Reversible Thermomagnetic Curves Research Articles

Archaeomagnetic analyses on fumiers burned under controlled experimental conditions

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Qn) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples’ data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest.

New high precision full-vector archaeomagnetic data from a roman kiln in Mérida (Spain)

This study presents new high precision age and full-vector archaeomagnetic data from a kiln excavated in the Roman archaeological site of Mitreo's house (Mérida, Badajoz, Spain). The age of the kiln was obtained by the stratigraphic method and by the ceramological study of the italic and sigillata pottery found with a very precise age date of 55 ± 15 years AD. Rock-magnetic experiments pointed towards magnetite as the main carrier of remanence and highly reversible thermomagnetic curves suggested that the studied samples were suitable for archaeoeointensity determinations.Paleomagnetic experiments including thermal and stepwise alternating field demagnetization yielded the following mean direction for the kiln: declination D = 0.6°; inclination I = 54.0°; (k = 481; α95 = 2.5°). Archaeointensityexperiments with the Thellier – Coe protocol on 28 samples yielded successful determinations in 27 cases. Anisotropy factors between 0.90 and 1.04 were obtained from anisotropy of thermoremanent magnetization (ATRM) experiments. A mean anisotropy-corrected archaeointensity value F = 56.3 ± 5.5 μT was obtained.The geomagnetic model SHA.DIF.14 k was used for an archaeomagnetic dating yielding a chronological interval between 40 BCE and 150 CE. This low age resolution when compared with the pottery-based age data is related to the behavior of the geomagnetic field in the Iberian Peninsula during the Roman period, which does not allow to differentiate well between results corresponding to those centuries. However, it is also related to the fact that the Iberian archaeomagnetic dataset in the analysed time range is highly scattered. A high quality full-vector data with a very precise age have been obtained which contribute to improve the Iberian secular variation curve and geomagnetic databases.

A paleointensity study of Cretaceous volcanic rocks from the Western Cordillera, Colombia

The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron.

An integrated magnetic, geochemical and archeointensity investigation of casting debris from ancient metallurgical sites of Michoacán, Western Mesoamerica

The ability of baked clay-elaborated objects to record the temporal variations of both direction and intensity of the geomagnetic field at the time of their elaboration or last use has been widely used during the last decades to fill the gaps left by available time-discrete volcanic-rock logs for a better knowledge of the evolution of the geomagnetic field. Further refinement of the secular variation curves has motivated the search for alternative non-conventional materials as attractive targets for archeomagnetic studies. Both clay and copper have been used since pre-Hispanic times for the elaboration of a wide variety of ornamental and utilitarian objects in Mesoamerica. Previous studies carried out in Europe and Israel have shown the usefulness of copper slags as one of these non-conventional materials. We present the results of an integrated study carried out on ten copper slags from seven ancient metallurgical sites of Michoacan. The analysis included a series of rock magnetism experiments, scanning electron microscope analysis, archeointensity determinations following the Thellier-Coe method and their corresponding dating. Based on the results obtained the slag-set analyzed can be divided in two main groups: low Fe concentrations (associated with high Cu concentrations) for slags comprising the first group, and high Fe concentrations (associated with low Cu concentrations) for slags from second group. Slags from the first group are characterized by almost reversible thermomagnetic curves, while those corresponding to the second group show a quite irreversible behavior. A crystalline (vitreous) structure within the massive surface of the slags is observed for those coming from the second (first) group. These striking features could reflect significant differences in the metallurgical process followed at these sites, which according to archeological and historical evidences correspond to Colonial and pre-Hispanic usage metallurgical sites; first and second group, respectively. The good experimental behavior exhibited by these materials demonstrates the suitability of copper slag to obtaining absolute archeointensity data which can be used to improve the existing Mesoamerican archeomagnetic record. This in turn, can be used both for geomagnetic field modelling purposes and archeomagnetic dating.

A parameter characterising the irreversibility of thermomagnetic curves

Previous attempts to quantify the degree of irreversibility of thermomagnetic curves have compared magnetisation during the heating and cooling cycles at a particular temperature, e.g. 100 °C. This may not give a true indication of the degree of irreversibility, as a coincidence at that temperature may occur when the thermomagnetic curve is irreversible. We suggest a new quantitative irreversibility parameter, IP, which is based on the area between the heating and cooling branches of the thermomagnetic curve. Accidental coincidences or crossovers of the branches at certain temperatures do not affect this parameter. For reversible thermomagnetic curves IP=0. In cases of strongly reduced magnetisation, e.g. due to oxidation processes during heating, IP may approach a value of −1, while for strongly increased magnetisation after heating, e.g. in the case of pyrrhotite-bearing samples, IP may exceed 1. The parameter may be also determined from high- and low-temperature variation curves of magnetic susceptibility.

Magnetic Properties of the Ophiolite Suites; Boso-Miura Peninsulas (Cenozoic) and Maizuru District (Paleozoic)

Magnetic properties of the pillow basalts, dike basalts and ultramafic rocks of the ophiolite suite of the Boso-Miura Peninsula (Cenozoic) are compared with those of the ophiolite suite in the Maizuru district (Paleozoic). From the dike basalt of Boso Peninsula to the least altered dike basalts of Miura Peninsula and Maizuru district, the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 1.5×10-3 to 8×10-4 (emu/cc).The Koenigsberger ratios (Qn) of these dike basalts range from 3.2 to 0.4. Thermomagnetic analyses for the dike basalts from the Cenozoic ophiolite suite exhibit the characteristic behavior of low temperature-oxidized cation deficient titanomagnetite or titanomaghemite.In contrast the dike basalt of the Paleozoic ophiolite suite does not have a clear irreversible thermomagnetic curve. The pillow basalts of the Cenozoic ophiolite have NRM intensities from 3×10-3 to 6×10-4 (emu/cc) and Qn-values from 8 to 5. Thermomagnetic analyses of the Cenozoic pillow basalts are quite similar to those of the Cenozoic dike basalts. In the Paleozoic ophiolite suite of the Maizuru district, pillow stractures are not developed in the basalt, but auto-brecciated pillow basalt are present. These auto-brecciated pillow basalts have NRM intensities of abont 1×10-4 (emu/cc) and Qn-values of about 0.3.The ultramafic rocks of the two ophiolite suites show wide range of NRM intensities from 1×10-3 to 1×10-4 (emu/cc). Thermomagnetic analyses of the ultramafic rocks have reversible thermomagnetic curves for pure magnetite.

The effects of alteration on the natural remanent magnetization of three ophiolite complexes: Possible implications for the oceanic crust.

Magnetic properties are compared for the pillow basalts and sheeted dike complexes for the following three ophiolite suites: Macquarie Island (-27 million years), Troodos Massif, Cyprus (∼middle Cretaceous), and Smartville complex, California (∼Jurassic). The magnetic properties of the pillow basalts strongly depend on their degree of alteration. From the least altered, low-temperatureoxidized, upper pillow basalts of Macquarie Island and the pillow basalts of the Troodos Massif (zeolite facies metamorphism) to the most altered pillow basalts of the Smartville complex (greenschist facies metamorphism) the average intensity of the natural remanent magnetization (NRM) decreases from a maximum of 80×10-4 to 0.50×10-4 gauss; the Koenigsberger ratio (Q) decreases from 6 to 0.3; the contribution due to viscous remanent magnetization (VRM) increases, and the NRM stability with respect to alternating fields decreases. For the most altered pillow basalts of the Smartville complex the NRM is probably predominantly a VRM, acquired during the present polarity epoch. Thermomagnetic analyses for the least altered pillow basalts exhibit the characteristic behavior of low-temperature-oxidized cation deficient titanomagnetites, while the more altered pillow basalts have reversible thermomagnetic curves with magnetite-like Curie points, probably caused in situ by thermally induced unmixing of the low-temperature-oxidized titanomagnetites. The sheeted dike complexes have NRM intensities of about 10×10-4 gauss and Q values of about 0.8, and most of their magnetic properties do not vary much between the three ophiolite complexes. All the sheeted dike samples have reversible thermomagnetic curves with magnetite-like Curie points. If these ophiolite complexes represent material which was originally formed at spreading centers and if their alteration represents ocean floor metamorphism, then with progressive alteration the pillow basalts lose their magnetic recording qualities and become poorer sources for the marine magnetic anomalies, and the underlying sheeted dike complexes become relatively more important. The most probable site for the occurrence of the sea floor metamorphism is in the relatively high temperature, high heat flow regions near spreading centers.

Magnetic properties of oceanic pillow basalts: evidence from Macquarie Island

Summary A combined pdaeomagnetic, hysteresis, thermomagnetic, and electron microprobe study has been carried out on pillow basalts from Late Tertiary oceanic crust exposed on Macquarie Island. Oriented cores were collected from four sites in the North Head region where the basalts are unmetamorphosed but have suffered pervasive seafloor weathering and at four sites near Langdon Point where the pillow lavas have experienced metamorphism to the greenschist facies. Field geologic and petrologic evidence suggests that the North Head samples are representative of the weathered zone of Late Tertiary oceanic crust while the specimens from Langdon Point are representative of pillow basalts from > 500m into the oceanic lithosphere. Geometric mean intensities of natural remanence (NRM) were 0.7 × 10−3 G for North Head samples and 0.6 × 10−3 G for specimens from Langdon Point. The geometric mean values of low field susceptibility (x) and Koenigsberger ratio (Q) for North Head samples (0.1 × 10−3 G Oe−1 and 9.8) are similar to those of DSDP basalts. The mean value of X and Q for Langdon Point samples (2.8 × 10−3 G Oe−1 and 0.3) is similar to those observed in metabasalt dredged from oceanic escarpments. Hysteresis, thermomagnetic and electron microprobe data indicate that the titanomaghemites in the North Head samples have suffered a high degree of low temperature oxidation. Langdon Point samples show reversible thermomagnetic curves with a magnetic Curie temperature (580° C), indicating that metamorphism has altered the magnetic mineralogy. This metamorphism is thought to occur at or near the ridge during the seafloor spreading process. Thus, the NRM of the metamorphosed pillow basalts would still record geomagnetic reversals and contribute to magnetic lineations. The low NRM intensities are ascribed to prolonged seafloor weathering of North Head samples and to metamorphism of the basalts from Langdom Point. Since the 10−3 G NRM intensities of pillow basalts are too low to account for the observed amplitude of marine magnetic anomalies, the underlying basaltic and doleritic sheeted dyke complex must also contribute to the anomalies.