir-Ru were found as idiomorphic inclusions in Cr-spinel and xenomorphic grains in intergrowths with laurite. They correspond to the early high-temperature magmatic solid-solution Os-Ir-Ru. Also, the phases (Os-Ir-Ru) of varying composition are common in the form of numerous micro - and nano-size inclusions in laurite-erlichmanite with osmium or ruthenium. Native Os<sup>o</sup> (Os> 80 wt.%) is recognized in polyphase aggregates, together with chalcocite, laurite, laurite-erlichmanite. Native Ru (Ru=93 wt.%) – occur in the polyphase, together with heazlewoodite, zaccarinite, Os-Ir-Ru solid solutions. Laurite and laurite- erlichmanite RuS<sub>2</sub> – (Ru, Os)S<sub>2</sub> are represented most widely.</p><p>There are two groups: 1) laurite-erlichmanite (Ru, Os)S<sub>2</sub>; 2) laurite RuS<sub>2</sub>- phase of variable composition. (Ru, Os)S<sub>2</sub> rarely forming independent grains, occurring more often in multi-component aggregates,  together with the laurites and contains a large number of rounded and rectangular micro-inclusions of native Os, (Os-Ir), and native Ru. Laurite has the reveal  stoichiometric composition (Ru=61,2 wt.%, S = 38.2 wt.%). It forms individual grains in chlorite and serpentine in association with irarsite, sulfides of Ni, Cu and rims around laurite-erlichmanite.</p><p>Solid solutions of (Os-Ir-Ru) and laurite-erlichmanite are forming before or simultaneously with Cr- spinel in the upper mantle at T=1200<sup>o</sup>C and P= 5-10 kbar.</p><p>Sulfoarsenides and arsenides of Ru, Ir, Rh, Ni are formed from the residual fluid phase at a post-magmatic stage, together with heazlewoodite. It is possible that in chromitites from Ulan-Saridag there are two generations of sulfides. 1-st PGM generation – magmatic solid solutions of laurite-erlichmanite. 2 -nd generation – the newly formed laurite, with primary laurite-erlichmanite or intergrowths with chalcocite, heazlewoodite and millerite confined to zones of chloritization. The predominance of  Os, Ru sulfides over the solid solutions of Os-Ir-Ru indicates a higher sulfur fugacity in the mantle source of Ulan-Sardag ultramafic-mafic massif. These results indicate the distinctive characteristics of PGM of Ulan-Sardag massif compared to PGM from the chromitites of the Northern and Southern branches of the ophiolites.</p><p>Ulan-Sardag ultrabasic massif occurred in three different geodynamic settings: mid-ocean ridges, primitive ensimatic and ensialic island arcs, subduction zone, and belongs to the Alaska type basic formation.</p><p>Mineral chemistry was determined at the Analytical Centre for multi-elemental and isotope research SB RAS. This work supported by RFBR grants: No. 16-05-00737a, 15-05-06950, 19-05-00764 and the Russian Ministry of Education and Science. </p>

scholarly journals P-velocity of the upper mantle

2021 ◽  
Vol 43 (2) ◽  
pp. 152-165
Author(s):  
V.V. Gordienko ◽  
L.Ya. Gordienko
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
A Priori ◽  
Velocity Profiles ◽  
P Wave ◽  
Depth Interval ◽  
Island Arcs ◽  
Thermal Anomalies ◽  
Ocean Ridges ◽  
Back Arc

The authors have constructed models featuring seismic P-wave velocity distribution in the upper mantle beneath oceanic, continental and transition regions, such as mid-ocean ridges, basins, trenches, island arcs, and back-arc troughs, Atlantic transitional zones, flanking plateaus of mid-ocean ridges, platforms, geosynclines, rifts, recent activation zones. The models are in agreement with the deep-seated processes in the tectonosphere as predicted in terms of the advection-polymorphism hypothesis. The models for areas of island arcs and coastal ridges are similar to those for alpine geosynclines disturbed by recent activation. The models for areas of mid-ocean ridges and back-arc troughs are identical. They fit the pattern of recent heat-and-mass transfer in the case of rifting, which, given the basic crust with continental thickness, leads to oceanization. The model for the basin reflects the effect of thermal anomalies smoothing beneath mid-ocean ridges or back-arc troughs about 60 million years later. The model for the trench and flanking plateau reflects the result of lateral heating of the mantle’s upper layers beneath the quiescent block from the direction of the island arc and basin (trench) and mid-ocean ridge and basin (flanking plateau). A detailed bibliography on regions covered by studies was presented in the authors’ earlier publications over past eight years. There are quite significant differences between models for regions of the same type that are described in publications of other authors. This is largely due to the fact that individual authors adopt a priori concepts on the velocity structure of the upper mantle. High variability of seismic P-wave velocities within the subsurface depth interval has been detected as a result of all sufficiently detailed studies. This variability is responsible for the sharp increase in the scatter of arrival times of waves from earthquakes at small angular distances. The corresponding segments of travel-time graphs were simply ignored, and the graphs started from about 3° after which the scatter of arrival time acquired a stable character. Accordingly, velocity profiles were constructed, as a rule, starting from depths of about 50 km. The constructed velocity profiles vary little from region to region with the same type of endogenous regimes. This enables us to maintain that the models represent standard (typical) VP distributions in the mantle beneath the regions, just as presumed in terms of the theory.

Sign in / Sign up

Export Citation Format

Share Document