Construction of a thermal demagnetization oven, and investigations on the Matahina ignimbrite

<p>A non-magnetic oven, and its ancillary equipment have been constructed and used to study magnetic properties of the Matahina ignimbrite, for which the following results have been established: 1. The directions of magnetization do not alter on heating in the oven. 2. The ignimbrite may be divided into sheets on the basis of magnetic properties. 3. Geological faulting has been revealed by divergent magnetization directions. 4. The T.R.M. acquired in the present earth's field is much greater than the N.R.M. This is possibly due to changes in minerals in the rock, either in the field since the rock was deposited, or on heating in the laboratory.</p>

Construction of a thermal demagnetization oven, and investigations on the Matahina ignimbrite

<p>A non-magnetic oven, and its ancillary equipment have been constructed and used to study magnetic properties of the Matahina ignimbrite, for which the following results have been established: 1. The directions of magnetization do not alter on heating in the oven. 2. The ignimbrite may be divided into sheets on the basis of magnetic properties. 3. Geological faulting has been revealed by divergent magnetization directions. 4. The T.R.M. acquired in the present earth's field is much greater than the N.R.M. This is possibly due to changes in minerals in the rock, either in the field since the rock was deposited, or on heating in the laboratory.</p>

An ultra-low magnetic field thermal demagnetizer for high-precision paleomagnetism

Thermal demagnetization furnaces are widely used paleomagnetic facilities for progressive removal of naturally acquired magnetic remanence or the imparting of well-controlled laboratory magnetization. An ideal thermal demagnetizer should maintain “zero” magnetic field in the sample chamber during thermal treatments. However, magnetic field noises, including the residual magnetic fields of the construction material and the induced fields caused by the alternating current (AC) in the heating element are always present, which can contaminate the paleomagnetic results at the elevated temperatures or especially for the magnetically weak samples. Here, we designed a new structure of heating wire named “straight core solenoid” to develop a new demagnetization furnace with ultra-low magnetic field noise. Simulation and practical measurements show that the heating current magnetic field can be greatly reduced by using the new technology. Thermal demagnetization experiments demonstrate that the new demagnetizer can yield low noise results even for weakly magnetic samples.

An ultra-low magnetic field thermal demagnetizer for high-precision paleomagnetism

Thermal demagnetization furnaces are widely used paleomagnetic facilities for progressive removal of naturally acquired magnetic remanence or the imparting of well controlled laboratory magnetization. An ideal thermal demagnetizer should maintain “zero” magnetic field in the sample chamber during thermal treatments. However, magnetic field noises, including the residual magnetic fields of the construction material and the induced fields caused by the alternating current (AC) in the heating element are always present, which can contaminate the paleoamgnetic results at the elevated temperatures or especially for the magnetically weak samples. Here, we designed a new structure of heating wire named “straight core solenoid” to develop a new demagnetization furnace with ultra-low magnetic field noise. Simulation and practical measurements show that the heating current magnetic field can be greatly reduced by using the new technology. Thermal demagnetization experiments demonstrate that the new demagnetizer can yield low noise results even for weakly magnetic samples.

Experimental test of the cooling rate effect on blocking temperatures in stepwise thermal demagnetization

SUMMARY Upon cooling, most rocks acquire a thermoremanent magnetization (TRM); the cooling rate at which this happens not only affects palaeointensity estimates, but also their unblocking temperatures in stepwise thermal demagnetization experiments, which is important, for example, to estimate volcanic emplacement temperatures. Traditional single-domain (SD) theory of magnetic remanence relates relaxation times to blocking temperatures— the blocking temperature is the temperature at which the relaxation time becomes shorter than the experimental timescale—and therefore strictly only applies to remanence acquisition mechanisms at constant temperatures (i.e. viscous remanent magnetizations, VRMs). A theoretical framework to relate (constant) blocking temperatures to (time-varying) cooling rates exists, but this theory has very limited experimental verification—partly due to the difficulty of accurately knowing the cooling rates of geological materials. Here we present an experimental test of this ‘cooling rate effect on blocking temperatures’ through a series of demagnetization experiments of laboratory-induced TRMs with controlled cooling rates. The tested cooling rates span about 1 order of magnitude and are made possible through (1) extremely accurate demagnetization experiments using a low-temperature magnetic properties measurement system (MPMS) and (2) the use of a ‘1-step-only’ stepwise thermal demagnetization protocol where the relaxation process is measured over time. In this way the relaxation time corresponding to the blocking temperature is measured, which can be done to much higher accuracy than measuring the blocking temperature directly as done in traditional stepwise thermal demagnetization experiments. Our experiments confirm that the cooling rate relationship holds to high accuracy for ideal magnetic recorders, as shown for a synthetic weakly interacting SD magnetoferritin sample. A SD-dominated low-Ti titanomagnetite Tiva Canyon Tuff sample, however, showed that natural samples are unlikely to be sufficiently ‘ideal’ to meet the theoretical predictions to high accuracy—the experimental data agrees only approximately with the theoretical predictions, which may potentially affect blocking temperature estimates in stepwise thermal demagnetization experiments. Moreover, we find a strongly enhanced cooling rate effect on palaeointensities for even marginally non-ideal samples (up to 43 per cent increase in pTRM for a halving of the cooling rate).

An ultra-low magnetic field thermal demagnetizer for high-precision paleomagnetism

Thermal demagnetization furnaces are widely used paleomagnetic facilities for progressive removal of naturally acquired magnetic remanence or the imparting of well controlled laboratory magnetization. An ideal thermal demagnetizer should maintain “zero” magnetic field in the sample chamber during thermal treatments. However, magnetic field noises, including the residual magnetic fields of the construction material and the induced fields caused by the alternating current (AC) in the heating element are always present, which can contaminate the paleoamgnetic results at the elevated temperatures or especially for the magnetically weak samples. Here, we designed a new structure of heating wire named “straight core solenoid” to develop a new demagnetization furnace with ultra-low magnetic field noise. Simulation and practical measurements show that the heating current magnetic field can be greatly reduced by using the new technology. Thermal demagnetization experiments demonstrate that the new demagnetizer can yield low noise results even for weakly magnetic samples.

Tracing of traffic-related pollution using magnetic properties of topsoils in Daejeon, Korea

The present study was designed to explore the possibility of roadside pollution screening using magnetic properties of topsoil samples in Daejeon, South Korea. Low-field magnetic susceptibility, frequency dependence of magnetic susceptibility, susceptibility of anhysteretic remanent magnetization, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRM were determined for roadside topsoil samples. Magnetic susceptibility measured on 238 samples from the upper 5 cm of the topsoils ranged from 8.6 to 82.5 × 10–5 SI with a mean of 28.3 ± 10.8 × 10–5 SI. The proximal zone, 55 m wide area situated on either side of the main street, exhibited an enhancement of magnetic susceptibility. In areas distant from the main street, low magnetic susceptibility (< 50 × 10–5 SI) was observed. The topsoil samples exhibited significant susceptibility contrasts, suggesting that two dimensional magnetic mapping was effective in identifying traffic-related pollution. A few magnetic hotspots with intensities of magnetic susceptibility near or over 50 × 10–5 SI might reflect the difference in topographic elevation and surface morphology. Among various IRM-related parameters, remanence of coercivity was most significant statistically. In most samples, IRM component analysis provided dual coercivity components. Thermal demagnetization of composite IRM and morphological observation of magnetic separates suggest angular magnetite produced by vehicle non-exhaust emissions spherical magnetite derived from exhaust emission to be the dominant contributors to the magnetic signal. It is likely that lower- and higher-coercivity components represent the presence of coarse-grained angular magnetite and fine-grained spherical magnetite, respectively.

An Ultra-low Magnetic Field Thermal Demagnetizer for High-precision Paleomagnetism

Thermal demagnetization furnaces are widely used paleomagnetic facilities for progressive removal of naturally acquired magnetic remanence or the imparting of well controlled laboratory magnetization. An ideal thermal demagnetizer should maintain “zero” magnetic field in the sample chamber during thermal treatments. However, magnetic field noises, including the residual magnetic fields of the construction material and the induced fields caused by the alternating current (AC) in the heating element are always present, which often greatly contaminate the paleoamgnetic results at the elevated temperatures or for the magnetically weak samples. Here, we designed a new structure of heating wire named named straight core solenoid to develop a new demagnetization furnace with ultra-low magnetic field noise. Simulation and practical measurement show that the heating current magnetic field can be greatly reduced by using the new technology. Thermal demagnetization experiments demonstrate that the new demagnetizer can yield low noise results even for weakly magnetic samples.