A discussion concerning the floor of the northwest Indian Ocean - An area on the crest of the Carlsberg Ridge: petrology and magnetic survey

1966 ◽  
Vol 259 (1099) ◽  
pp. 198-217 ◽  
Keyword(s):  
Remanent Magnetization ◽  
Ultramafic Rocks ◽  
Magnetic Survey ◽  
Vector Fitting ◽  
Uniform Magnetization ◽  
Ocean Floor Spreading ◽  
Carlsberg Ridge ◽  
History Of ◽  
Magnetic Gradients ◽  
Basalt Glasses

Rocks collected in the vicinity of a transcurrent fault cutting the crest of the Ridge have been affected by brecciation and, in some cases, metamorphism and hydrothermal action. These processes have led to the formation of spilites from crystalline basalts, and ultramafic rocks from basalt glasses Further hydrothermal action has taken the form of replacement of some ultramafic rocks by quartz, ending in a nearly pure quartzite. The mineralogy is characteristic of greenschist facies meta-morphism. Fresh basalts were collected from a nearby hill, which seems to be a recent volcano post-dating the faulting and metamorphism. The magnetic survey reveals a marked parallelism between the anomalies and the trend of the ridge, regardless of bathymetry. Computations confirm that uniform magnetization of the material represented by the bathymetry can in no way simulate the observed anomalies. Application of the vector fitting technique suggests that the remanent magnetization of this material is often reversed and from this a very crude and simple model is developed to account for the observed anomalies. The model is consistent with an ocean floor spreading hypothesis and periodic reversals in the earth's magnetic field. If substantiated it would have important implications in deducing the history of the ocean basins. Above all it provides a plausible explanation to account for the magnetic gradients and amplitudes observed over ridges without implying improbable magnetic contrasts, structures, or changes in petrology.

scholarly journals Mineralogy and Geochemistry of Ultramafic Rocks from Rachoni Magnesite Mine, Gerakini (Chalkidiki, Northern Greece)

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 934
Author(s):  
Evangelos Tzamos ◽  
Micol Bussolesi ◽  
Giovanni Grieco ◽  
Pietro Marescotti ◽  
Laura Crispini ◽  
...  
Keyword(s):  
Raw Materials ◽  
Mining Area ◽  
Ultramafic Rocks ◽  
Northern Greece ◽  
Icp Ms ◽  
Time In Literature ◽  
History Of ◽  
First Time ◽  
The Eu

The importance of magnesite for the EU economy and industry is very high, making the understanding of their genesis for the exploration for new deposits a priority for the raw materials scientific community. In this direction, the study of the magnesite-hosting ultramafic rocks can be proved very useful. For the present study, ultramafic rock samples were collected from the magnesite ore-hosting ophiolite of the Gerakini mining area (Chalkidiki, Greece) to investigate the consecutive alteration events of the rocks which led to the metallogenesis of the significant magnesite ores of the area. All samples were subjected to a series of analytical methods for the determination of their mineralogical and geochemical characteristics: optical microscopy, XRD, SEM, EMPA, ICP–MS/OES and CIPW normalization. The results of these analyses revealed that the ultramafic rocks of the area have not only all been subjected to serpentinization, but these rocks have also undergone carbonation, silification and clay alteration. The latter events are attributed to the circulation of CO2-rich fluids responsible for the formation of the magnesite ores and locally, the further alteration of the serpentinites into listvenites. The current mineralogy of these rocks was found to be linked to one or more alteration event that took place, thus a significant contribution to the metallo- and petrogenetic history of the Gerakini ophiolite has been made. Furthermore, for the first time in literature, Fe inclusions in olivines from Greece were reported.

The Iceland Research Drilling Project crustal section: stable remagnetization below 3 km crustal depth

1988 ◽  
Vol 25 (8) ◽  
pp. 1304-1315 ◽  
Author(s):  
James M. Hall ◽  
Brian E. Fisher
Keyword(s):  
Grain Size ◽  
Hydrothermal Alteration ◽  
Remanent Magnetization ◽  
Drill Hole ◽  
Lava Flows ◽  
Crustal Layer ◽  
Transition Zones ◽  
Mixed Polarity ◽  
History Of ◽  
Drilling Project

A section at below 3.1 km depth in Icelandic crust, sampled in the 1978 Icelandic Research Drilling Project drill hole, contains a number of subaerially deposited lava flows showing both downwards and probably original upwards inclinations of cleaned, stable remanent magnetization. Such "mixed polarities" are inconsistent with an initial cooling thermoremanent origin for the magnetization. An attempt is made to identify the factors involved in producing these mixed polarities and to consider the possible wider importance of the results. The mixed-polarity flows have experienced intense hydrothermal alteration, followed by the widespread deposition of secondary magnetite. Secondary magnetite, which is formed in relatively anhydrous conditions associated with dike intrusion, dominates primary magnetite volumetrically where dike density locally exceeds about 30%.Where secondary magnetite is very dominant or is the only type of magnetite present, directional remagnetization appears to be uniform and complete. Where secondary and primary magnetite are both important, relatively high remanence and saturation magnetizations, total magnetite and primary magnetite grain size, and low deuteric oxidation state of primary magnetite are all associated with downwards directional remagnetization. It appears that a complex balance of the properties and history of primary and secondary magnetite, in addition to the relative abundances of these phases, controls the final stable polarity of samples.If the narrow transition zones between little-altered extrusives, greenschist-facies flows and dikes of the Troodos (Cyprus) ophiolite, and DSDP hole 504B are typical of oceanic crust, a narrow ~0.2 km interval of mixed polarities may be underlain in some locations by an intermediate crustal layer in polarity opposition with the uppermost, little-altered, extrusive layer.

scholarly journals Correlation of Elastic Moduli and Serpentine Content in Ultramafic Rocks

2019 ◽  
Author(s):  
Aida Farough ◽  
Alexander Karrasch
Keyword(s):  
Physical Properties ◽  
Lower Crust ◽  
Elastic Moduli ◽  
Oceanic Lithosphere ◽  
Ultramafic Rocks ◽  
Seismic Velocities ◽  
Rock Composition ◽  
Metamorphic History ◽  
History Of ◽  
Thin Crust

Understanding the physical properties of ultramafic rocks is important for evaluating awide variety of petrologic models of the oceanic lithosphere, particularly upper mantle and lower crust. Hydration of oceanic peridotites results in increasing serpentine content, which affects lithospheric physical properties and the global bio/geochemical cycles of various elements. In understanding tectonic, magmatic and metamorphic history of the oceanic crust, interpreting seismic velocities, rock composition and elastic moduli are of fundamental importance. In this study we show that as serpentine content increases, density decreases linearly with a slope of 7.85. We also correlate increase in serpentine content with a linear decline in shear, bulk and Young’s moduli with slopes of 0.48, 0.77, 0.45 respectively. Our results show that increase in serpentine content of lower crust and forearc mantle could decrease elasticity of lithospehere and result in break-offs. Therefore tectonic processes at peridotite rich slow spreading ridges may be strongly affected by serpentine content, particularly serpentinization may be responsible for discontinuities in thin crust, and formation of weak fault zones.

scholarly journals Convection in the Moon

1972 ◽  
Vol 47 ◽  
pp. 377-383
Author(s):  
S. K. Runcorn
Keyword(s):  
Heat Transfer ◽  
Solid State ◽  
Remanent Magnetization ◽  
Second Harmonic ◽  
Crystalline Rocks ◽  
Hydrostatic Equilibrium ◽  
Iron Core ◽  
The Moon ◽  
History Of ◽  
The Moment

It is natural to inquire whether thermal convection is occurring in the Moon through solid state creep processes. The primary evidence is the departure of the Moon from the figure of hydrostatic equilibrium, but certain difficulties in the thermal history of the Moon are eased by assuming heat transfer by convection. If convection exists in the Moon it must have a second harmonic pattern, otherwise the lunar moments of inertia would not differ.Two important predictions of the marginal theory of convection: the existence of a core of radius 0.06–0.3 of the lunar radius (for a second ergee harmonic) and the value of 0.4 for the ratio of the dynamical to surface ellipticities now have support, the latter from the data of the heights of the lunar surface. The former prediction is compatible with the value of the moment of inertia factor now found if the Moon's interior is ‘hot’.Further the existence of a fluid iron core 3400 m.y. ago seems required as a result of the remanent magnetization of the crystalline rocks of the maria basins inferred from the remanent magnetization of the returned Apollo samples and the fields measured by the Apollo 12 and Explorer magnetomers.

Structure du plateau continental armoricain

1975 ◽  
Vol 279 (1288) ◽  
pp. 109-121 ◽  
Keyword(s):  
Ground Level ◽  
Magnetic Anomalies ◽  
Magnetic Survey ◽  
Positive Anomaly ◽  
The South ◽  
The North ◽  
Basement Rocks ◽  
Average Accuracy ◽  
Magnetic Field Model ◽  
History Of

Between 1965 and 1970 there has been conducted, in collaboration with the hydrographic service of the French navy, a magnetic survey of the Armorican continental plateau. The zone, covered with 48000 km of profiles, stretches between the parallels of 46° 10' N and 49° 30' N and between the meridians 4° W and 7° 15' W. Position fixing was carried out by means of Toran, with an average accuracy of 50 m. Maps of the total field intensity, reduced to epoch 1967.5, and the magnetic anomalies have been drawn to the scale 1:250000. The magnetic anomalies have been calculated by means of a magnetic field model defined by Jensen and Cain. Two major zones of anomaly can be identified: a northern one, bounded to the south by the 48° N parallel, with strong relief, where the magnetic anomalies are numerous and include major ones at the entrance to the Channel; a southern zone, with considerably less relief, but where the anomalies present several major directions. Several of these anomalies have been interpreted by three different methods with the intention of determining the depth of the sources of disturbance and of specifying possible relationships between these sources and the structural geology of the Armorican plateau. From the south to the north of the region studied, the floor of the continental plateau appears to be divided into compartments by large SW-NE horst and graben structures of Hercynian age. In each compartment the depth to basement increases from east to west by step-faulting with a NW-SE orientation, such faulting has played a role throughout the geological history of the continental plateau. Finally, it emerges from this study that the magnetic anomalies are, for the most part, related to the major structural and tectonic lines of the metamorphic basement visible at ground level. Several areas of high positive anomaly may be associated with basic intrusions emplaced along the lines of major faults in the basement rocks of the Armorican platform.

Apparent magnetization mapping in the presence of strong remanent magnetization: The space-domain inversion approach

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. J11-J24 ◽  
Author(s):  
Lianghui Guo ◽  
Lei Shi ◽  
Xiaohong Meng ◽  
Rui Gao ◽  
Zhaoxi Chen ◽  
...  
Keyword(s):  
Remanent Magnetization ◽  
North China ◽  
Magnetic Layer ◽  
Real Data ◽  
Magnetic Data ◽  
Magnetization Distribution ◽  
Cross Sectional ◽  
Space Domain ◽  
Uniform Magnetization ◽  
Domain Inversion

Apparent magnetization mapping is a technique to estimate magnetization distribution in the subsurface magnetic layer from the observed magnetic data, of benefit in identifying lithologic units and delineating magnetic geologic boundaries. The conventional approaches for apparent magnetization mapping usually neglect effects of remanence, resulting in large geologic deviation and the occurrence of negative magnetization when the magnetic layer contains strong remanent magnetization. We have developed a space-domain inversion approach for apparent magnetization mapping based on the amplitude of magnetic anomaly (AMA), the analytic signal (AS), and the normalized source strength (NSS) to reduce effects of remanent magnetization. The AMA, AS, and NSS are three common quantities insensitive or weakly sensitive to the remanence transformed from the magnetic total field anomaly or components. The magnetic layer underground is first divided into a regular grid of vertical rectangular prisms, each having a cross-sectional area of one grid square and a uniform magnetization. Then, an iterative algorithm is adopted to invert each quantity of the AMA, AS, and NSS to obtain an optimum value of magnetization of each prism in the magnetic layer. The inversion approach permits the top and bottom surfaces of the magnetic layer to be constant or variable in depth, and requires no prior information of magnetization directions. Our tests on the synthetic and real data from the metallic ores area in the southern margin of North China have proved the feasibility and robustness of the presented inversion approach. All of the AMA, AS, and NSS inversions produced nonnegative magnetization distribution in the magnetic layer. Also, the AS and NSS inversions produced a better resolution of magnetization distribution than that of the AMA.

scholarly journals Neumayer in Australia: his scientific legacy

2011 ◽  
Vol 123 (1) ◽  
pp. 11
Author(s):  
Patrick Quilty
Keyword(s):  
Major Influence ◽  
Magnetic Survey ◽  
Scientific Legacy ◽  
Magnetic Parameters ◽  
Terrestrial Magnetism ◽  
Scientific Establishment ◽  
History Of ◽  
Major Figure ◽  
First Time

Georg von Neumayer (1826-1909) is a major figure in the history of Australian and Antarctic science. He came to Australia twice, in 1852 and 1857–1864, the first time as a sailor and the second as the scientist who established the Flagstaff Observatory in Melbourne. He came here at a time when the scientific tradition was firmly established in Europe (its home) but new to Australia where there was little or no homegrown scientific establishment. His main contributions are in the fields of terrestrial magnetism, the early days of oceanography, and the potential of polar research. He built and managed the Flagstaff Observatory, conducted a magnetic survey of Victoria, visited Tasmania to re-measure the magnetic parameters at Rossbank Observatory, worked to identify the most efficient sailing routes for shipping between Europe and Australia and collaborated with other scientists and artists during his sojourn here. On return to Europe, he became a major influence in the Heroic Era of Antarctic exploration.

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