P. J. Wasilewski

The Global Magnetic Field of Mars and Implications for Crustal Evolution

The Mars Global Surveyor spacecraft obtained globally-distributed vector magnetic field measurements approximately 400 km above the surface of Mars. These have been compiled to produce the first complete global magnetic field maps of Mars. Crustal magnetization appears dichotomized, with intense magnetization mainly confined to the ancient, heavily cratered highlands in the south. The global distribution of sources is consistent with a reversing dynamo that halted early in Mars evolution. Intense crustal magnetization requires an increased oxidation state relative to mantle-derived rock, consistent with assimilation of an aqueous component at crustal depths.

The Moho as a magnetic boundary revisited

We have re-examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non-magnetic mantle. Whereas the original conclusion was based on limited data, the present study is based on about 400 globally-distributed xenolith samples. While it is now clear that the “Moho” represents a maximum velocity gradient in a complex transition from crust to mantle, and while crustal magnetization is highly variable, the original concept of a “magnetic crust” overlying a non-magnetic mantle is strongly supported by the new data. A large upper mantle xenolith suite of ultramafic composition is characterized by non-magnetic chrome spinels and magnesian ilmenites. Such rocks are magnetic only where demonstrably altered. The most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature. Prograded rocks have characteristic magnetite Curie points, and are strongly magnetic, especially the more mafic compositions, while the mafic rocks formed by crystallization at depth have Curie points between 400°–570°C and are strongly magnetic. Induced magnetizations are commonly several A/m, and can readily account for long-wavelength magnetic anomalies measured by satellite and aircraft; thus, there is no need to search for exotic sources for “missing” crustal magnetization. No metal or other peculiar magnetic species have been observed by us in any of our (unaltered) samples. Mineral assemblages (including magnetic components) are consistent with oxidizing conditions close to the FMQ buffer.

Unique thermoremanent magnetization of multidomain sized hematite: Implications for magnetic anomalies

Intense magnetic remanence (100^1000 A/m) associated with MD hematite and/or titanohematite and associated with high Koenigsberger ratios (40^1000) indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant. Titanohematite behaves similarly to hematite and, thus, the grain size dependence of TRM acquisition in hematite is considered as a generalization. The transition between truly MD behavior and tendency towards SD behavior in hematite has been established to be between grain sizes of 0.1 and 0.05 mm. In contrast to magnetite and titanomagnetite, hematite exhibits inverse grain size dependence, with MD hematite acquiring a relatively intense TRM in the geomagnetic field, comparable to sub-micrometer sized magnetite and only an order of magnitude less than SD magnetite. Consequently MD hematite (and by analogy titanohematite) remanence may be of significance as a source of magnetic anomalies at all scales. MD hematite exhibits TRM weak field acquisition behavior that is different from all other magnetic minerals, being the only magnetic mineral having an REM (TRM/SIRM) value E0.1 for TRM acquisition in the geomagnetic field. The very different TRM behavior of MD hematite in contrast to magnetite is due to two factors. The first is the lesser influence of demagnetizing energy with respect to wall pinning energy, at temperatures almost up to the Curie temperature for hematite. The second is the greater importance of the magnetostatic energy in the applied field, which for hematite dominates the total energy at high temperatures. fl 2000 Published by Elsevier Science B.V. All rights reserved.

Grain size limit for SD hematite

Grain sizes in the range (10 −4 to 10 −1 mm) are common in some rocks. Because thermal and/or chemical remanent magnetization of hematite in this range approaches intensities of single domain (SD) magnetite, careful exploration of this transition, may serve to develop new applications in rock magnetism that relate to magnetic anomaly source identification, and various paleomagnetic and grain size-dependent investigations. Grain size-dependent magnetic behavior of hematite reveals a SD–multidomain (MD) transition at 0.1 mm. This transition is recognized by variation in magnetic coercivity and susceptibility and is related to an anomaly in remanence recovery when cycling through the Morin transition. The coercivity decrease with increasing grain size occurs much more gradually above 0.1 mm than below this value. Magnetic susceptibility of the grains smaller than 0.1 mm has negligible dependence on the amplitude of the applied alternating magnetic field. For the larger grains a new amplitude-dependent susceptibility component is observed. The grain size of 0.1 mm is also associated with loss of most of the remanence when cycling through the Morin transition. This behavior is ascribed to a transition from the metastable SD to the MD magnetic state. The increase in magnetized volume causes the demagnetizing energy to destabilize the SD state, resulting in a transition where the demagnetizing energy is reduced by nucleation of the domain wall for grains larger than 0.1 mm. The 0.1 mm transition has no significant effect on shape of the temperature-dependent coercivity and saturation magnetization.

Magnetic petrology of deep crustal rocks—Ivrea Zone, Italy

Magnetic petrology is an extension of petrology, integrating magnetic property studies with conventional petrology for the purpose of understanding the development and modification of the magnetization in rocks. The magnetic properties of a suite of samples from the Ivrea Zone, northern Italy, are described with the aim of developing a magnetic petrology for deep crustal rocks. Samples studied include a variety of metamorphosed mafic (granulites, granofelses, plagioklasfelse, amphibolite, pyriclasite) and pelitic/quartzo-feldspathic rocks (acid granulites, stronalite and kinzigite gneisses, schists), and representative ultramafic rocks (phlogopite peridotite, pyroxenite, hornblendite). Ilmenite is the chief oxide mineral in a majority of the samples, but in a few mafic rocks, magnetite is the principal oxide constituent. Cr,Al-spinel is common in ultramafic samples and rutile is present in some metapelites. Sulfides are generally subordinate in abundance to oxide minerals. The mafic rocks in the Ivrea section have the broadest range of magnetic properties and include the most strongly magnetic samples. The Ivrea ultramafic rocks are moderately magnetic—the magnetite present appears to be secondary and is associated with crustal alterations—while pelitic and quartzo-feldspathic lithologies are dominantly paramagnetic or only weakly magnetic. The main ferromagnetic mineral in all strongly magnetic samples is magnetite that is nearly pure Fe3O4 in composition. Consequently, Curie temperatures are 565–580°C for these rocks. Pyrrhotite contributes to the magnetism in several ultramafic and mafic rocks, and it is the sole ferromagnetic mineral in the pelitic and quartzo-feldspathic samples. The results of our study indicate that certain mafic rock types (amphibolites, mafic granulites, pyriclasites) are the most likely source of the Ivrea Zones regional scale magnetic anomalies. If the Ivrea Zone represents a tectonically exposed cross section of continental crust, the geographic distribution of these rock types suggests the presence of multiple and variably thick, strongly magnetic layers, with magnetite Curie temperatures, in the deep crust. Petrographic evidence suggests, however, that the magnetic mineralogy of the sampled Ivrea rocks might have been changed subsequent to peak metamorphic conditions. Therefore, considerable care must be taken in evaluating the magnetic record in these crustal cross sections, particularly if they are to be used to model the magnetic characteristics of the lower crust.

Lodestone: Natures only permanent magnet‐What it is and how it gets charged

Magnetite and Titanomagnetite exhibit magnetic properties which are attributable to the micro-structures developed during oxidation and exsolution: All magnetite iron ores which are lodestones contain maghemite. These lodestones have Hc between 10 and 30 mT, SIRM between 8 and 18 Am²kg¹ and RI between 0.10 and 0.26. Magnetite, titanomagnetite and metals have REM values (ratio of NRM to SIRM) < 0.05. Samples (called fulgarites) obtained from the Smithsonian Institution have REM values ranging from 0.45 to 0.92. The REM value serves as a witness parameter to the magnetic fields associated with the lightning bolt. If a high REM value (say ≫ 0.1) can be verified as not to be due to contamination by man and does not contain MD hematite then the rock has LRM (lightning remanent magnetization). The magnetic field associated with lightning can be revealed from an isothermal remanent acquisition (RA) curve.

Magnetic characterization of the new magnetic mineral tetrataenite and its contrast with isochemical taenite

Abstract Taenite with 48–57% Ni is magnetically soft, but when transformed via atomic ordering below 320°C it becomes the new magnetic mineral tetrataenite with considerable magnetic hardness and other distinctive magnetic properties. Tetrataenite is ubiquitous in chondrite meteorites, occurring as discrete grains, as a clear taenite rim, and in the etched cloudy zone. Measured remanent coercivities range from ∼ 30 mT in Appley Bridge (LL4) to ∼ 600 mT in some Bjurbole (L4) chondrules. This range suggests a considerable variation in tetrataenite properties that might be related to degree of order, size and properties of the ordered regions, possible shock induced effects, and other causes, including the initial grain size, particularly in the cloudy zone. Tetrataenite is characterized during heating to the apparent Curie point near 550°C, and taenite is characterized during subsequent cooling to room temperature. This record is possible because of the large magnetic anisotropy contrasts between tetrataenite and taenite and the fact that heating to the apparent Curie point destroys the magnetic anisotropy of tetrataenite, thereby allowing a taenite cooling record. Failure to recognize the presence of tetrataenite can result in considerable misinterpretation of thermomagnetic analysis since the curve shapes for tetrataenite and taenite can be different. Magnetic hysteresis loops, thermomagnetic curves, coercivity-temperature curves, and saturation remanence thermal demagnetization curves provide unquestionable records of two vastly different magnetic materials. Classical magnetic techniques can be used to monitor the progress of disordering of the tetrataenite during isothermal annealing. Magnetic hysteresis loop measurements of a Bjurbole chondrule at 488°C clearly record the magnetic parameter changes associated with disordering. Because of the large coercivity and other distinctive properties, tetrataenite exerts significant control on the bulk magnetic properties of meteorites regardless of its volumetric significance. Paleofield estimates to date have not considered the role of tetrataenite and should therefore be accepted with caution.

Magnetic Anomalies on the Tree Trunks

Magnetic measurements of soil and tree bark adjacent to a busy highway revealed a significant variation in the concentration of magnetic particles with distance from the highway. Further more, forest-facing tree-bark contains significantly more magnetic particles than road-facing tree-bark. Magnetic particles were detected both on the bark of the maple trees and in the first centimeter of the soil cover (O/A horizon). Stability of saturation isothermal magnetization (SIRM) and hysteresis parameters of the soil indicates the presence of single domain (SD/PSD) magnetic carriers. Measurements of the tree bark hysteresis parameters and SIRM detect a significant lower coercivity component that we interpret to be an indication of more abundant pseudo-single domain (PSD) type magnetic grains. Magnetic measurements around the perimeters of eight tree trunks reveal magnetic carriers whose distribution is antipodal to the source direction (highway). We interpret our observation by adopting an air circulation model, where suspended PSD/SD particles are carried in the air stream. The air stream from the heavy traffic lowers the amount of moisture on the tree trunk surfaces facing the highway and thus reduces an adhesive potential on this side. Therefore, more particles can stay on the moist side of the trunk protected from the direct airflow.

A magnetic study of the serpentinization process at Burro Mountain, California

Abstract A study has been made of the magnetic properties of a suite of continental serpentinites from Burro Mountain, California. The chemistry of this set of samples has been previously studied, enabling the magnetic properties to be compared to the chemical changes which occurred during serpentinization. Two distinct magnetic phases have been recognized. The first is extremely stable but does not appear to contribute significantly to the natural remanent magnetization of the most strongly magnetized samples. The second phase is clearly multi-domained magnetite having a well-defined transition in its coercivity near 120°K. However, this second phase is not apparent in either the least serpentinized or the most serpentinized of the samples studied. The magnetic data argue strongly for the existence of two types of serpentinites; the first is magnetized dominantly by a stable component which we suggest may be Ni 3 Fe, the second is magnetized Fe 3 O 4 with unstable magnetization. There is no clear connection between the appearance of the stable component and the amount of serpentinization.

The role of hematite-ilmenite solid solution in the production of magnetic anomalies in ground- and satellite-based data

Abstract Remanent magnetization (RM) of rocks with hematite–ilmenite solid solution (HISS) minerals, at all crustal levels, may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys. The possibility that lower thermal gradient relatively deep in the crust can result in exsolution of HISS compositions with strong remanent magnetizations (RM) was studied for two bulk compositions within the HISS system. Samples from granulite-terrane around Wilson Lake, Labrador, Canada contains titanohematite with exsolved ferrian ilmenite lamellae. Other samples from the anorthosite-terrane of Allard Lake, Quebec, Canada contain ferrian ilmenite with exsolved titanohematite lamellae. In both cases, the final exsolved titanohematite has similar Ti content and carries dominant magnetic remanence with REM (=NRM/SIRM, where NRM is the natural remanent magnetization and SIRM is the saturation isothermal remanent magnetization) that is comparable to the Ti-free end member. The RM was acquired prior to exsolution and the ilmeno-hematite-rich rock possesses thermal remanent magnetization (TRM), whereas rocks with hemo-ilmenite possess chemical remanent magnetization (CRM). In both cases, we found fairly large homogeneous grains with low demagnetizing energy that acquired intense RM. The magnetism of the ilmeno-hematite solid solution phases is not significantly perturbed by the continuous reaction: ilmeno-hematite≧titanohematite solid solution. Hence, the occurrence of HISS in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large REM.