Anisotropy Of Low-field Susceptibility Research Articles

Investigating the provenance of Italian archaeological obsidian tools based on their magnetic properties

Reconstructing the distribution of ancient obsidian tools is one of the few ways to trace ancient trade routes during the Neolithic. The use of magnetic properties for obsidian provenance studies has already been applied as a useful inexpensive and non-destructive tool. It is mainly based on the variation of the type, concentration and grain size of the magnetic particles within the vitreous matrix coming from different sources. In this study, we present the results of a rock magnetic investigation carried out on archaeological obsidian tools collected from six Neolithic sites situated in Northern Italy (Castello d’Annone, Brignano Frascata, Cascina Chiappona, Casalnoceto, Garbagna and Parma). A total of 57 archaeological samples were analysed by measuring several magnetic parameters such as low-field and anhysteretic susceptibility, saturation isothermal remanent magnetization at room and liquid nitrogen temperature, remanence and saturation magnetization from hysteresis cycles and anisotropy of low-field susceptibility. The obtained results were compared with the magnetic properties of geological samples from five Mediterranean islands (Lipari, Sardinia, Palmarola, Pantelleria and Melos). Cluster analysis was applied to the whole set of parameters, allowing the correlation of the pertinent group of artefacts and geological obsidians. Such analysis shows that most of the studied archaeological tools come from Lipari, with few exceptions consisting of samples coming from Pantelleria and Sardinia. Our results are in good agreement with other studies based on chemical analyses that also show that Lipari is the most common Neolithic obsidian source in Northern Italy, despite its longer distance in respect to other obsidian sources.

A case of time-dependent anisotropy of low-field susceptibility (AMS)

A suite of non-tectonized, Meso-Proterozoic siliceous crystal tuffs and volcanic breccia with a visible stratification has a modal bulk susceptibility ( k) ∼ 160 μSI [mean; S.D. = 141 μSI; 44 μSI]. Normally, such susceptibilities suffice to make reliable measurements of anisotropy of low field susceptibility (AMS) using AC induction-coil instruments. However, for this suite, time-dependent susceptibility – variations during measurement are large in comparison to susceptibility – differences along different axes through a specimen. Thus, in many specimens, AMS axes determined by routine induction coil measurement in a (Sapphire Instruments SI2B; 19,200 Hz, 60 A/m) measurement procedures are not reproducible. The apparent variation of specimen susceptibility during a single, four minute AMS measurement can > 2 μSI whereas in the same interval the noise-level of our instrument is < 0.2 μSI. Thus, the time-dependence of the specimen-susceptibility is an intrinsic phenomenon due to the characteristics (grain size, domain structure) of the magnetite which dominates susceptibility during measurement and handling. Two procedures improved the reproducibility and stability of AMS axial orientations in some specimens. First, for some specimens, one or two cycles of LTD or AF demagnetization (≤ 15 mT) stabilized AMS axes. (Previous workers have observed that LTD and AF demagnetization may change slightly the AMS of polydomainal magnetite). Of course, exposure to alternating fields is preferably avoided before any AMS study. Second, for some specimens AMS measurement were improved by shielding the induction coil instrument in a large magnetically shielded space (ambient field < 5 nT). Further improvements were achieved by permitting the specimens to relax in side a magnetic shield for 24 h before measurement. The occurrence of time-dependent bulk susceptibility, especially noticeable in its AMS axial orientations is certainly a rare phenomenon and the procedures we used to improve measurement are excessive and unnecessary safeguards in routine AMS work; in any case, they mostly failed to produce a desirable degree of reproducibility. However, a simple test to detect for the presence of such time-dependent AMS is to repeat measurements at intervals of several hours, between which the specimen is permitted to relax in different orientations with respect to the geomagnetic field, or in a shielded space.

Miocene to Quaternary deformation in NE Slovenia: complex paleomagnetic and structural study

Combined paleomagnetic and structural research was carried out in the Mura-Zala Basin including the western and southern surrounding hills in northeastern Slovenia. The Mura-Zala Basin was formed due to ENE–WSW trending crustal extension in the late Early Miocene (18.3–16.5 Ma). First, marine sedimentation took place in several more or less confined depressions, then in a unified basin. During thermal subsidence in the late Miocene deltaic to fluvial sediments were deposited. After sedimentation, the southernmost, deepest depression was inverted. Map-scale folds, reverse and strike-slip faults were originated by NNW–SSE compression. This deformation occurred in the latest Miocene–Pliocene and is reflected also in the magnetic fabric (low field susceptibility anisotropy). After this folding, the Karpatian sediments of the Haloze acquired magnetization, then suffered 30° counterclockwise rotation relative to the present north (40° counterclockwise with respect to stable Europe). This Pliocene (Quaternary?) rotation affected a wide area around the Mura-Zala Basin. The latest Miocene to Quaternary folding and subsequent rotation may be connected to the counterclockwise rotation of the Adriatic microplate.

Limitations of tensor subtraction in isolating diamagnetic fabrics by magnetic anisotropy

The anisotropy of low field susceptibility (AMS) represents the orientation distribution of all minerals in a rock, whereas the anisotropy of magnetic remanence (AMR, preferably anhysteretic) isolates that of the accessory remanence-bearing minerals. The subtraction of normalized AMR from AMS, in theory and under limited practical circumstances, may isolate the paramagnetic+diamagnetic anisotropy contribution and thus the orientation distribution of the matrix minerals ( Borradaile et al., 1999. Geol. Soc. Lond., Sp. Publ. 151, 139–145). Limitations include the great sensitivity of the subtraction process to the precision of the definition of the respective (AMS, AMR) tensors, and a requirement that single-domain and superparamagnetic grains are absent. The latter is particularly important for superparamagnetic minerals because iron oxides may be part of the orientation distribution of the main group of remanence-bearing minerals, although they would be excluded from the AMR fabric. Low ratios of saturation isothermal remanence to induced susceptibility characterize those rare rocks in which superparamagnetic behavior is a significant contribution.

Possible cryptic tectono-magnetic fabrics in ‘post-tectonic’ granitoid plutons of the Canadian Shield

Two late Archean granitic plutons forcefully intruded and thermally metamorphosed Archean schists. Feldspar magacrysts aligned concentrically with the margins during inflation. Substantial tectonic deformation never affected the plutons. Nevertheless, both plutons show the same, consistently oriented, cryptic magnetic fabric revealed by anisotropy of low field susceptibility (AMS) from 134 samples. They have an NE-SW vertical magnetic foliation and a magnetic lineation trending 060/15. Eigenvalues of the orientation distribution of principal susceptibilities show that the magnetic fabrics have almost identical strengths, orientations and symmetry in both plutons. We propose a post-magmatic reactivation of the earlier regional tectonic shortening. This imparted a ‘cryptic tectonic’ magnetic fabric on the plutons that overprinted the magmatic AMS fabric in most outcrops. The plutons' AMS fabrics are subparallel to the much older schistosity of the country rocks. Because multidomain magnetite provides > 99% of the low field susceptibility of the rocks, it controls the AMS. However, we see no alignment of the magnetite grains. Thus, magnetic anisotropy may be due to stress alignment of intragranular domain walls and not controlled by grain shape, as usually assumed for magnetite.

Rock magnetism of deformed Upper Triassic limestones from the Lagonegro Basin (Southern Apennines, Italy)

The stable component of the natural remanent magnetization (NRM) of the Upper Triassic “Calcari con Selce” limestone formation at Marsico Nuovo (Lagonegro Basin, Southern Italy) has an inclination which is about 45° too steep for the expected paleogeographic position. In order to explain this abnormal magnetization direction, several rock magnetic properties such as the dependence of susceptibility on temperature and alternating field frequency, anisotropy of low field susceptibility, anisotropy of anhysteretic remanent magnetization (ARM) and characteristics of acquisition and thermal decay of isothermal remanent magnetization (IRM) have been investigated. Strong inverse fabrics of both susceptibility and ARM have been recognized, which are due to magnetite in the superparamagnetic state on the one hand and in the stable single domain grain size on the other hand. Although internal deformation in the investigated rocks has not been reported in the geological literature, this may be the cause of the observed anisotropies. The anisotropy is probably responsible for the anomalous NRM directions of these limestones.

Self-reversal of isothermal remanent magnetization in a pyrrhotite (Fe 7S 8) crystal

The remanent magnetization induced at room temperature (IRM) was measured for powder and massive specimens separated from a Fe 7S 8 crystal. To characterize the crystal, several methods were used including Mössbauer spectroscopy, thermomagnetic analysis, low-field susceptibility anisotropy and magnetic colloid method. Self-reversals of IRM were revealed in the massive crystal fragment after its magnetization in magnetic fields of the order of 10–10 2 mT. Both normal and reverse IRM vectors lay in the maximum susceptibility plane perpendicular to the crystallographic c′-axis of the crystal. A twinning also perpendicular to the crystal's c′-axis and a very low coercivity in each twin (monocrystallic layer) probably support the origin of the reverse IRM in the crystal.

A study of colour changes in purple-green slate by petrological and rock-magnetic methods

The purple Cambrian slates of North Wales show colour changes to green in bedding-parallel patches and ellipsoidal spots. These are referred to as reduction spots but the process is more complex; the colour change is not simply associated with the reduction of iron. The green discoloration is accompanied by diffusion of Fe 3+ away from, and possibly by Mg and Ca towards the green material. The actual colour change is brought about by the replacement of the fine metamorphic hematite pigment with magnetite. However, coarser clastic hematite persists in the green slate. The green slate has twice as much chlorite and traces of magnetite, despite having less total Fe. The additional chlorite is chiefly responsible for the doubling of the magnetic susceptibility of the green slate with respect to the purple slate. Chlorite and epidote have the same compositions in both green and purple slates despite the difference in whole rock Fe content. Both purple and green slates behave predominantly as paramagnets between −194°C and 700°C. The anisotropy of low-field susceptibility (“magnetic fabric”) is increased by heating and therefore made easier to measure and the principal directions of anisotropy are preserved. Heating to 700°C increases the saturation isothermal remanence dramatically and increases its anisotropy due to the growth of single-domain magnetite. However, heating is not universally recommended as a means of enhancing otherwise feeble magnetic fabrics. Some other slates and schists show regressive changes in magnetic fabric on heating with erratic changes in principal directions.

Field‐impressed anisotropies of magnetic susceptibility and remanence in minerals

The application of alternating (AF) or direct (DF) magnetic fields to samples containing dispersed particles of magnetite, titanomagnetite, or maghemite has been found to alter significantly the measured low‐field susceptibility anisotropy even though the particle orientation is unchanged. The acquisition of isothermal remanent magnetization (IRM) is also strongly dependent on the previous AF treatment given to the samples. A method of quantifying the changes brought about by application of the fields is described. The samples studied were intrinsically isotropic or only weakly anisotropic, and the computed impressed susceptibility or IRM ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis. The size and shape of the field‐impressed ellipsoids were strongly dependent upon particle size. These effects might thus form the basis of a rapid, nondestructive means of estimating particle size in rocks. The results also have important implications for magnetic fabric analysis and the static AF demagnetization of rocks. In particular, to ensure that the intrinsic magnetic fabric is measured without any field‐impressed component of anisotropy, samples should not be subjected to static AF demagnetization prior to the determination of either low field susceptibility anisotropy or IRM anisotropy. Samples should also not carry a significant remanence (such as an IRM acquired in a field greater than about 5 mT) prior to low field susceptibility anisotropy analysis.

Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland)

Magnetite as well as ilmenohematite are contained by the Bergell granitic rocks, but the fabric of low field susceptibility is due to the magnetite only and is equivalent to the macroscopic biotite fabric. Microscopic observations and high field anisotropy measurements show that the ilmenohematite trigonal axes are aligned parallel to the directions of minimum low field susceptibility anisotropy. The deflection of the stable direction of natural remanent magnetization caused by the alignment of ilmenohematite is corrected using the susceptibility anisotropy data. The resulting virtual palaeopole position deviates from other Oligocene to Miocene pole positions of stable Europe suggesting that since the time of intrusion some 25 my ago the Bergell massif has been rotated anti-clockwise by an angle of ∼30° about a vertical axis.

Low Field Susceptibility Anisotropy in the Basal Plane of Hematite (α-Fe2O3) and its Dependence on the Remanent Moment

Measurements of room temperature susceptibility in the basal plane of natural single crystals of α-Fe2O3 in which a high field isothermal remanence has been induced reveal a substantial anisotropy whose minimum is coincident with the direction of remanence. The susceptibility anisotropy appears to be related primarily to the low coercivity fraction of the remanent moment. Sizeable discontinuous changes in the susceptibility are sometimes present as the specimen is rotated. Recent domain wall studies have disclosed the presence in synthetic crystals of 180° walls exclusively. The present work suggests that the magnetization process within the basal plane of natural crystals of hematite is more complex.