ALTERNATING FIELD AND THERMAL DEMAGNETIZATION

Principle

Recognition and removal of secondary NRM is the major goal of paleomagnetic laboratory work. Thus, paleomagnetists stress the importance of isolating the characteristic component of NRM by selective removal of secondary NRM. Partial demagnetization experiments, using either AF or thermal demagnetization, are routinely performed in the laboratory using various instruments to isolate ChRM.

Alternating Field Demagnetization

Demagnetization consists of applying a decaying alternating magnetic field to a sample. In the absence of external direct magnetic fields and significant distortion in the applied AF, the sample will be "cleaned" of any remanent magnetization of coercivity less than the peak intensity of the applied AF. This cleaning is the result of randomizing the mobile magnetic domains along the axis of the applied field.

Because it is decaying, the amplitude of each half-cycle of the applied AF is smaller than its predecessor. With each half-cycle, the domains whose coercivities are less than the applied field align themselves with the field. During each half-cycle of the AF, a small percentage of these mobile domains will have coercivity greater than the following half-cycle and will therefore become fixed in direction. In this way, equal numbers of domains will be magnetized in the positive and negative directions oriented along the axis of demagnetization, resulting in a net zero remanent field on the sample.

AF demagnetization is often effective in removing secondary NRM and isolating characteristic NRM (ChRM) in rocks with titanomagnetite as the dominant ferromagnetic mineral. In such rocks, secondary NRM is dominantly carried by multidomain (MD) grains, whereas ChRM is retained by single-domain (SD) grains. MD grains have coercivity (hc) dominantly <20 mT (200 Oe), whereas SD grains have higher hc. AF demagnetization thus can remove a secondary NRM carried by the low-hc grains and leave the ChRM unaffected. AF demagnetization is a convenient technique because of speed and ease of operation and is thus preferred over other techniques when it can be shown to be effective.

Thermal Demagnetization

Relaxation Time and Blocking Temperature

In order to discuss the theory behind thermal demagnetization of a specimen, it is necessary to understand the principles of relaxation time and blocking temperature for a SD grain. Relaxation time, the time over which remanent magnetization of an assemblage of SD grains decays, may vary over many orders of magnitude. Relaxation time for SD grains of a given material at a constant temperature depends on grain volume (v) and microscopic coercive force (hc). Grains with low product (vhc) have short relaxation time, whereas grains with high product (vhc) have long relaxation time. Ultimately, these properties help to define the range over which an SD grain will remain stable.

Relaxation time, however, has strong temperature dependence. For instance, relaxation time for an elongate SD magnetite grain with a length of 0.1 the age of Earth at 510C. With decreasing temperature, this grain changes behavior from superparamagnetic (unstable; it will decay to zero very soon after removal of the magnetizing field) to stable SD at 550C. The temperature at which this transition occurs is the blocking temperature. Between the Curie temperature (TC ; temperature at which saturation magnetization becomes zero; 580C for magnetite) and the blocking temperature (TB), the grain is superparamagnetic. Below TB, relaxation time increases rapidly during continued cooling. SD grains with short relaxation time also have low TB.

Rocks have distributions of ferromagnetic grain sizes and shapes yielding distributions of TB between TC and surface temperatures. The strong dependence of relaxation time on temperature and the transition in behavior from superparamagnetic above TB to stable below TB are critical to understanding acquisition of thermoremanent magnetism.

Theory of Thermal Demagnetization

The procedure for thermal demagnetization involves heating a specimen to an elevated temperature (Tdemag) below the Curie temperature of the constituent ferromagnetic minerals and then cooling to room temperature in zero magnetic field. This causes all grains with blocking temperature (TB) < Tdemag to acquire a "thermoremanent magnetization" in H = 0, thereby erasing the NRM carried by these grains. In other words, the magnetization of all grains for which TB < Tdemag is randomized, as with low hc grains during AF demagnetization.

SD grains with short relaxation time also have low TB and can more easily acquire secondary NRM, whereas SD grains with long relaxation time are stable against acquisition of secondary NRM. Thus, thermal demagnetizers are effective in selectively erasing secondary NRM when Tdemag > TB of grains carrying secondary NRM, leaving unaffected the ChRM carried by grains with longer relaxation time (= higher TB).

Schonstedt TSD-1 Thermal Demagnetizer

The TSD-1 (Fig. F20) is used to provide progressive thermal demagnetization of rock specimens by heating them to any specified temperature up to 800C and then cooling them in a low magnetic field environment (<10 nT). External fields are attenuated by the shield assembly so that the instrument can be operated in a laboratory environment. Figure F21 shows the magnetic field profile of the TSD-1 and the position in the cooling chamber between 70 and 105 cm in which the magnetic field is the lowest.

The instrument consists of two separate chambers: one for heating and one for cooling. They are arranged coaxially so that as soon as specimens in the furnace chamber reach thermal equilibrium, the specimen holder can be pushed directly into the cooling chamber. Complete chamber isolation and individual controls allow one specimen batch to be blower cooled while another batch is being heated. An outstanding feature of the instrument is its processing speed. The low thermal mass of both the furnace and sample holder promotes rapid heating and cooling of specimens. The short cycle time helps to minimize the amount of chemical change in specimens by limiting their exposure to high temperature.

The basic sample holder is an open assembly with two inconel tubes on which lies a quartz tray for placing specimens. The quartz tray can hold up to 10 specimens, each 2.5 cm in diameter x 2.5 cm in length. Scientists on board the JOIDES Resolution often find it convenient to heat/cool a batch of specimens in the TSD-1 while measuring a second batch on the cryogenic magnetometer and vice versa, thereby speeding up the processing of data on thermally demagnetized samples.

TSD-1 Operation Procedures

  1. Place samples on the quartz tray aligned in the same orientation and place the tray on the inconel specimen holder. Open the door to the oven on the forward (left) end and gently insert the holder into the oven just far enough so that the door can be closed. Close the door.
  2. Depress the white "POWER" button on the control panel and set the desired temperature with the thumbwheel. Depress the red "HEAT" button to begin heating the samples. The "HEAT" button will initially glow and then will cycle when the oven approaches the final temperature. The average rock specimen reaches equilibrium (heated to its center) between 5 and 10 min after the oven reaches its final temperature.
  3. Once the specimens have reached thermal equilibrium, open the door and push the boat into the cooling chamber with a fire resistant, nonconductive rod (we use the wooden rod placed in front of the oven on the shelf, but DO NOT leave the rod in the oven). The boat should be pushed in ~1 in beyond the mouth of the cooling chamber. At this point, another boat of specimens may be inserted into the heating chamber if desired. Close the door and activate the cooling blower by pushing the blue "COOL" button. Cooling time is about half of heating time. Note: It is recommended that specimens be moved into the cooling chamber as quickly as possible so as to avoid freezing in undesired remanents.
  4. Remove samples after they have been sufficiently cooled.
  5. Measure the samples on the SRM and then repeat the process.

Caution: NEVER place plastic sample cubes in oven when heating above 200C. Instead, remove sediment from the cube carefully and wrap in aluminum foil.

Sedimentary rocks may explode at high temperatures if they contain water. It is best to dry these samples at a moderate temperature before exposing them to higher temperatures.

Do not turn the power off while the samples are still in the oven or cooling chamber, as this may impart spurious magnetizations to the samples.

When handling samples, avoid placing hands on the quartz boat if possible. Fingerprints or smudges may cause permanent marks on the quartz when heated.

Be sure to engrave orientation arrows into the specimen before heating. Indelible ink will not remain visible at higher temperatures.

DTECH Model D-2000 Demagnetizer

A DTECH alternating field demagnetizer, model D-2000, is available for demagnetization of discrete samples of rock or sediment. The D-2000 unit consists of a an AF demagnetizer coil and sample access tube and is enclosed within a mu-metal shield (Fig. F22). The demagnetizer unit is connected to a D-2000 electronics controller and a Crest CA-9 power amplifier. The D-2000 hardware is entirely controlled via a user-friendly, windows-based software application located on a Compaq PC laptop computer.

The unit can demagnetize five to six samples simultaneously at peak AFs of up to 200 mT. The user may also choose a decay rate of the applied demagnetization field intensity within a range of 0.1–0.001 mT/half-cycle. An extremely thorough and useful online help manual is provided with the software.

DTech Operation Procedures

Startup Procedure
  1. The DTech 2000 demagnetizer is controlled via an RS232 cable that connects the controller to the small Compaq laptop located at the forward end of the laboratory. Make certain this cable is plugged into the rear of the computer.
  2. Click on the "D2000v30" icon on the desktop to start the program.
  3. Turn on power to the D-2000 controller (switch in back).
  4. Turn on power to the Crest power amplifier.
  5. Turn both Crest power amplifier channels fully clockwise to "0dB."
Sample Loading and Demagnetization
  1. Extract the green sample holder from the demagnetizing unit and place samples in the cutout in the sample holder insert. Be consistent and orient the samples similarly. The D-2000 coils will demagnetize specimens along the Z-axis of the unit (fore to aft).
  2. Choose Demag Info "Settings" and enter in values for peak demagnetization field and decay rate. Choose "Add" and then "Accept."
  3. Choose "Run Demag" and wait for the run to complete.
  4. Remove samples and measure on the cryomagnetometer. If desired, repeat the process and enter in a higher peak field demagnetization value.
  5. Consult the on-line Help menu for more detailed information concerning the operation of the DTech 2000.
Shutdown Procedure
  1. Choose "Halt" if demagnetization is in progress.
  2. Important: Turn both Crest power amplifier channels to "–80dB."
  3. Power off the Crest power amplifier. Important: The coil fans are powered from the D-2000 controller power supply. In order to avoid overheating the demagnetizing coil, the fans should be left running for several minutes after extensive demagnetizing at high field levels.
  4. Power off the D-2000 controller.
  5. Exit from the D-2000 control program.

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