USGS-GD-Scientific Capabilities - PALEOMAGNETISM ANALYSES Equipment

PALEOMAGNETISM
ANALYSES

Equipment

 

Menlo Park Laboratory:

  • Superconducting rock magnetometer. This instrument measures the remanent magnetic moment vector of specimens up to 20 cc in volume with a practical sensitivity from 1 emu to 5 x 10- 8 emu. The magnetometer is housed in a shielded room that reduces the ambient field to 1% of its normal value, thereby reducing measurement time and minimizing contamination from stray magnetic fields. Typical measurement time per sample is 2 minutes; data are logged and reduced by online microcomputer.
  • Spinner magnetometers. The laboratory maintains a shielded flux-gate magnetometer for high-sensitvity work and a high- speed spinner with coil detector for strongly magnetized materials. The flux-gate instrument, equipped with digital data logging and reduction, has a practical sensitivity of 5 x 10-7 emu. Typical measurement time per sample is 6 minutes.
  • Demagnetization equipment: Alternating field and thermal methods The AF unit operates at 400 Hz, producing a maximum magnetic field strength of 1000 Gauss (0.1 Tesla). The sample is held in a reciprocating tumbler to reduce spurious magnetization induced by AF treatment. Thermal demagnetization is carried out in a shielded furnace (internal magnetic field less than 5 x 10-5 Gauss) which holds up to 40 standard specimens per 4-hour heating/cooling cycle. Heatings are conducted in air atmosphere. Both demagnetization devices are housed in the shielded room near the cryogenic magnetometer, minimizing contamination from stray magnetic fields during transport between devices.
  • Paleointensity apparatus Thellier experiments, that determine the strength of ancient magnetizing fields, are carried out in a system of six furnaces; each can apply a set axial magnetic field or can be run in a near-zero field. Ambient field reduction is supplied by a Rubins coil array with automatic feedback to keep the working-area field below 1 x 10-4 Gauss. The furnaces can be operated in vacuum or in an inert atmosphere (argon or nitrogen) to inhibit oxidation of the specimens. Up to 30 specimens can be loaded in the system per heating/cooling cycle.
  • Curie balance Strong-field magnetization and Curie temperatures are measured in this electromagnet/balance system. The specimen (typically 0.5-1.0 gram; solid or granular) is suspended between the electromagnet pole pieces which produce a magnetic gradient force on the specimen. A balance automatically records apparent weight, which is proportional to magnetization, as the specimen is heated. Heatings can be performed in air, vacuum, or inert gas. The electromagnet field limit is 8000 Gauss (0.8 Tesla). The furnace and magnet are controlled automatically, and the magnetization/temperature data are recorded digitally. A complete heating/cooling cycle up to 700 degrees C runs in 2.25 hours. Strong-field magnetization curves can be run manually, giving values of saturation magnetization.
  • Electromagnet Isothermal remanent magnetization is imparted to the specimen at room temperature with an electromagnet, capable of producing DC fields up to 7000 Gauss (0.7 Tesla). Magnetic remanence is then measured with one of the magnetometers. Saturation remanent magnetization and coercivity of remanence are measured with this system.
  • Susceptibility meter Low-field magnetic susceptibility is measured with a Sapphire SI-2 system. The system accommodates specimens up to 20 cc in volume; a larger pass- through detector is also available for use with long sediment cores up to 10 cm in diameter. Bulk susceptibility and anisotropy of susceptibility can be measured. The data are recorded and reduced automatically by an online microcomputer.
  • Rock drills and other field equipment. The laboratory maintains a full complement of portable rock drills, sample orienting equipment, and sediment sampling tools.
 


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URL http://geology.cr.usgs.gov/capabilities/paleom/equip.html