-
A = 0.5
r2
The permanent magnetism present in a rock before any treatment is termed the NRM. Often the magnetization remaining after various demagnetization treatments, but not after imparting artificial magnetization, is also referred to as the NRM. Because many artificial magnetization components are commonly demagnetized and analyzed by paleomagnetists today (e.g., ARM and IRM), the usage for NRM has become somewhat complicated. To be specific that the remanence being measured is that before any laboratory treatment, terms like "original NRM" or "NRM prior to demagnetization" are commonly used, although some paleomagnetists would still simply use the term "NRM." Similarly, common usages for the remanent magnetization following some level of demagnetization are, for example, "NRM following AF demagnetization at 20 mT," "NRM after 20 mT demagnetization," or "NRM after thermal demagnetization at 250°C." Simply stating "remanent magnetization after 20 mT demagnetization" is ambiguous because the remanence could have been imparted by a variety of means. The use of "natural" in front of remanence magnetization as in "NRM after 20 mT demagnetization" clarifies that the investigator is not looking at an artificially imparted magnetization that has been subsequently demagnetized.
The NRM is usually the vector resultant of the primary magnetization acquired when the rock was formed and the secondary magnetization (which may include several components) acquired during subsequent geological time, or during coring and sampling. The major part of all paleomagnetic investigations involves the recognition and separation of these magnetic components. Magnetic components are commonly isolated by means of partial demagnetization of the natural remanence (thermally or in an AF), as the primary component will normally have a different stability to demagnetization procedures than the secondary components. The ways in which most natural materials acquire their primary remanence is by no means simple and, ideally, each sample needs to be examined in detail to determine the nature and origin of its remanence prior to any interpretation. Because it is often difficult to conclude a magnetization is primary without uncertainty, many paleomagnetists prefer use the term characteristic remanent magnetization (ChRM), which avoids genetic connotations. The different types of remanence acquisition and the magnetization of natural materials are discussed in every standard paleomagnetic textbook (e.g., Stacey and Banerjee, 1974; Piper, 1987; Butler, 1992; Opdyke and Channell, 1996; McElhinny and McFadden, 2000).
The NRM can be affected by the coring process, core splitting, sampling, and the magnetic shipboard environment. Although these overprints are generally soft and can be successfully removed by demagnetization techniques, cases where a removal of the coring-induced overprint was not possible have been described (e.g., Curry, Shackleton, Richter, et al., 1995; Fuller et al., 1998; Acton et al., 2002). In addition, the NRM may be ephemeral because alteration of magnetic minerals can be rapid in some sediments when they are exposed to oxygen after being buried in reducing conditions (e.g., Richter et al., 1999; Yamazaki et al., 2000). A detailed discussion of the overprint and other problems is provided in "Problems with Magnetic Measurements in the Shipboard Environment."
The ChRM of sediment cores is primarily used for initial magnetostratigraphic interpretation in consultation with biostratigraphy to establish an age model. If it is possible to find a proper normalizer to account for variations in the concentration of magnetite and to fulfill a set of established reliability criteria (Tauxe, 1993), magnetic intensity can be used to determine variations in the relative geomagnetic paleointensity. Magnetic remanence is also an essential tool in tectonics and geodynamics because magnetic inclination is directly related to the latitude at which the magnetization was acquired and changes in the magnetic declination record small- or large-scale block rotations. Declination may also be an important tool for azimuthal core orientation.
The most sensitive device for magnetic field detection is the Superconducting Quantum Interference Device (SQUID). The device has been developed for traditional low-temperature superconductors requiring cooling with liquid helium to 4 K (–269°C) and is commercially available from several suppliers.
The key device is a Josephson junction, which is made by sandwiching a thin layer of a nonsuperconducting material between two layers of superconducting material. The devices are named after Brian Josephson, who predicted in 1962 that pairs of superconducting electrons could "tunnel" right through the nonsuperconducting barrier from one superconductor to another.
To understand the unique and important features of Josephson junctions, it is first necessary to understand the basic concepts and features of superconductivity. Many metals and alloys exhibit a phase transition at very low temperatures (within 20° or less of absolute zero). At this critical temperature (Tc), the metal goes from what is known as the normal state, where it has electrical resistance, to the superconducting state, where there is essentially no resistance to the flow of direct electrical current. What occurs is that the electrons in the metal become paired. Above Tc, the net interaction between two electrons is repulsive. Below Tc, though, the overall interaction between two electrons becomes very slightly attractive as a result of the electrons' interaction with the ionic lattice of the metal.
This very slight attraction allows them to drop into a lower energy state, opening up an energy "gap." Because of the energy gap and the lower energy state, electrons can move (and therefore current can flow) without being scattered by the ions of the lattice. When the ions scatter electrons, it causes electrical resistance in metals. There is no electrical resistance in a superconductor and therefore no energy loss. There is, however, a maximum supercurrent that can flow, called the critical current. Above this critical current the material is normal. There is one other very important property for our purposes: when a metal goes into the superconducting state, it becomes perfectly diamagnetic and expels all magnetic fields, as long as the magnetic fields are not too large.
Until a critical current is reached, a supercurrent can flow across the nonsuperconducting barrier that separates the two superconductors; electron pairs can tunnel across the barrier without any resistance. But when the critical current is exceeded, another voltage will develop across the junction. That voltage will depend on time; that is, it is an AC voltage. This in turn causes a lowering of the junction's critical current, causing even more normal current to flow and a larger AC voltage.
The frequency of this AC voltage is nearly 500 gigahertz (GHz) per millivolt across the junction. So, as long as the current through the junction is less than the critical current, the voltage is zero. As soon as the current exceeds the critical current, the voltage is not zero but oscillates in time. Detecting and measuring the change from one state to the other is at the heart of the many applications for Josephson junctions.
Josephson junctions can be fashioned into circuits called SQUIDs (an acronym for Superconducting Quantum Interference Device). These devices are extremely sensitive and very useful in constructing extremely sensitive magnetometers and voltmeters.
A SQUID consists of a ring with two Josephson junctions interrupting the loop. A SQUID is extremely sensitive to the total amount of magnetic field that penetrates the area of the loop—the voltage that you measure across the device is strongly correlated to the total magnetic field around the loop. In a cryogenic magnetometer, a current is induced into the superconducting ring when a magnetized specimen is placed within it. The magnitude of the generated flux can then be measured and processed by a computer to determine the direction and intensity of magnetization of the specimen. For a complete account of the theory see Goree and Fuller (1976).
In the early 1970s, SRMs, also known as cryogenic magnetometers ("cryo"), were developed that can measure weakly magnetized samples more quickly than spinner magnetometers. Superconducting magnetometers contain magnetic sensors that are composed of pick-up coils with SQUID detectors, which are superconducting at liquid helium temperatures (4 K). The magnetic sensors are surrounded by an insulated evacuated space cooled by helium. Pick-up coils connected to SQUID detectors are employed to measure the NRM along one axis or simultaneously along two or three mutually perpendicular axes. Mumetal shields are employed to blanket external field variations, and automated systems have been developed to complete AF demagnetizations within this field-free environment. The deployment of superconducting cryogenic technology and the development of long-core magnetometers have important implications for the shipboard paleomagnetic investigations:
The paleomagnetism laboratory aboard the JOIDES Resolution is equipped with the world's only seagoing cryogenic magnetometer (Figs. F1, F10). The system, provided by 2G Enterprises of Mountain View, California, consists of a 2G 750R SRM as the principal instrument.
The first cryogenic magnetometer was installed in the shipboard laboratory in April 1985 during the Leg 103 port call. The built-in AF demagnetization capacity of the long-core magnetometer was 9 mT prior to Leg 123 and was limited to 30 mT until the addition of the new magnetometer at the Leg 168 port call (June 1996).
To extend the time needed between liquid helium fills, the new SRM magnetometer is fitted with a CTI Cryogenics refrigeration unit capable of cooling the magnetometer's inner (vapor-cooled) thermal shield to 18 K, just a few degrees above the liquid helium temperature of 4.2 K. The refrigerator consists of a cold head, mounted on the magnetometer, which is basically a heat pump fed compressed helium by the compressor unit and the compressor itself, which is in turn cooled by the ship's chill water supply. External electronics include the three SQUID consoles and their power supply console, controls and power supply for the Compumotor sample-handling system (Fig. F11), the Compumotor itself, and a power supply and relays for the in-line demagnetization coils.
The cryogenic magnetometer lies inside three concentric mu-metal cylinders. Its AF coil assembly is also contained within these shields. The AF field is limited to 80 mT. A superconducting lead shield surrounds the sensing region and maintains an absolutely stable field within that region (Fig. F12).
The paleomagnetism Marine Specialist monitors the vital signs of the instrument on a regular basis to ensure that temperatures remain constant within the instrument and to check helium boil-off. The cryocooler adsorber is changed annually.
The magnetometer system, including the sample track movement, the energizing of the demagnetization coils, and the measurement of moment, is controlled through a dedicated Pentium PC. The controlling program is called 2G Long Core and is written in Labview. A detailed user guide to the Labview software is available in the laboratory. The main Long Core control panel screen is shown in Figure F13. Sample holders for whole/split cores and for discrete samples are available and can be exchanged easily.
The 2G 750R SRM is a three-axis instrument configured for split- and whole-core pass-through measurements. The three SQUIDS and their sensors operate simultaneously and continuously, providing the user with a constant voltage output on the digital panel meters of the axis consoles.
The magnetic moments are obtained from the SQUID measurements (see Goree and Fuller, 1976). The SQUID output (Sx, Sy, and Sz) is given as a voltage, which is estimated from the number of flux counts (i.e., flux quanta) plus a proportion of a single flux count that occurred when the sample was in the sensor region. This proportion of a single flux count has been called the analog signal (fa). For the new magnetometer, fa is a fairly small part of the total signal (i.e., the number of flux counts will generally be >>1). The voltage (V) is directly related to the analog flux; thus, the SQUID output can be considered to be in units of flux quanta or voltage. SQUID voltage is converted to a magnetic moment in units of emu by multiplying by a calibration constant (supplied by 2G Enterprises) for each coil (Cx, Cy, and Cz). Calibration constants used with the system configuration as of October 1996 are Cx = 8.21 x 10–5 emu, Cy = 8.34 x 10–5 emu, and Cz = 4.32 x 10–5 emu.
Thus, to obtain the magnetic moments in emu units (Mx, My, and Mz) the SQUID output needs to be multiplied by the calibration constants
The magnetization per unit volume J is obtained by normalizing the magnetic moments by the volume of the discrete sample (V):
The volume normalization for whole and split cores is more difficult because the volume of core measured by each coil varies because each coil senses a slightly different length of core. In addition, the z-axis of the sample is long relative to the other sample axes, and the z-axis sensor senses a longer interval than the other sensors. This length is referred to as the effective sensor length. To determine the effective sensor length for each coil, a small sample (effectively a dipole point source) is moved through the sensor region at 1-cm intervals. The variation in the moment as a function of position along the three axes is obtained (Fig. F14). The curves are normalized and the area under each curve is estimated. If the curves have negative side lobes (x- and y-axes), the area under the side lobes is subtracted from the total area. The area under the normalized curves gives the effective sensor length (Lx, Ly, and Lz). The effective sensor lengths for the x-, y-, and z-axes, respectively, as estimated during Leg 168 are:
The effective volume (Ve) and intensities for each sensor are determined as follows:
where A is the cross-sectional area of a long sample and r the radius:
r2
for split cores and
r2
for whole cores. ODP core liners have an internal radius of 3.3 cm.
The magnetization per unit volume J is obtained by normalizing the magnetic moments by the effective volume:
Magnetic inclination and declination are then calculated from Jx, Jy, and Jz using standard procedures as outlined in any paleomagnetic reference book and the Long Core User's guide that is available in the shipboard laboratory.
A cube standard purchased from Geofyzika, which has a volume of 8 cm3 and an intensity of 7.62 A/m (moment of 6.096 x 10–5 Am2), is available on the ship for testing the calibration of the magnetometers. Tests conducted on Leg 200 confirmed that all three axes of the SRM agree to >1% with this standard. In addition, the automated tray positioning was checked by putting the standard at known positions and measuring the tray. The position indicated by the software was found to be good to within 1 cm, which is reasonable given the stretch in the pulley system used to move the sample boat (Shipboard Scientific Party, 2003).
Based on tests conducted during Leg 186 and 200, the background noise level of the magnetometer in the shipboard environment is about 2 x 10–9 Am2 (Shipboard Scientific Party, 2000, 2003). When the discrete sample tray is placed in the sample boat, the noise level increases, being slightly greater than x 10–9 Am2 before cleaning (where cleaning consists of using window cleaner and AF demagnetization) and slightly less after. The clean discrete sample tray measurement with the addition of empty plastic cubes in the tray results in noise that can exceed x 10–9 Am2. From repeated tests made with sample boats and trays before and after cleaning, it can be concluded that dirt on the sample boat or tray dominates the background signal. Even with diligence, it is difficult to keep the trays clean given the amount of core material measured on a typical leg. Furthermore, the split-core sections are in a plastic core liner that generally has been stored in dusty conditions prior to coring and that resides in a metal core barrel just prior to core collection. The noise associated with the core liners will therefore likely be several times greater than the noise associated with the sample boat.
Under favorable clean conditions, the noise level will be thus be about x 10–9 Am2 or higher. For a split core, given the large volume of core material within the sensing region of the magnetometer, which is ~100 cm3, the minimum measurable remanent intensities will be greater than ~2 x 10–5 A/m. For discrete samples, which typically have volumes of 6–10 cm3, the minimum measurable remanent intensities will be greater than ~4 x 10–4 A/m. Results from measurements from several cruises indicate that accurate measurements are likely to be obtained when both split-core and discrete samples have intensities approximately two to five times higher than the background noise level or when they have intensities greater than ~10–4 A/m and 10–3 A/m, respectively.
Additional directional uncertainty occurs because the tray rotates a few degrees one way or the other as it is pulled through the magnetometer. This affects the accuracy of the moments for the x- and y-axes and, hence, the declination. On Leg 209, Jeff Gee and Miguel Garces noted that the rotation of the tray was systematic. Using calibration standards from Scripps, they determined that a rotation of 7° about the z-axis was necessary to remove the bias when measuring discrete samples. How systematic the rotation is with time and different types of samples is unknown.
Standard queries through a Web browser (Fig. F15) allow discrete and long-core data to be downloaded in a format that can be used for data analysis and representation with commercial plotting software.
The Web queries allow the user to
ZPlot files have the following format:
A detailed manual for the Long Core software and user manuals for the Zplot program are available in the paleomagnetism laboratory. The Zplot manual is also available on the Web (www-odp.tamu.edu/isg/appsdev/docs/zplot.pdf).
The principle of spinner instruments is the generation of an alternating voltage by the continuous rotation of a magnetized sample within or near a coil or fluxgate system. For a given sensor configuration, the amplitude of the output voltage is proportional to the component of magnetic moment perpendicular to the rotation axis, and the phase of the voltage is utilized to relate the direction of the measured component to a reference direction in the sample. The total vector is determined by spinning the sample about a second orthogonal axis, although in practice the sample is rotated successively about three axes to obtain average values of the NRM components and reduce the effect of inhomogeneity.
The Molspin Minispin spinner magnetometer (Fig. F16) is a basic, portable field unit interfaced with a computer (currently a laptop) for control and data acquisition. The software (PMagic) driving the Minispin executes spin sequences and calculates declination, inclination, and intensity corrected for the volume of the sample. PMagic also contains procedures for statistical analysis and plotting. A series of measurements is made on each sample as it is run through a demagnetization sequence. Ordinarily, six separate spin orientations are required to produce an accurate measurement. In general, the processing rate will vary with the NRM intensity and in response to demagnetization of the samples from a particular lithologic unit.
The Minispin can measure both rock and sediment samples up to 2.54 cm (1 in) cubed in size. According to the manual, the noise level varies from 0.2 mA/m (short spin) to 0.1 mA/m (long spin) for a 12.87-cm3 sized sample. The minimum measurable intensity is on the order of 0.1 mA/m and the maximum is on the order of 2500 mA/m. Parameters that can be specified in the PMagic software include short (6 s for 24 spins) vs. long (25 s for 120 spins) integration time and sensitivity range and four or six spin positions per sample. In general four spins are sufficient. Six spins are used when the operator is trying to get the maximum accuracy for a weak rock.
The absolute calibration of the Minispin is carried out with a standard calibration specimen provided by the manufacturer of the instrument. Turn the power supply and the Minispin on, and the motor will spin for ~1 s and the liquid crystal display (LCD) will display "32," indicating that the instrument is functioning correctly. Allow 5 min equilibration time before calibrating.
Start the PMagic software and select the "Spin" menu option to start the spinner session. Select calibration and mount the calibration standard with the V-nick toward the operator and the arrow on the standard pointing away. Select position 1 on the attenuator and select short spin. Rotate the white ring around the sample access clockwise or counterclockwise to adjust the declination to only the white ring, not the entire shield cylinder. If the declination value is significantly off (i.e., ~180°), do not try to move the ring all the way around. Something is wrong; likely the standard is backward or the sample holder is not rotating smoothly.
The intensity of the standard needs to be entered next. The value of the current standard in use is 761 mA/m. The Minispin will remeasure the standard and calibrate itself. If the intensity value is not close enough, the calibration procedure can be repeated until the values are satisfactory. Molspin recommends calibrating the Minispin every hour of operation.
After finishing the calibration procedure the Minispin is ready for the measurement of rock specimens.
Note that spinner data are rarely collected and that the ODP relational database model does not include Minispin data. Thus, an interface between the Minispin and the ODP relational database does not exist.
A detailed manual for the PMagic software and user manuals for the Minispin are available in the paleomagnetism laboratory.
A Hall effect gaussmeter (model MG-5DP), capable of measuring DC and AC fields over three ranges of kGauss (capacity in each range) is available in the laboratory for calibration of demagnetization coils and measurement of strong DC fields.
This portable gaussmeter operates according to the Hall effect principle. When a current-carrying conductor is subjected to a magnetic field that is perpendicular to the current flow, a drift current is produced that is perpendicular to both the control current and the magnetic field. If the conductor is a semiconductor, a substantial voltage is produced in the direction of the Hall drift current. The Hall voltage produced is directly proportional to the product of the control current and the strength of the magnetic field.
Precalibrated transverse and axial probes are supplied with the instrument for measuring fields perpendicular and parallel to the axis of the probe, respectively. The instrument is equipped with a peak-reading mode that, when selected, provides peak readings of either positive or negative magnetic field levels. The MG-5DP model will operate either from AC or from sealed lead acid batteries.
The portable APS520 fluxgate magnetometer in the shipboard laboratory enables measurement of small ambient magnetic fields over the range of 10–7 Oe up to 1000 mOe and a sensitivity of 10–6 Oe (0.1 gamma). The field is measured simultaneously along three orthogonal directions and is displayed on three front panel digit LCDs.
The fluxgate system consists of a magnetic field measuring probe connected by a 3-m interconnect cable to a power supply and electronic readout console. The small probe size permits measurements to be made in restricted spaces, such as the sample access tube of the cryogenic magnetometer. Other applications for the fluxgate system include measurement of magnetic fields inside steel and mu-metal enclosures, measurement (and zeroing) of the magnetic field trapped in superconducting shields, study of the geomagnetic field, and measurement of magnetic properties of materials.
