Local Magnetic Anomalies Research Articles

Palaeointensity study of the Oshima 1986 lava in Japan: implications for the reliability of the Thellier and LTD-DHT Shaw methods

We have performed a palaeointensity study of the Oshima 1986 lava in order to examine the reliability of the Thellier and LTD-DHT Shaw methods with special reference to high-temperature oxidation states of magnetic grains. Coe’s version of the Thellier method was applied to fifteen specimens from five block samples. The palaeointensities determined by this Thellier method ranged from 44.8 to 59.0 μT with an average of 51.0 ± 4.1 μT ( N = 15). They are systematically higher than the expected intensity (45.5 μT from the DGRF 1985). Since the in situ magnetic survey at the sampling site indicates no strong local magnetic anomaly, these high palaeointensities are overestimation, probably due to some rock magnetic property and/or some thermal alteration during the experiments. Microscopic observations show that the samples, where some titanomagnetites surrounding vesicles were classified with intermediate oxidation indices of III–V, gave 10–30% higher palaeointensity values. Therefore, the erroneously high Thellier palaeointensities have a relation to the high-temperature oxidation state. For eight of the fifteen specimens used in the Thellier experiments, the applied DC field was perpendicular to the primary NRM component for quantitative evaluation of laboratory-induced chemical remanent magnetisation (CRM). Directional changes of NRM thermally demagnetised during the Thellier experiments indicate that most of the samples more or less acquired CRM due to laboratory heating suggesting that some magnetic phase with high blocking temperatures was produced. For the specimens of intermediate oxidation level, the CRM appears to have been gradually acquired at relatively low temperatures (≤400 °C). It would appear to be difficult to detect this gradual thermal alteration with the standard pTRM check, weak-field susceptibility measurements and hysteresis parameter changes. We also applied the double heating technique of the Shaw method combined with low temperature demagnetisation (LTD-DHT Shaw method) to seven specimens from the same block samples. The palaeointensities determined by the LTD-DHT Shaw method yielded an average of 46.4 ± 4.7 μT ( N = 6), which agrees with the expected intensity indicating no dependence on the high-temperature oxidation states.

Radio emission from mini-magnetospheres on the Moon

[1] Small-scale, localized magnetic anomalies on the surface of planetary satellites effectively create “mini-magnetospheres”, which can lead to standoff shocks analogous to planetary bow shocks. This gives rise to the tantalizing possibility that radio emission can be produced in the vicinity of such mini-magnetospheres, since it is well known that radio emissions are directly associated with the foreshock regions of planetary bow shocks and travelling interplanetary shocks. In this letter, we generalize a theoretical model for foreshock radio emissions in order to determine whether observable radio fluxes can also be generated near mini-magnetospheres. We show that, when applied specifically to the Moon, this model predicts that mini-magnetospheres should produce radio fluxes above current instrument sensitivity limits. Moreover, the predicted strong dependence of the radio emission on the upstream plasma properties may have important implications for the model's applicability to other planetary satellites.

Stress-Induced Changes in the Electrical Conductivity of Igneous Rocks and the Generation of Ground Currents

If we can ever hope to understand the non-seismic signals that the Earth sends out before major earthquakes, we need to understand the physics of rocks under increased levels of stress. In particular we need to understand the generation of electrical currents in the ground. We have begun to study how electrical conductivity of igneous rocks changes under stress and what types of charge carriers are involved. We show that quartz-rich granite and quartz-free anorthosite both generate electronic charge carriers when subjected to stress. The charge carriers are positive holes (p-holes), i.e., defect electrons on the oxygen anion sublattice. They spread out of the stressed rock volume, the “source volume”, into the surrounding unstressed rocks. Time-varying ground currents are required to generate pre-earthquake local magnetic field anomalies and low-frequency electromagnetic emissions. We posit that stress-induced activation of p-hole charge carriers and their outflow from the source volume is the basic process by which ground currents can be generated in the Earth’s crust. We propose that the arrival of p-holes at the Earth’s surface leads to changes in the ground potential that may induce ionospheric perturbations. We further propose that the build-up of high electric fields at the ground surface can ionize the air, hence cause ion emission and corona discharges. When p-holes recombine at the ground surface, they are expected to form vibrationally highly excited 0-0 bonds. The de-excitation of these 0-0 bonds will lead to stimulated mid-IR emission, which may explain the reported pre-earthquake “thermal anomalies” identified in satellite images.

RESEARCH REPORT: MAGNETIC INVESTIGATIONS AND THE AGE OF A MEDIEVAL KILN AT KUNGAHÄLLA (SOUTH‐WEST SWEDEN)*

The results from a magnetic survey and archaeomagnetic investigation of a medieval brick kiln at Kungahälla in Bohuslän (south‐west Sweden) are reported. Detailed magnetic total field and magnetic gradient measurements over known traces of the kiln showed marked local magnetic anomalies of up to 200 nT, revealing the rectangular shape of the kiln remains. Palaeomagnetic investigations of the remanent magnetization of 12 bricks from the kiln floor gave precise definitions of both the direction of the archaeomagnetic field [(Dm, Im) = (66.7°, 8.8°), k = 655, α95= 1.7°] and the palaeointensity [Bm= 69.0 ± 3.6 µT]. An archaeomagnetic date is obtained by comparing the direction of the archaeomagnetic field found in the kiln with a geocentral dipole‐transformation of the British master‐curve for secular variation. Of two possible archaeomagnetic dates (ad 1280 ± 50 or ad 1480 ± 50), the ad 1280 ± 50 age is in good accordance with the C‐14 ages and archaeological dates as well as with direct historical evidence, whereas a c. 100 years younger TL date appears to be too young.

Moon‐related nonthermal ions observed by Nozomi: Species, sources, and generation mechanisms

The 3‐D velocity distribution functions of nonthermal ions observed in the vicinity of the Moon have been analyzed to understand the interaction between the solar wind and the Moon. The observation was made during the lunar swing‐by of the Nozomi spacecraft, and the closest approach was about 1.6 RL from the Moon's surface. Both Nozomi and the Moon were located in the solar wind, and nonthermal ions were observed only when the spacecraft was very close to the Moon. The nonthermal ions were measured by a Particle Spectrum Analyzer/Ion Spectrum Analyzer (PSA/ISA) on board the Nozomi. After analyses and careful examination of the 3‐D velocity distribution of the nonthermal ions, the characteristics of the nonthermal ions were revealed as follows: The nonthermal ions have a partial ring structure in the phase space, they are protons, their source location is the dayside of the Moon, and they have large velocities when they are generated. We propose the following scenario of a solar wind interaction with the Moon: The electromagnetic field in the vicinity of the Moon must have a dynamic structure, possibly a miniature bow shock associated with a local magnetic anomaly, where some of the solar wind protons are deflected violating their first adiabatic invariant. After the deflection, they move under the force of the convection electric field and gyrate around the magnetic field in the solar wind. This motion forms a partial ring structure with large initial velocities in the velocity phase space.

Magnetic Position Determination by Homing Pigeons?

How pigeons return home from unfamiliar release sites is a long-standing puzzle in animal behaviour. Walker (1998, 1999) has described a “vector summation model” which “identifies a novel coordinate that pigeons could use with magnetic total intensity to determine position”. The model is not applicable in a magnetic field generated simply by a geocentric dipole, but requires a field perturbed by higher-order sources. Tests are devised to simulate the addition of both regional and local magnetic anomalies to a geocentric dipole field, and to calculate the directions of the home loft from a number of release sites. The results indicate that a pigeon would be unlikely to derive useful information from the model.

Aeromagnetic And Aeroradiometric Response To Hydrothermal Alteration

In Precambrian terrains all regional and most local intensive magnetic anomalies are produced by magnetite. Monoclinic pyrrhotite is responsible for some local, but often intensive, magnetic anomaly patterns. Both magnetite and pyrrhotite are affected by hydrothermal alteration processes in various ways, resulting in changes either in abundance or in grain fabric. These changes are recorded in the magnetic properties of the altered rock units and reflected in their aeromagnetic signatures. Hydrothermal alteration in deformed bedrock zones is commonly controlled by structural or tectonic features. Regional high-resolution aerogeophysical surveys can be utilized, in both re- gional and detailed investigations, to map the overall geological and tectonic setting or to estimate local changes in magnetic mineralogy and the relative abundance of radionuclides. Magnetite is most commonly destroyed in alteration processes, such as biotitization, carbonation, sulfidization and silicification. The progressive destruction of magnetite begins at grain margins and results first in broken and cracked grain texture and smaller grain size, then progresses to total disap- pearance of magnetite. Alteration in magnetite- bearing rock units may be recognized by decreased magnetic intensity and by the broken, disrupted magnetic pattern. The abundance of monoclinic pyrrhotite is enhanced by reducing hydrothermal fluids, and typical crystal anisotropy is developed due to tectonic stress. The relative contents of radioelements are changed in the same hydrothermal processes and partly for the same reasons as the ferrimagnetic minerals. Potassic alteration often results in elevated K radiation particularly for mafic rocks, and then anomalous K/Th ratios along local shear or fracture zones may be indicative of gold-bearing mineralization. On the other hand, high U/Th ratios within metasedimentary units may point out prospects for sulphidization. Although variation of U/Th ra- tios largely reflects the environmental conditions during primary diagenesis or a later deformational phase, mainly the decrease in Th radiation close to sulphide mineralization seems to be responsible for the elevated U/Th ratios.

Magnetic data processing for hydrocarbon exploration in the Pannonian Basin, Yugoslavia

Because magnetic minerals may directly indicate the presence of oil and gas deposits, magnetic methods are applied to hydrocarbon exploration in oil‐bearing sedimentary basins. The basic problem in applying these methods is the isolation of weak magnetic anomalies sourced by low concentrations of the magnetic minerals present. These weak anomalies are often masked by much stronger magnetic anomalies caused by underlying magnetic rocks and/or by rocks in the basin sediments. Weak local anomalies can efficiently be isolated by applying selective 1‐D digital frequency filters. The method of filtering has been checked by data obtained using simple models of magnetic sources and using a model representative of the local geology in our study area in the southern Pannonian basin, Yugoslavia, The magnetic field frequency content was analyzed by applying the power spectral density estimation, using the maximum entropy method. The digital filters were designed using the window function method. The best results were obtained by the Kaiser window function for the chosen range of the band‐pass filter. In our study area, me isolated local magnetic anomalies have amplitudes of ±10 nT and trend in an east‐west direction parallel to the predominant structural grain. These anomalies correlate very well with the known oil and gas fields. As an example, filter processing of magnetic anomaly data, combined with the 3‐D seismic data gained in the filtered magnetic field, correlate well with one of the oil fields. The next phase of the project will concentrate on the anomalies occurring outside the established gas and oil fields.

On the smooth surface of the northern bow shock of Mars, based on Mars Global Surveyor's measurements

A very smooth and time-invariable bow shock of Mars is revealed using Mars Global Surveyor's data. The bow-shock position has been identified using magnetic and electron flux data obtained by the Magnetometer and Electron Reflectometer (MAG/ER) experiment aboard Mars Global Surveyor, in the time period between days 87 and 255 of 1998. From the magnetic field and the electron flux measurements, 148 bow-shock crossings were detected, concentrated mostly on the northern hemisphere of the planet. With these results, a 3D configuration of the bow shock is constructed and presented. The results show that part of the observed bow shock is a surprisingly smooth surface. It is possible that the bow shock is smooth only in the northern hemisphere, since the southern surface is characterized by local magnetic anomalies. Its real shape can only be revealed in a 3D representation in the planetary centered solar ecliptic coordinate system and questions the theoretically expected variation of the bow shock.

New directions in archaeomagnetism

Since the first magnetic analyses of archaeological materials were carried out over a century ago, archaeomagnetic reference curves are now available covering the last few millennia. It would seem to be an appropriate time to examine the archaeomagnetic record to see how it can be improved. For directional studies the disturbing factors include magnetic refraction, mechanical deformation, local magnetic field anomalies, and magnetic anisotropy. In the complex field of archaeointensity determination there is a real need for faster and more reliable methods. The use of sediments on Palaeolithic sites will be increasingly important for the dating of early hominids.

Paleointensity record in zero-age submarine basalt glasses: testing a new dating technique for recent MORBs

Thellier–Thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows. Two were erupted along the Juan de Fuca ridge at around 46°N and one along the East Pacific Rise South at around 18°S. The within-sample dispersion of paleointensity results from the ‘Animal Farm’ flow (EPR south) is very low and leads to a well-defined mean value of 35.6±1 μT (95% error on the mean) based on 11 glass chips from four independent samples. Today’s geomagnetic field intensity in the area is 31.2 μT. Comparing Animal Farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 A.D. and 1950 A.D. (taking into account different sources of errors). This is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid-ocean ridge basalts (MORBs). However in the Juan de Fuca area results show a more erratic pattern with samples varying by up to 30% higher and lower from the expected value of about 55 μT. The dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit. Local magnetic anomalies should thus always be checked when doing paleointensity on MORB samples which should also be distributed as widely as possible in a flow unit. When no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the EPR axis during the last several hundred years.

The plasma environment of Mars: from the shocked solar wind down to the ionosphere

Despite the great number of missions devoted to the exploration of the planet Mars (including: Mariner 4, in 1965; Mars 2, 3, & 5, from 1971 to 1974; Viking landers, in 1976; and Phobos 2, in 1989), actually little is known about the near-Mars plasma environment and its interaction with the interplanetary medium. Mars Global Surveyor has recently confirmed that no intrinsic magnetic field could be invoked to stand off significantly the solar wind. Nevertheless, complex localized magnetic anomalies have been reported. They could be the signature of a past intrinsic field fossilized in rocks. It has long been recognized that the Mars–solar wind interaction strongly depends on solar activity, solar zenith angle, and altitude, as well. Unfortunately, the limited space and time coverages of the observations do not allow us to characterize the full nature of interaction. The solar wind pressure is thought to be balanced by the ionosphere and/or upper atmosphere, in analogy with Venus and comets. While the atmosphere of Venus is 10 000 times denser than that of Mars near the surface, the densities of the neutral atmospheres of the two planets are indeed of the same order of magnitude at altitudes of 250– 450 km . In addition, mass loading of the solar wind by the ionization of suprathermal atmospheric neutrals is likely to play a role comparable to a comet one. Similar to Venus there is evidence that there are important inhomogeneities inside and outside the Martian ionosphere (plasma clouds and holes) and it is likely that there is also a strong axial asymmetry in the plasma distribution around the wake. Erosion processes produced by the solar wind are claimed to be important in the quantification of the Mars atmospheric mass budget. Frictions exerted on the wake planetosphere plasma have been observed. They should not be restricted to the outermost region and might also occur deep inside so that the amount of volatiles lost in the past through this phenomenon could be important.

Effects of seamounts on an interplate earthquake at the Suruga trough, Japan

Detailed magnetic surveys carried out in the Suruga trough area, where the Philippine Sea plate subducts beneath Japan, revealed some local magnetic anomalies on the overriding plate side. We first show that these anomalies reflect seamounts sitting on and subducting with the Philippine Sea plate, since no magnetic sources corresponding to the anomalies are identified in the seafloor topography. An important issue to be addressed is how these seamounts affect the generation of a great interplate earthquake at the Suruga trough (often referred to as the Tokai earthquake). So we made a numerical simulation of earthquake generation, applying the rate- and state-variable friction law to the plate interface. Here we incorporated the seamounts in the simulation as an additional normal stress exerted locally to the interface. We then found how the seamounts affect the generation of an interplate earthquake depends on whether or not they are located in the seismogenic zone. The most notable effect is that the amount of slip immediately before the main shock, often called a pre-slip, tends to be enhanced if they are located inside the zone.

Oceanic basement structure, sediment thickness, and heat flow near Hole 504B

A new seismic reflection survey around Hole 504B, the deepest borehole in ocean crust, reveals active faulting, possible volcanic centers, and a lateral change in the relationship of heat flow and basement structure near the borehole. Migration of single channel and multichannel seismic profiles collected in a 25 by 25 km grid with a 1 km line spacing significantly improved the resolution of basement structure and sediment thickness. West of Hole 504B, heat flow is high above east‐west lineated basement ridges, whereas heat flow to the east is normal above ridges and high above two buried basement knolls. The difference is probably due to lateral variations in sediment thickness. Small, buried basement knolls are common and may have been point sources for lava flows. Hole 504B lies in a flat‐floored basin that slopes gently upward to the west. A recently active fault 1.1–1.2 km south of Hole 504B is indicated by sediment reflector discontinuities that extend up to the seafloor. The fault strikes east‐west and crosses a buried volcanic knoll where Holes 678B and 896A were drilled. Regionally, basement relief north of Hole 504B is 100 to 150 m lower than to the south, which we attribute to an increased spreading rate obtained from dating published local magnetic anomaly patterns with a recent timescale. We find at least five graben structures resembling failed rifts which may have formed in response to asymmetric spreading or to the change in tectonic stress accompanying the spreading rate change. South facing scarps on basement ridges are as common as north facing scarps. Sediment thickness is highly correlated to basement depth due to preferential deposition in topographic lows when the crust was 1–2 Ma old and to later winnowing.

Tectonomagnetic field observations in central Italy 1989–1995

It has been reported that earthquakes and volcanic eruptions produce significant variations in the geomagnetic field. To investigate possible tectonomagnetic effects related to seismic activity, the Istituto Nazionale di Geofisica installed a seismomagnetic network in central Italy; the area was selected after a magnetic noise survey. Geomagnetic field intensity data are simultaneously recorded at four magnetometer stations and at L'Aquila Geomagnetic Observatory, which is located in the same area. The data are differentiated in order to detect local magnetic field anomalies. The present study reports geomagnetic field observations in central Italy from July 1989 to March 1995 and also discusses the detectability of a tectonomagnetic effect in this area. A variation of about 5 nT over a two-month period (January–February 1990) has been observed in the geomagnetic field. No other anomaly has been observed, although moderate seismicity occurred in central Italy during the study period.

The reliability of aeroplane attitude determination using the main geomagnetic field with application to tensor VLF data analysis

The attitude angles of an aeroplane used for geophysical measurements are necessary in order to calculate field components in a fixed reference frame. In this paper it is shown analytically how the measurement of static magnetic field components can be used to determine the attitude angles. Error analysis shows that when magnetic anomalies are small to moderate (less than 5% of the total field), the attitude angles can be determined to within a few degrees at high latitudes. For a given area the maximum error is linearly related to the magnitude of the anomaly.The technique is illustrated on a tensor VLF data set from an area in Sweden with local magnetic anomalies less than 1% of the regional total magnetic field. Attitude errors propagated into tensor VLF transfer functions are of the same order as their random errors.

Magnetic petrology of lower crust and upper mantle xenoliths from McMurdo Sound, Antarctica

A suite of xenoliths was collected from Ross Island in the McMurdo Sound area, Antarctica. The purpose of the study of these xenoliths was to determine the level of magnetization in lower crust and upper mantle lithologies and to discover the mineralogical basis for the magnetization records. The majority of xenoliths are pyroxene granulites of mafic composition; a few hornblendites occur, and upper mantle dunites are fairly common. The primary mineral assemblage of the pyroxene granulites is plagioclase + calcic pyroxene ± orthopyroxene ± olivine ± ilmenite; mineral stability relations together with two-pyroxene geothermometry indicate derivation from depths of 12 to 20 km. The pyroxene granulites show evidence of partial melting, presumably related to incorporation of the xenoliths in the host basaltic magmas. Prior to their ascent to the surface, many underwent metamorphic reactions producing secondary hornblende, biotite and titanomagnetite. On the whole, the Antarctic xenoliths are weakly magnetic, but a few ilmenite-rich pyroxene granulites have NRM > 40 × 10 −4 A m 2/kg and X o > 0.005 SI units. The latter contain 1–3% titanomagnetite, which is largely of secondary origin and occurs chiefly as tiny quench crystals concentrated in fine-grained melt areas and as coronas about primary ilmenite crystals. Oxidation of Fe-Ti oxides is pervasive as revealed by the frequent occurrence of wispy ferrian rutile lamellae in ilmenite and trellis-type ilmenite oxyexsolution lamellae in titanomagnetite. The oxidized nature of the samples is further indicated by the presence of hematite and pseudobrookite-titanohematite intergrowths. The oxidation has caused a shift in titanomagnetite toward Fe 3O 4-rich compositions and in ilmenite toward higher Fe 2O 3. As a consequence, the pyroxene granulites exhibit thermal unblocking in the vicinity of 500–570°C. The high geothermal gradient, characteristic of a continental rift setting, argues for about 12 km of magnetic crust before the Curie point is exceeded. The primary oxide of consequence in this rift environment appears to be ilmenite, which may be a common lower crust oxide mineral in this type of tectonic setting. No evidence was found in thermomagnetic experiments or in petrographic studies to suggest the presence of native iron. The secondary origin of much of the titanomagnetite now present suggests that unaltered, in situ granulite lower crust in this area would be weakly magnetic even in a situation where the geothermal gradient was lower or where a tectonic reconfiguration would render the lower crust above the Curie geotherm. The strongest magnetizations reside in Cenozoic volcanic cap rocks, and these are expected to be the major source of local magnetic anomalies in the area.

Paleomagnetic correlation of Late Quaternary lava flows in the lower east rift zone of Kilauea Volcano, Hawaii

Paleomagnetic data reflecting paleosecular variation (PSV) of the geomagnetic field are used to correlate individual lava flows in the lower east rift zone of Kilauea Volcano. This area has been recently mapped based on petrography, stratigraphy, soil development, and degree of weathering of the flows; eight 14C dates also provide a framework for the relative age assignments. Site‐mean directions of remanent magnetization for historic flows in this region indicate that local magnetic anomalies do not mask the PSV signal, that within‐flow angular differences are generally <5°, and that temporal resolution of the paleomagnetic directions is of the order of 100 years. The paleomagnetic correlations do not conflict with the observed stratigraphic relationships, and PSV reference curves from dated lava flows and lake sediments have also been used to help determine the sequence of directional groups. Paleomagnetic grouping of flows into eruptive events indicates a different and perhaps more refined eruptive history for the lower east rift zone than implied by the geologic mapping alone. The number of individual flows based on petrographic distinctions is likely the same, but the frequency of events (including petrographically distinct flows) is apparently lower than previously thought. Flows from ridge crest vents bifurcated by the rift indicate formation of the central graben in this region after ∼300 years ago, possibly related to the 1790 eruptions along dual fractures.

Local Magnetic Anomalies Associated with Active Faults in Beppu Bay, Southwest Japan.

Many examples of local magnetic anomalies in the vicinity of active faults have been reported, although the association of magnetic anomalies with active faults has not been well established. Aiming at studying local magnetic anomalies, we made magnetic surveys in Beppu Bay, Southwest Japan, where many active faults have been found through acoustic sounding. Along individual survey profiles, we found many magnetic anomalies seemingly associated with active faults, but in the contour maps of the geomagnetic total intensity and the shaded relief maps, no clear lineation-like feature could be identified. Profile data could be interpreted in terms of either a model of spatial magnetization variation or a model of magnetic basement undulation. The former model implies the correspondence of a low magnetization zone to a fractured zone, whereas the latter model suggests the depression of magnetic basement.

Mise en évidence de corps d'origine cosmique probable au sein de la formation de Carnot (République Centrafricaine)

Two independent investigations, made on the Mesozoic fluvial Carnot Formation, yielded magnetic microspherules in borehole residues and a localized (270 m long) magnetic anomaly profile. The spherules are ferrous with sometimes traces of Al, Ti and Mn; their morphology is similar to the morphology of cosmic spherules (with mosaic, octahedral, spongy or with “Astrakhan coat”-like fabrics). The magnetic anomaly is related to a body buried at approximately 30–40 metres, i.e., in the Cretaceous Carnot Formation. This body could be modelled by a sphere or by an assemblage of dimensional polygons, with strong remanent magnetization. This indicates that the anomaly source has undergone a fusion and cooling process under the Cretaceous geomagnetic field. As there is no evidence of volcanic activity, this process is considered to be related to meteoritic falling. The magnetic spherules and the source of magnetic anomaly correspond to micrometeorites and meteorites which fell in the west of Central African Republic between Albian and Maastrichtian times. However, without geochronological data, it is not possible to indicate if the two perturbations are the result of the same extraterrestrial event or of two different events.