MAGNETIC SUSCEPTIBILITY

Principle

Physical Background

Magnetic susceptibility is the degree to which a material can be magnetized in an external magnetic field. If the ratio between the induced magnetization and the inducing field is expressed per unit volume, volume susceptibility (k) is defined as

k = M/H,

where M is the volume magnetization induced in a material of susceptibility k by the applied external field H. Volume susceptibility is a dimensionless quantity. The value depends on the measurement system used:

k(SI) = 4 k(cgs) = 4 G Oe–1,

where G and Oe are abbreviations for Gauss and Orstedt, respectively. The SI system should be used.

Mass, or specific, susceptibility is defined as

= k/r,

where r is the density of the material. The dimensions of mass susceptibility are therefore cubic meters per kilogram.

The commonly used MS is measured at very low fields, usually not exceeding 0.5 mT, which have no influence on the NRM. It is therefore referred to as low-field susceptibility (klf). For comparison, fields of ~50 mT are required to change orientation in magnetite domains. High-field susceptibilities are measured in fields of a few hundred millilteslas and require equipment that is not available in the shipboard laboratory.

In practice, volume susceptibility is generally measured with core logging devices, for which calibration factors must be established to account for the specific geometry and effects of core conveyors and core liners. In the case of discrete specimen measurements, the mass of the specimen can be determined more accurately than volume and specific susceptibility is directly obtained. If average grain density and moisture content of the specimen are known, the specimen measurements can be compared with core logging measurements. Susceptibility values can then be normalized to mass and volume corrected for porosity. This can make susceptibility data more useful for quantitative estimates in conjunction with other mineral phases, such as carbonate, which are always normalized to dry mass.

Anisotropy of Magnetic Susceptibility

MS is a symmetrical second rank tensor that relates the induced magnetization M to the inducing field H:

Mi = kij x Hj.

Six independent components must be determined to define the susceptibility ellipsoid. AMS measures the shape, preferred crystallographic orientation, preferred particle orientation, or preferred distribution of ferr(i)omagnetic and paramagnetic grains. Analysis and interpretation of AMS data requires a detailed characterization of the sources of MS. Studies of AMS are occasionally carried out on hard-rock legs using the Kappabridge. The AMS of sediments is generally too weak to be measured in the magnetically noisy shipboard environment, and the higher rate of core recovery usually limits the work in the laboratory to routine measurements. Although the equipment for AMS measurements is available onboard, the software and analysis programs are very basic and not linked to the Janus database.

Environmental Effects

Cores should be equilibrated to room temperature because MS of most materials is a temperature-dependent property. The temperature dependence of paramagnetic minerals such as clays is described by the Curie-Weiss law, k = c/T, where c is the Curie constant and T is the temperature in Kelvin. Assuming a room temperature of 20C the MS of pure paramagnetic material that is 5C (10C; 20C) below room temperature will be 1.7% (3.5%; 7.1%) higher than the room temperature susceptibility. The temperature dependence of other materials in the temperature range between 0C and 20C is less significant.

Use of Magnetic Susceptibility

MS is used primarily as a relative proxy indicator for changes in composition that can be linked to paleoclimate-controlled depositional processes. The high precision and sensitivity of susceptibility loggers makes this measurement extremely useful for core-to-core and core-downhole log correlation. The physical link of MS to particular sediment components, ocean or wind current strength and direction, or provenance usually requires more detailed magnetic properties studies in a specialized shore-based laboratory.

Magnetic susceptibility is monitored on discrete samples during thermal demagnetization experiments to recognize changes in the magnetic mineralogy caused by phase transitions or oxidation that may occur during heating.

AMS measures the preferred orientation, distribution, or shape of ferromagnetic, paramagnetic, or ferromagnetic and paramagnetic minerals, which can be linked in many cases to bottom currents, compaction, or deformation (Tarling and Hrouda, 1993). The interpretation of AMS measurements usually requires a detailed characterization of the magnetic properties in order to determine the sources of MS.

Bartington MS2C Logger for Whole Cores

Equipment

A Bartington Instruments MS2C system is integrated in the MST for whole-core logging. The main unit is the widely used, versatile MS2 susceptometer for rapid measurements with a number of sensors. The unit has a measuring range of 1 x 10–5 to 9999 x 10–5 (SI, volume specific) or 1 x 10–8 to 9999 x 10–8 (SI, mass specific). It has five front panel controls: on-off switch, sensitivity range switch, SI or cgs unit switch, zero button, measure button, and continuous measurement switch. None of these controls needs to be operated because the instrument is controlled by the MST program. The unit switch should always be on SI. The range switch is typically on the lower sensitivity (1.0), which allows rapid 1-s measurements. The MST program allows the collection of multiple 1-s measurements, which are immediately averaged. This is useful if the sampling period is set, for example, at 3 s for the GRA measurement and there is time to take three susceptometer readings simultaneously. The MS2C loop sensor has an internal diameter of 80 mm, which corresponds to a coil diameter of 88 mm. It operates at a frequency of 0.565 kHz and an AF intensity of 80 A/m (= 0.1 mT). Temperature drift is <10–5 SI/hr. The resolution of the loop is 2 x 10–6 SI on the 0.1 range (9 s measuring time).

Dual-Frequency Measurements

Fine-grained magnetic material (single domain; diameter = ~0.003 quency dependence can be determined from measurements in dual-frequency mode. The high frequency used is 5.65 kHz. This mode of measurement is rarely used in general and is therefore not implemented for routine measurements in the MST program.

Drift Correction

The Bartington instrument is automatically zeroed at the beginning of each run, before the core enters the loop. Instrument drift may occur during the period of a core section scan. To correct for the drift, a zero-background measurement (MSbkgd) is taken at the end of a core section log. The drift is corrected under the assumption that it is linear over the time of interest (~10 min). The time elapsed between the zeroing of the instrument at the beginning of the run and the background measurement, tbkgd, is measured. For each measurement within the core (MSmeas) the elapsed time (t) is also measured, and the background-corrected susceptibility, Mscorr, is calculated as

MScorr = MSmeas + MSbkgd/tbkgd x t.

Absolute Susceptibility Values

The Bartington instrument output values are relative, volume-specific susceptibilities (krelative), which must be corrected before they can be reported in SI units. Currently, no correction is implemented for standard queries from the database. Two ways of correcting the susceptibilities are described here (see also Blum, 1997).

  1. Bartington correction factors. Theoretically, the instrument output is in volume-specific SI units for cores with diameters (d) passing exactly through the coil diameter (D) (i.e., if d/D = 1). Bartington provides a table relating values of d/D to correction factors that must be applied to the relative susceptibility readings from the meter. For d = 66 mm and D = 88 mm, d/D = 0.75 and the corresponding correction factor is 1.48. Then,
    k = krelative /1.48 x 10–5 = 0.68 x 10–5 krelative.
    This correction does not take into account other effects such as those from core liner and core conveyer boat, and so on.
  2. Calibration with laboratory measurements. Absolute susceptibility is easily measured on sample cubes in shore-based or shipboard laboratories (Kappabridge). These measurements can be compared with corresponding readings from the Bartington instrument. Empirical correlation from Leg 154 and Leg 162 data gave correction factors of 7.7 x 10–6 and 8.0 x 10–6, respectively. On Leg 154, volumes of specimens were not exactly determined and may have been slightly smaller than assumed, which would underestimate the factor.

Performance

Precision is 2 x 10–6 (SI). Susceptibility values in natural marine sediment samples over an interval of only a few meters (Milankovitch or millennial-scale cyclicity) can range from a few tens to several thousands of 10–6 SI units. Typically, variations are two to three orders of magnitude greater than the precision. This makes MS one of the most precise proxies for stratigraphic changes and extremely useful for core-to-core correlation. Accuracy is 5% (according to Bartington).

Spatial Resolution

Blum (1997) determined the full-width-half-maximum (FWHM) response from measurements of four thin discs with varying amounts of iron dust and determined that the widths associated with half-maxima ranged from 4.0 to 4.4 cm. The width along the core axis corresponding to >99% response is ~15 cm. It is recommended that the first and last measurement in each core section be taken 3–4 cm away from the edge to avoid any deconvolution of edge effects.

Bartington Sensor for Discrete Samples

Equipment

The paleomagnetism laboratory is equipped with a Bartington Instruments magnetic susceptibility meter (model MS2), a power supply, and a dual-frequency sensor unit (MS1.B 10 cm3 single sample well) for performing discrete measurements of MS (Fig. F17).

The MS1 sensor contains an inductor, which, together with a capacitor and a current-controlled oscillator, creates a low-frequency alternating magnetic field at a frequency of 0.565 KHz and an AF intensity of 80 A/m (= 0.1 mT). Any magnetic material brought within the influence of this field will effect a change in frequency of oscillation. This change is transmitted to the MS2 meter via a coaxial interconnection cable.

The MS2 meter converts the pulse information from the sensor into an initial susceptibility value. Power for the instrument may be supplied from either a 12-V DC adaptor or from internal rechargeable NiCd batteries.

Dual-Frequency Measurements

Fine-grained magnetic material (single domain; diameter = ~0.003 quency dependence can be determined from measurements in dual-frequency mode. The high frequency used is 5.65 kHz.

Absolute Susceptibility Values

The instrument is preset to display the susceptibility value directly in one of the dimensional systems, thus producing a basic mass or volume specific unit of:

 
Mass
Volume
Symbol X k
S.I. (used by ODP) 10–8 (kg/m3) 10–5
cgs 10–6 (g/cm3) 10–6

Susceptibility values assume a sample volume of 10 cm3. Displayed susceptibility values (kdisplay) have to be corrected for their volume (V in cm3) by

k = kdisplay x 10/V x 10–5

for volume specific susceptibility in the SI system.

Performance

Two sensitivity positions are provided with the instrument: range 1 or 0.1. Accuracy on the order of 1.0 x 10–5 SI can be obtained with a 1-s measurement cycle (1 range) and an accuracy of 0.1 x 10–5 SI with the 10-s cycle (0.1 range). The Zero (Z) and Measure (M) functions are selected on a three-position toggle switch. Measurements are displayed on a four-digit LCD panel.

Kappabridge KLY-2

The Kappabridge KLY-2 magnetic susceptibility system (Fig. F18) measures MS and AMS of hard rock or sediment samples at sensitivities of 0.05 x 10–6 SI to 200,000 x 10–6 SI within a series of 11 ranges. The instrument's operation is based on measurements of inductivity changes in a coil due to a rock specimen. The semiautomatic inductivity bridge is operated in conjunction with a Pentium PC and manufacturer-supplied software. The software package includes tensor calculation and statistics, graphic data display and printout, and data storage.

For magnetic anisotopy determinations, susceptibilities are measured in 15 directions (Fig. F19); the susceptibility tensor is determined using the least-squares method, and the accuracy of the anisotropy is determined according to the methods of Hext (1963) and Jelinek (1978). The standard sample size is 10 cm3, although fragments may be measured in a 40-cm3 container. The front panel contains the power and start/reset switches, a range selector, a status display, a zero setting, and a digital data display. Details of instrument operation and data analysis are provided in a manual supplied by the manufacturer. Other accessories include a computer interface manual, a step-by-step operation manual, and reports from users on previous legs. Also, various specimen holders and a calibration standard can be found in a black briefcase stored in the laboratory.

Note: The instrument's high accuracy, fast measuring rate, and outstanding sensitivity makes it possible to measure specimens with very weak magnetic properties. But as a shipboard instrument onboard the JOIDES Resolution, the Kappabridge receives relatively little use because of its very high sensitivity amidst magnetically noisy surroundings. Thus, the location of the sensor is an important factor when using the system. Avoiding placement of the sensor near computer terminals, other instruments, and metal fixtures will certainly improve its performance. Overall, the best results come from measuring samples that have fairly strong magnetic properties (Range 5 and higher).

Zeroing and Calibration

After turning the Kappabridge on, the instrument needs to be zeroed with the Im (on the meter) and Re (on the sensor) knobs until the needles in the two displays on the left side of the meter point to zero. Detailed instructions are provided in the manual and in "Appendix B." A calibration standard is provided in the black briefcase together with other accessories. The instrument is very stable and calibration should not be necessary, but it is good practice to measure the standard occasionally to make sure the instrument is properly calibrated ("Appendix B").

Maintenance

Turn the susceptibility bridge off when not in use. Keep the bridge and sensor covered when not in use.

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