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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Geophysics - Magnetic
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
Articles tagged as Geophysics - Magnetics tend to deal with measuring the magnetic variations within the earth vertically and horizontally (usually through aeromagnetic surveys), which involves two entirely different dimensions, one being the relative magnetic intensity of rocks, and the other being the magnetic polarity of the rocks. The earth has a history of switching the north-south polarity of its magnetic field, which geologists first figured out when trying to understand the magnetic zebra pattern of oceanic crust. It turns out that in a magma "crystals" point in every possible direction, but when they chill they line up according to the prevailing polarity of the magnetic field. One thus has to be cautious about magnetic "lows" and "highs" due to reversed polarities of the rock's magnetic minerals, and due to the relative abundance of magnetic minerals in different adjacent rock types. But these articles can also be about what magnetic surveys have revealed about the nature of the earth not just as it is configured now, but also as it evolved. In fact, some articles are about the rocks reveal about the history of the earth's magnetic field which is drive by the inner core. Magnetic geophysical surveys are not relevant to diamonds themselves, but they are very important to exploration for mantle derived intrusions such as lamproites and kimberlites which intruded the upper crust rather quickly and chilled very quickly.
Crustal electrical conductivity in north central Saskatchewan: the North American Central Plains anomaly and its relation to a Proterozoic plate margin.
Canadian Journal of Earth Sciences, Vol. 21, pp. 533-43.
Sharma, R., Muthry, Ch.V.V.S., Nagaraju, B.V., Gouda, H.C., Singh, R.K.
Interpretation of aeromagnetic dat a of Panna and adjoining areas for evaluating of structural patterns favourable for emplacement of KCRs and depth magnetics
Geological Society of India, Bangalore November Meeting Group Discussion on Kimberlites and Related Rocks India, Abstract p. 121-122.
India, Madhya Pradesh, Aravalli Bundelkhand Craton
Journal of African Earth Sciences, in press available
Africa, West Africa
Geophysics - magnetics
Abstract: Studies of mafic dyke swarms may simultaneously provide information on the mechanical, geochemical, geochronological and magnetic environments at the time of their formation. The mafic intrusive history of different cratons can also be potentially used to unravel their assembly into their current configuration. The identification and classification of dykes is a first step to all these studies. Fortunately, even in regions with poor outcrop, we can use the strong magnetic response of mafic dykes to identify and map their extent. In West Africa the first maps of mafic dyke distribution were made over 40 years ago, but there are still large areas where there are almost no published data. In this paper we present a significantly updated map of mafic dykes for the West Africa Craton based in large part on new interpretations of the regional airborne magnetic database. This map includes the locations of over three thousand dykes across the craton, which locally shows several orientation clusters that provide a minimum estimate for the total number of dyke swarms in this region. Whilst we will have to wait until systematic dating of the different swarms is completed, we can demonstrate that there is a long and complex history of mafic magmatism across the craton, with up to 26 distinct dyke swarms mapped based according to their orientation. The mapping and dating of these swarms will provide key constraints on the assembly of the fragments that make up the modern continents.
International Journal of Earth Sciences, in press available, 27p.
Africa, Zimbabwe
Geophysics - magnetics
Abstract: Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.
Abstract: New work from Carnegie’s Peter Driscoll suggests Earth’s ancient magnetic field was significantly different than the present day field, originating from several poles rather than the familiar two. It is published in Geophysical Research Letters. Earth generates a strong magnetic field extending from the core out into space that shields the atmosphere and deflects harmful high-energy particles from the Sun and the cosmos. Without it, our planet would be bombarded by cosmic radiation, and life on Earth’s surface might not exist. The motion of liquid iron in Earth’s outer core drives a phenomenon called the geodynamo, which creates Earth’s magnetic field. This motion is driven by the loss of heat from the core and the solidification of the inner core. But the planet’s inner core was not always solid. What effect did the initial solidification of the inner core have on the magnetic field? Figuring out when it happened and how the field responded has created a particularly vexing and elusive problem for those trying to understand our planet’s geologic evolution, a problem that Driscoll set out to resolve. Here’s the issue: Scientists are able to reconstruct the planet’s magnetic record through analysis of ancient rocks that still bear a signature of the magnetic polarity of the era in which they were formed. This record suggests that the field has been active and dipolar—having two poles—through much of our planet’s history. The geological record also doesn’t show much evidence for major changes in the intensity of the ancient magnetic field over the past 4 billion years. A critical exception is in the Neoproterozoic Era, 0.5 to 1 billion years ago, where gaps in the intensity record and anomalous directions exist. Could this exception be explained by a major event like the solidification of the planet’s inner core? In order to address this question, Driscoll modeled the planet’s thermal history going back 4.5 billion years. His models indicate that the inner core should have begun to solidify around 650 million years ago. Using further 3-D dynamo simulations, which model the generation of magnetic field by turbulent fluid motions, Driscoll looked more carefully at the expected changes in the magnetic field over this period. “What I found was a surprising amount of variability,” Driscoll said. “These new models do not support the assumption of a stable dipole field at all times, contrary to what we’d previously believed.” His results showed that around 1 billion years ago, Earth could have transitioned from a modern-looking field, having a “strong” magnetic field with two opposite poles in the north and south of the planet, to having a “weak” magnetic field that fluctuated wildly in terms of intensity and direction and originated from several poles. Then, shortly after the predicted timing of the core solidification event, Driscoll’s dynamo simulations predict that Earth’s magnetic field transitioned back to a “strong,” two-pole one. “These findings could offer an explanation for the bizarre fluctuations in magnetic field direction seen in the geologic record around 600 to 700 million years ago,” Driscoll added. “And there are widespread implications for such dramatic field changes.” Overall, the findings have major implications for Earth’s thermal and magnetic history, particularly when it comes to how magnetic measurements are used to reconstruct continental motions and ancient climates. Driscoll’s modeling and simulations will have to be compared with future data gleaned from high quality magnetized rocks to assess the viability of the new hypothesis.
International Journal of Earth Sciences, Vol. 105, 8, pp. 2175-2201.
Africa, Zimbabwe
Geophysics - magnetics
Abstract: Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.
Abstract: The Central Indian region is having complex geology covering the Godavari Graben, the Bastar Craton (including the Chhattisgarh Basin), the Eastern Ghat Mobile Belt, the Mahanadi Graban and some part of the Deccan Trap, the Northern Singhbhum Orogen and the Eastern Dharwar Craton. The region is well covered by reconnaissance scale aeromagnetic data, analyzed for the estimation of basement and shallow anomalous magnetic sources depth using scaling spectral method. The shallow magnetic anomalies are found to vary from 1 to 3 km whereas magnetic basement depth values are found to vary from 2 to 7 km. The shallowest basement depth of 2 km corresponds to the Kanker granites, a part of the Bastar Craton, whereas deepest basement depth of 7 km is for the Godavari Basin and the southeastern part of the Eastern Ghat Mobile Belts near the Parvatipuram Bobbili fault. The estimated basement depth values correlate well with the values found from earlier geophysical studies. The earlier geophysical studies are limited to few tectonic units whereas our estimation provides detailed magnetic basement mapping in the region. The magnetic basement and shallow depth values in the region indicate complex tectonic, heterogeneity and intrusive bodies at different depth which can be attributed to different thermo-tectonic processes since Precambrian.
Geochemistry, Geophysics, Geosystems: G3, in press available
Technology
geophsyics - magnetics
Abstract: Remanent magnetization in geological samples may record the past intensity and direction of planetary magnetic fields. Traditionally, this magnetization is analyzed through measurements of the net magnetic moment of bulk millimeter to centimeter sized samples. However, geological samples are often mineralogically and texturally heterogeneous at submillimeter scales, with only a fraction of the ferromagnetic grains carrying the remanent magnetization of interest. Therefore, characterizing this magnetization in such cases requires a technique capable of imaging magnetic fields at fine spatial scales and with high sensitivity. To address this challenge, we developed a new instrument, based on nitrogenvacancy centers in diamond, which enables direct imaging of magnetic fields due to both remanent and induced magnetization, as well as optical imaging, of room-temperature geological samples with spatial resolution approaching the optical diffraction limit. We describe the operating principles of this device, which we call the quantum diamond microscope (QDM), and report its optimized image-area-normalized magnetic field sensitivity (20 µT?µm/Hz½), spatial resolution (5 µm), and field of view (4 mm), as well as trade-offs between these parameters. We also perform an absolute magnetic field calibration for the device in different modes of operation, including three-axis (vector) and single-axis (projective) magnetic field imaging. Finally, we use the QDM to obtain magnetic images of several terrestrial and meteoritic rock samples, demonstrating its ability to resolve spatially distinct populations of ferromagnetic carriers.
Abstract: The Central Indian region has a complex geology covering the Godavari Graben, the Bastar Craton (including the Chhattisgarh Basin), the Eastern Ghat Mobile Belt, the Mahanadi Graben and some part of the Deccan Trap, the northern Singhbhum Orogen and the eastern Dharwar Craton. The region is well covered by reconnaissance-scale aeromagnetic data, analysed for the estimation of basement and shallow anomalous magnetic sources depth using scaling spectral method. The shallow magnetic anomalies are found to vary from 1 to 3 km, whereas magnetic basement depth values are found to vary from 2 to 7 km. The shallowest basement depth of 2 km corresponds to the Kanker granites, a part of the Bastar Craton, whereas the deepest basement depth of 7 km is for the Godavari Basin and the southeastern part of the Eastern Ghat Mobile Belt near the Parvatipuram Bobbili fault. The estimated basement depth values correlate well with the values found from earlier geophysical studies. The earlier geophysical studies are limited to few tectonic units, whereas our estimation provides detailed magnetic basement mapping in the region. The magnetic basement and shallow depth values in the region indicate complex tectonic, heterogeneity, and intrusive bodies at different depths, which can be attributed to different thermo-tectonic processes since Precambrian.
Canadian Journal of Earth Sciences, Vol. 55, pp. 295-307.
Canada, Alberta
geophysics - electromagnetics, magnetotellurics
Abstract: The study of ancient plate boundaries can provide insights into the past and present-day tectonic processes. Here, we describe a magnetotellurics (MT) study of the Precambrian basement of the Hay River Fault (HRF) in northwest Alberta, which is the southwest segment of the Great Slave Lake shear zone. New broadband MT data were collected to give a clearer image of the crustal structure. The Western Canada Sedimentary Basin was imaged as a low-resistivity layer above the resistive crystalline basement. Four basement conductors were defined, and correlate with the terrane boundaries delineated with aeromagnetic data. These are (1) a major conductor in the Kiskatinaw domain, (2) a conductor on the boundary of the Ksituan and Chinchaga domains, (3) a conductor on the boundary of the Chinchaga and Buffalo Head domains, and (4) a conductor near the HRF. Both (1) and (2) correspond to areas of high seismic reflectivity. The low resistivity can be explained by interconnected grain boundary graphite or sulfide phases deposited by metamorphic fluid migration. The HRF was not definitively located in previous studies. The new data show that the HRF could be thin (1 km) or wide (10 km) and located at the boundary of the contrasting aeromagnetic anomalies or further to the north. Various tectonic processes are proposed to interpret the possible locations of the HRF. No electrical anisotropy structure is required to interpret the MT data in this study.
Abstract: Transition from a weak and erratic geomagnetic field to a more stable one around 560 million years ago, inferred from palaeomagnetic measurements, suggests that the inner core may have solidified around that time, much later than thought.
Journal of South American Earth Sciences, Vol. 94, 102203 14p. Pdf
South America, Brazil
geophysics - magnetics
Abstract: Accretionary orogens are considered as the result of the major crust production process, and terranes are accreted material representing distinctive assemblages regarding age and evolution. Scientific advances in the last years show that the southernmost São Francisco Craton includes Archean, Paleoproterozoic and Neoproterozoic areas. We used aerogeophysical data and field geology to understand the extension of individual pieces of this tectonic puzzle. We described five magnetometric lineaments. A-, B- and C-lineaments are related to dyke swarms of different ages. D- and E-lineaments represent regional-scale tectonic structures. In a tectonic perspective, we have identified the following terranes: (i) the Archean São Tiago crust (2.67?Ga), which is part of the São Francisco proto-craton; two Siderian juvenile arcs, represented by the (ii) Cassiterita (2.47?Ga) and the (iii) Resende Costa/Lagoa Dourada (2.36-2.35?Ga) orthogneisses; and two Rhyacian arcs, the (iv) juvenile Serrinha (2.22-2.20?Ga) and the continental Ritápolis (2.19-2.10?Ga) arcs. Tectonic terranes and five magnetic subdomains were identified on the basis of (i) association of shear zones/faults with quartz veins in the field; (ii) low- and high-intensity magnetic anomalies; (iii) sharp contrast in Euler solution intensities; and (iv) high-contrast in radioelement contents in the gammaspectrometric maps. Processing of aerogeophysical data permitted us to propose a new scenario on the evolution of the southern São Francisco Craton, and in particular of the Mineiro belt. The integration between aerogeophysical, new and compiled geologic information, provides a robust model for the understanding of individual tectonic pieces of the studied area.
Abstract: Dike swarms are igneous structures of continental expression accounting for major episodes of magmatism in igneous provinces, mantle plume heads, and continental breakup. In regional magnetic maps, dike swarms are recognized by high-amplitude lineaments indicative of lengthy and juxtaposed magnetized bodies. High-anomaly amplitudes from such tabular (2D) bodies tend to obscure lower amplitude contributions from localized 3D sources, representative of magmatic structures that once served as magma plumbing and storage. The recognition of such subtle signals with conventional filtering techniques is prevented due to spectral overlapping of individual contributions. We have developed a processing scheme to remove contributions from elongated, homogeneous sources to make clear contributions from 3D sources located below, in the middle of, or above a framework of elongated homogeneous sources. The canceling of 2D fields is accomplished by evaluating the horizontal component of the magnetic anomaly along the lineament strike, which for true elongated and homogeneous sources gives a null response. The gradient intensity of the transformed field is then evaluated to enhance residual fields over 3D sources. Lineaments thus removed identify tabular bodies with homogeneous magnetization, interpreted as being indicative of the uniform distribution (mineral type, concentration, and grain-size distribution) of magnetic carrier content in the rock. We evaluated our technique with synthetic data from multiple 2D-3D interfering sources and then applied it to interpret airborne data from the Ponta Grossa Dike Swarm of the Paraná-Etendeka Magmatic Province in Southeastern Brazil.
Abstract: Microscopic minerals excavated from an ancient outcrop of Jack Hills, in Western Australia, have been the subject of intense geological study, as they seem to bear traces of the Earth’s magnetic field reaching as far back as 4.2 billion years ago. That’s almost 1 billion years earlier than when the magnetic field was previously thought to originate, and nearly back to the time when the planet itself was formed. But as intriguing as this origin story may be, an MIT-led team has now found evidence to the contrary. In a paper published today in Science Advances, the team examined the same type of crystals, called zircons, excavated from the same outcrop, and have concluded that zircons they collected are unreliable as recorders of ancient magnetic fields. In other words, the jury is still out on whether the Earth’s magnetic field existed earlier than 3.5 billion years ago.
Abstract: Uncertainty exists in the configuration and extent of the Midcontinent Rift System (MRS) because of deficiencies in geophysical data and limited information from outcrops and basement drill holes. Additional ambiguity is caused by misunderstanding the definition of continental rifts. Six principal problematic regions in mapping the MRS are described. Gravity and magnetic data, supported by drill hole and seismic reflection data, show that the Eastern Lake Superior rift segment of the MRS continues south from Lake Superior and connects to a much narrower rift in northern Lake Michigan. The eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data, but is interpreted to occur near the U.S. - Canada border. The rift segment in southeastern Michigan intersects the Grenville Front and likely continues eastwards in modified form to near the boundary with Canada. Cross-cutting gravity and magnetic signatures may reflect Grenvillian overthrusts near the terminus of the MRS in Michigan. The proposed southerly extensions of both branches of the rift system into Oklahoma and Ohio are based primarily on positive gravity anomalies, but neither postulated extension appears to be associated with rifted troughs. Rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by Grenville orogenesis. Recent paleomagnetic investigations, in conjunction with high-resolution radiometric dating, imply that the MRS developed during the rapid southward movement of Laurentia during a quiescent period along its eastern continental margin. Massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin. The heated crust was made more ductile, fostering rifting due to extensional stresses. The Nipigon Embayment remains as a possible candidate for an early "third branch" of the MRS, but current evidence is insufficient to include the Fort Wayne "rift" as part of the MRS. Future studies of the MRS would be well-served by new age-dating and high-resolution seismic studies of the lithosphere.
Pure and Applied Physics, Vol. 177, pp. 5263-5274.
Mantle
geophysics - magnetics
Abstract: Theoretical magnetotelluric (MT) soundings are investigated for a stratified (five-layered) Earth model consisting of two transitional layers with conductivity varying linearly with depth, and three homogeneous layers with constant conductivity. The analytical expressions for the tangential electric and magnetic fields as well as the surface impedance are derived in terms of Airy functions. The effect of the thicknesses of the two transitional layers and the interlayer between them on the MT responses (apparent resistivity and impedance phase) is examined in detail.
GEM: International Journal on Geomathematics, open access 38p. Pdf
Mantle
geophysics - magnetics
Abstract: We discuss the resolving power of three geophysical imaging and inversion techniques, and their combination, for the reconstruction of material parameters in the Earth’s subsurface. The governing equations are those of Newton and Poisson for gravitational problems, the acoustic wave equation under Hookean elasticity for seismology, and the geodynamics equations of Stokes for incompressible steady-state flow in the mantle. The observables are the gravitational potential, the seismic displacement, and the surface velocity, all measured at the surface. The inversion parameters of interest are the mass density, the acoustic wave speed, and the viscosity. These systems of partial differential equations and their adjoints were implemented in a single Python code using the finite-element library FeNICS. To investigate the shape of the cost functions, we present a grid search in the parameter space for three end-member geological settings: a falling block, a subduction zone, and a mantle plume. The performance of a gradient-based inversion for each single observable separately, and in combination, is presented. We furthermore investigate the performance of a shape-optimizing inverse method, when the material is known, and an inversion that inverts for the material parameters of an anomaly with known shape.
Earth and Planetary Letters, Vol. 550, 116507, 11p. Pdf
Russia, Canada
geophysics - magnetics
Abstract: A simplified two-fold pattern of convection in the Earth's core is often used to explain the non-axisymmetric magnetic flux concentrations in the present day geomagnetic field. For large lateral variations in the lower mantle heat flux, however, a substantial east-west dichotomy in core convection may be expected. This study examines the effect of a large lateral variation in heat flux at the outer boundary in cylindrical annulus experiments that achieve approximate geostrophy of the convection as well as in rapidly rotating spherical shell simulations. In either geometry, the imposed boundary heat flux is derived from the seismic shear wave velocity in the lowermost mantle. The pattern of large-scale convection in the simulations closely follows that in the annulus experiments, which suggests that the lateral buoyancy at the equator essentially determines the structure of core convection. In particular, the location of a coherent downwelling that forms beneath Canada in mildly driven convection entirely switches over to the Siberian region in strongly driven states. Spherical dynamo models in turn show that this eastward migration of convection causes the relative instability or even the disappearance of the high-latitude magnetic flux in the Western hemisphere. Finally, large radial buoyancy causes homogenization of convection, which may place an upper bound for the Rayleigh number in the core.
Physics of the Earth and Planetary Interiors, doi.org/10.1016/ j.pepi.2020.106638 51p. Pdf
Mantle
geophysics - magnetics
Abstract: Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200-300?km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1?wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.
Abstract: Geophysical interpretation of potential field data plays an important role in the integration of geological data. Estimation of density and magnetic susceptibility variations within the upper crust helps evaluating the continuity of geological structures in the field. In the present study we use gravity and magnetic data in NW Amazonian Craton in Colombia. Total horizontal gradient of the reduction to magnetic pole were used to delineate magnetic lineaments and domains showing four zones, each with its own features. Multiscale edge detection (worming) of the data help delineate upper crustal structures that we interpret as tectonic boundaries that correlate with the four zones identified. 3D density and magnetic susceptibility inversion showed high density and/or high magnetic susceptibility sources correlated with these crustal structures. Zone (1) is located south of the Guaviare River, with predominant NW-SE and NE-SW magnetic lineaments; zone (2), located from south of the Guaviare River to the north, present nearly E-W magnetic lineaments and a deep E-W edge interpreted as a possible shear zone parallel to Guaviare, Orinoco and Ventuari rivers; zone (3) from south of the Vichada River to the north, with NE-SW and NW-SE lineaments; N-S zone (4) cuts the zones (2) and (3), characterized by high density/magnetic susceptibility source bounded by N-S deep edges. A more complete tectonic evolution interpretation requires further work, but we speculate that the zone (4) could indicate an aborted rift/collision suture and that the zone (2) is indicative of a younger deformation event. Shear direction at (2) is not clear: geological maps show NEE-SWW right-lateral faulting, but geophysical anomalies suggest left-lateral displacement, highlighted by left dislocation of the Orinoco River. We also speculate that a N-S edge located at the SE of the area can be related with the Atabapo Belt and the limit of Ventuari-Tapajós and Rionegro geochronological provinces.
Journal of African Earth Sciences, Vol. 179, 104204, 17p. Pdf
Africa
EMAG2
Abstract: Data from the Earth Gravitational Model (EGM2008) and the Earth Magnetic Anomaly Grid (EMAG2) were used to develop a continental scale crustal thickness model for Africa, and to estimate the depth to the bottom of the magnetic layer (DBML) and the geothermal gradient and heat flow. The results are: (1) the estimated DBML from the magnetic data varies from ~23.0 to ~37.2 km. The shallowest DBML values are located in the northern, eastern, and western parts of the continent, whereas the deepest values are observed in the central and southern regions. (2) The estimated crustal thickness based on gravity data varies from ~29.9 km in the northern and western parts of Africa to ~48.0 km in its southern regions, with an average thickness of 35.1 km for the whole continent. (3) The estimated heat flow varies between high values of 46-59 mW/m2, observed in the northern, eastern, and western regions to low values of ~< 41 mW/m2, observed in the central and southern parts of the continent. (4) The geothermal gradient values vary between 14.5 and 23.6 °C/km (5) The East African rift zone is underlain by shallow DBML characterized by high heat flow values that vary between 42 and 59 mW/m2 (6) The heat flow anomalies in Egypt and Libya may be associated with the zone of the Pelusium megashear system, and it shows heat flow values that vary between 36.3 and 59.0 mW/m2. The current study has taken advantage of the availability of the EGM2008 and EMAG2 datasets to map crustal thickness variations and DBML beneath the continental landmass of Africa.
Abstract: The development of sophisticated sample environments to control temperature, pressure, and magnetic field has grown in parallel with neutron source and instrumentation development. High-pressure apparatus, with high- and low-temperature capability, novel designs for diamond cells, and large volume presses are matched with next-generation neutron sources and moderator designs to provide unprecedented neutron beam brightness. Recent developments in sample environments are expanding the pressure-temperature space accessible to neutron scattering experiments. Researchers are using new capabilities and an increased understanding of the fundamentals of structural and magnetic transitions to explore new territories, including hydrogenous minerals (e.g., ices and hydrates) and magnetic structural phase diagrams.
Proceedings of the VIII International Scientific Conference "Geophysics and Geodynamics" held Lviv, Ukraine., 5p. Pdf
Europe, Israel
geophysics - magnetics
Abstract: The eastern Mediterranean is a tectonically complex region evolving in the long term located in the midst of the progressive Afro-Eurasian collision. Despite years of investigation, its geological-geophysical structure is not completely known. At the same time, the recent discovery of large gas deposits has attracted the attention of many researchers to this region. For instance, the latest U. S. Geological Survey estimates using conventional assessment methodology suggest that there are on the order of 1.7 billion barrels of recoverable oil and more than 4 trillion m3 of recoverable gas in the Levant Basin [1]. This highlights the need for analysis of the paleogeographical conditions that can yield deep paleotectonic criteria for oil and gas discovery in this region. For this purpose, isopach maps of the Middle-Upper Jurassic and Lower Cretaceous were generated from detailed examinations of numerous well sections and the most sig- nificant outcroppings in the eastern Mediterranean. The maps confirm an earlier model of continental accretion [2]. In particular, abrupt changes in the trend and thickness of the Early Mesozoic formations coincide with the terrane boundaries. These compiled isopach maps also pinpoint significant distinctions between the Arabian and Sinai plates on the one hand and the Syrian arc on the other. A new tectonic map of the eastern Mediterranean is presented that first of all integrates geophysical satellite-derived gravity and airborne magnetic fields, as well as tectonic-structural, paleo-geographical and facial analyses. The results have clear implications for hydrocarbon prospecting in this region.
Journal of Geochemical Exploration, Vol. 23, 13p. Pdf
Africa, South Africa
geophysics - magnetics
Abstract: In this study, the southern margin of the Archean Kaapvaal Craton in South Africa is selected to investigate the occurrences of potential kimberlite bodies. Ground magnetic survey was conducted to identify potential targets for further exploration. Euler 3D deconvolution and Keating correlation coefficients are the processing techniques that were applied to the ground magnetic data to identify geological features that correspond to kimberlite models. These techniques revealed several potential targets that resemble geologic models of kimberlite bodies. Seven of these targets (Targets A, B, C, D, E, F and G), spatially cluster along or at the intersections of linear structures, which is consistent with one of the primary geological conditions that control the emplacement of kimberlite bodies. A follow-up soil sampling and analysis was conducted to assess the geochemistry of soils at the seven targets. The major element geochemistry of soil samples suggest that Target B possesses residual soils that originated from kimberlite intrusion, while the remaining six samples show geochemical characteristics of soils derived from crustal rocks. The composition of compatible (Ni, Cr, V, Co, Cu and Zn) and incompatible (Nb, Zr, Sr, Ce, and Ba) trace elements of Target B shows similarity with the known Group 1 Kimberlites locally and globally, thus confirming that Target B most probably belongs to Group 1 Kimberlite petrological clan. This, however, does not preclude the possibility of modification of kimberlite chemistry by crustal assimilation during ascent and post-emplacement processes such as weathering as demonstrated by elevated concentrations of SiO2, Al2O3 and Rb and low content of MgO. In light of this, Target B may be interpreted as a potentially contaminated and weathered Group 1 Kimberlite pipe which intruded the Karoo Supergroup at the intersection of linear structures. This finding is consistent with the presence of large number of known diamondiferous kimberlites in this region, which post-date the formation of the Karoo Supergroup.
Abstract: The Orosirian paleopoles from the three circum-Slave basins (i.e., the Great Slave, Coronation, and Kilohigok Basins) of the Slave craton show large (?110°) and back-and-forth swings at 1.96-1.87 Ga, known as the Coronation loops. The Coronation loops, taken at face value, would imply rapid and substantial spin motions of the Slave craton, which is at odds with modern plate tectonics. Alternatively, the Coronation loops have been interpreted as a product of basin-scale rotations, local-scale vertical-axis rotations, or inertial interchange true polar wander (IITPW). One way to differentiate these models is to take advantage of the well-correlated stratigraphy in three circum-Slave basins and directly compare the time-equivalent paleomagnetic results. In this study, we collected ?300 samples from nine formations from the Goulburn Supergroup of the Kilohigok Basin, in shallowly dipping autochthonous sections east of the Bathurst Fault. We provide seven new reconnaissance-level paleopoles of the Slave craton, namely from the Kenyon, Hackett, Rifle, Beechey, Link, Kuuvik, and Brown Sound Formations of the Goulburn Supergroup. Our results and the compiled Orosirian paleomagnetic data of the Slave craton suggest that although basin-scale rotation or local vertical-axis rotation in fault zones are able to explain some of the disagreements among time-correlative paleopoles, they could not account for the large declination variation within the homoclinal sections in individual basins. Notably, our results from the ?1963 Ma Rifle Formation show progressive changes in declination through the stratigraphy, which cannot be explained by either basin-scale or local vertical-axis rotations. Selective remagnetization is also considered unlikely to be the cause. Instead, we suggest that IITPW could potentially be responsible for the Coronation loops, which could also provide an explanation for some discrepant paleomagnetic data observed globally during the Orosirian time.
Abstract: Magnetic monopoles play a central role in various areas of fundamental physics, ranging from electromagnetism to topological states of matter. While their observation is elusive in high-energy physics, monopole sources of artificial gauge fields have been recently identified in synthetic matter. String theory, a potentially unifying framework that encompasses quantum mechanics, promotes the conventional \emph{vector} gauge fields of electrodynamics to \emph{tensor} gauge fields, and predicts the existence of more exotic \emph{tensor monopoles} in 4D space. Here we report on the characterization of a tensor monopole synthesized in a 4D parameter space by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we characterize the tensor monopole by measuring its quantized topological charge and its emanating Kalb-Ramond field. By introducing a fictitious external field that breaks chiral symmetry, we further observe an intriguing transition in the spectrum, characterized by spectral rings protected by mirror symmetries. Our work represents the first detection of tensor monopoles in a solid-state system and opens up the possibility of emulating exotic topological structures inspired by string theory.
Abstract: The eastern Mediterranean is a tectonically complex region evolving in the long term located in the midst of the progressive Afro-Eurasian collision. Despite years of investigation, its geological-geophysical structure is not completely known. At the same time, the recent discovery of large gas deposits has attracted the attention of many researchers to this region. For instance, the latest U. S. Geological Survey estimates using conventional assessment methodology suggest that there are on the order of 1.7 billion barrels of recoverable oil and more than 4 trillion m3 of recoverable gas in the Levant Basin [1]. This highlights the need for analysis of the paleogeographical conditions that can yield deep paleotectonic criteria for oil and gas discovery in this region. For this purpose, isopach maps of the Middle-Upper Jurassic and Lower Cretaceous were generated from detailed examinations of numerous well sections and the most significant outcroppings in the eastern Mediterranean. The maps confirm an earlier model of continental accretion [2]. In particular, abrupt changes in the trend and thickness of the Early Mesozoic formations coincide with the terrane boundaries. These compiled isopach maps also pinpoint significant distinctions between the Arabian and Sinai plates on the one hand and the Syrian arc on the other. A new tectonic map of the eastern Mediterranean is presented that first of all integrates geophysical satellite-derived gravity and airborne magnetic fields, as well as tectonic-structural, paleogeographical and facial analyses. The results have clear implications for hydrocarbon prospecting in this region.
