Diana Jordanova

Anisotropy of magnetic susceptibility of heated rocks

Abstract Heating produces changes, which does not always correspond to simple enhancement of the magnetic fabric. Two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating. The first diagonalizes the tensor from the difference between each tensor term before and after heating. The second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment. When the same magnetic fabric is obtained from several thermal steps, it cannot be related to randomly oriented ferrimagnetic minerals. Instead, the newly formed fabric must be related to characteristics of the pre-existing rock. By comparing this ferrimagnetic minerals fabric with the initial whole rock fabric, we can distinguish cases where heating simply enhances pre-existing fabric from those where thermal treatment induces a different fabric. Relative to the pre-heating fabric, this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction, related to petrostructural elements other than those defining the initial fabric.

Magnetic Characteristics of Different Soil Types from Bulgaria

Magnetic studies of different soil types can provide valuable information about palaeoenvironmental conditions at the time they were formed. Results of investigations of rock-magnetism of genetically different soil types, which developed over varying time intervals (Meadow Chernozem - formed during the last 6000 years BP; Leached Cinnamonic soil - formed since the 3rd-2nd century BC and Pellic Vertisoil - the oldest, formed since the Late Pliocene) are presented. The soil profiles of the Leached Cinnamonic and Pellic Vertisoil are characterized by lower values of magnetic susceptibility, as compared to that of the parent materials. It is shown that using percentage frequency-dependent susceptibility (Xfd%) and viscous remanent magnetization (VRM), pedogenic alteration could be detected even in such circumstances. The variations of the parameters measured along the depth of the studied Meadow Chernozem soil profile are characterized by gradual smooth magnetic enhancement, pointing to the absence of secondary redistribution of pedogenic magnetites. In contrast to this case, the two other profiles (Leached Cinnamonic and Pellic Vertisoil) show magnetically enhanced lower (illuvial) horizons due to processes of acid destruction and re-precipitation of the original pedogenic ferromagnetic minerals down the profiles. These specific magnetic properties are of particular importance in using susceptibility variations as a palaeoclimatic proxy record.

Origin of magnetic fabric in bricks: its implications in archaeomagnetism

Comparison with the magnetic anisotropy of unbaked (only dried) and baked loam bricks, hand moulded in a rectangular frame, reveals that the same moulding technique had been applied to produce the bricks of a Medieval brick kiln that was archaeomagnetically dated at 1650 AD [Geoarchaeology (to be published)]. Anisotropy of magnetic susceptibility (AMS) measurements show that the unbaked and baked bricks have a shape-related magnetic fabric, induced during the moulding process, with average Kmax occurring in the greatest faces along the direction of the longest edges and Kmin perpendicular to the greatest faces of the bricks. The anisotropy of thermoremanence (ATRM) is high, indicating that the remanence directions of bricks may accuse large deviations from the geomagnetic field direction responsible for it. However, anisotropy seems unlikely to be the cause for the apparent discrepancy between the archaeomagnetical and archaeological date of the brick kiln, the latter presumably about half a century older. Besides AMS, also the anisotropy of anhysteretic remanence was examined as a possible substitute for ATRM and to obtain information on the magnetic state of the minerals contributing to the remanence anisotropy. � 2002 Elsevier Science Ltd. All rights reserved.

The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study

The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study Several plutons located in the southwestern part of the Sredna Gora Zone — Bulgaria are examples of the Apuseni-Banat-Timok-Sredna Gora type of granites emplaced during Late Cretaceous (86-75 Ma) times. The studied intrusive bodies are spatially related to and deformed by the dextral Iskar-Yavoritsa shear zone. The deformation along the shear zone ceased at the time of emplacement of the undeformed Upper Cretaceous Gutsal pluton, which has intruded the Iskar-Yavoritsa mylonites. A clear transition from magmatic foliation to high-, moderate- and low-temperature superimposed foliation and lineation in the vicinity of the Iskar-Yavoritsa and related shear zones gives evidence for simultaneous tectonics and plutonism. Away from the shear zones, the granitoids appear macroscopically isotropic and were investigated using measurements of anisotropy of magnetic susceptibility at 113 stations. The studied samples show magnetic lineation and foliation, in agreement with the magmatic structures observed at a few sites. Typical features of the internal structure of the plutons are several sheet-like mafic bodies accompanied by swarms of mafic microgranular enclaves. Field observations indicate spatial relationships between mafic bodies and shear zones as well as mingling processes in the magma chamber which suggest simultaneous shearing and magma emplacement. Structural investigations as well as anisotropy of magnetic susceptibility (AMS) data attest to the controlling role of the NWSE trending Iskar-Yavoritsa shear zone and to the syntectonic emplacement of the plutons with deformation in both igneous rocks and their hosts. The tectonic situation may be explained by partitioning of oblique plate convergence into plate-boundary-normal thrusting in the Rhodopes and plate-boundary-parallel transcurrent shearing in the hinterland (Sredna Gora).

Paleomagnetism in northwestern Bulgaria: geological implications of widespread remagnetization

Abstract Two different paleomagnetic directions have been obtained in several formations in northwestern Bulgaria. One of them, found only in Upper Permian–Lower Triassic red sandstones, is very likely a primary magnetization. A secondary magnetization of Eocene age appears as either pre-, syn- or post-folding depending on the site. The identification of the timing of remagnetization with respect to folding allows us to distinguish areas among the first deformed during Tertiary tectonics. Within each superimposed structural unit of the West Balkan, the deformation spread from south to north. The boundary between the Srednogorie and Balkan zones appears to be a major tectonic structure in Bulgaria. The Balkan likely underwent a clockwise rotation relative to stable Europe since the beginning of the main Middle Eocene orogenesis.

Magnetic fabric of bulgarian loess sediments derived by using various sampling techniques

SummaryThe anistropy of low field magnetic susceptibility has been studied for seven outcrops of loess sediments in North-Eastern Bulgaria. Different sampling methods were applied in order to choose the best technique for obtaining the primary magnetic fabric of such unconsolidated sediments. AMS results show significant changes in the petrofabric of samples collected by the first technique which disturbs the original sedimentary fabric. The second applied technique does not cause such a strong deformation but some disturbance of the magnetic fabric is probable. Typical sedimentary fabric is obtained from hand samples and it is therefore concluded that this represents the best method for obtaining reliable AMS results from soft sediments.

Thermomagnetic Behavior of Magnetic Susceptibility—Heating Rate and Sample Size Effects

Thermomagnetic analysis of magnetic susceptibility k(T) was carried out for a number of natural powder materials from soils, baked clay and anthropogenic dust samples using fast (11oC/min) and slow (6.5oC/min) heating rates available in the furnace of Kappabridge KLY2 (Agico). Based on the additional data for mineralogy, grain size and magnetic properties of the studied samples, behaviour of k(T) cycles and the observed differences in the curves for fast and slow heating rate are interpreted in terms of mineralogical transformations and Curie temperatures (Tc). The effect of different sample size is also explored, using large volume and small volume of powder material. It is found that soil samples show enhanced information on mineralogical transformations and appearance of new strongly magnetic phases when using fast heating rate and large sample size. This approach moves the transformation at higher temperature, but enhances the amplitude of the signal of newly created phase. Large sample size gives prevalence of the local micro- environment, created by evolving gases, released during transformations. The example from archeological brick reveals the effect of different sample sizes on the observed Curie temperatures on heating and cooling curves, when the magnetic carrier is substituted magnetite (Mn0.2Fe2.70O4). Large sample size leads to bigger differences in Tcs on heating and cooling, while small sample size results in similar Tcs for both heating rates.

Changes in Magnetic Properties of Baked Archaeological Samples. Implications for Palaeointensity Determination

Rock magnetic investigations of archaeological materials of burnt clay from Eneolithic ovens (∼4500 years BC) showed particular changes with time in the magnetic mineralogy of samples, stored under normal conditions. Our results indicate that well-burnt clay from the archaeological materials contains a significant amount of very fine magnetic grains, which could notably influence the rock magnetic properties and behavior at room temperature. The main observations after 4 years of storage under laboratory conditions are as follows: 1) decrease in the final unblocking temperature of NRM from ∼600–620°C to ∼580°C and 2) increase in the capacity of laboratory TRM acquisition. The most probable mechanism responsible for the observed changes is supposed to be fast low-temperature oxidation of the finest (superparamagnetic) grains and the development of the maghemite shell in coarser single-domain grains. The Thellier palaeointensity experiments, carried out at the beginning of the study, showed very good results, which satisfy all acceptance criteria, applied to evaluation of the results, quite well. Palaeointensity determinations repeated 4 years later on samples from the same material showed the experimental results to be of significantly inferior quality. The main difference is the presence of the significant deviation (change in the slope) on the Arai diagram after T>350–400°C. The calculated palaeointensity is either higher than the one obtained before, or similar, but evaluated with large uncertainty. Therefore, we conclude that the possibility to obtain biased palaeointensity values increases during short-time storage (i.e. several years) due to the low-temperature changes of the material.