Lecture 24

• thermoremanent magnetization (TRM)

  rocks cooling in the Earth field
• detrital remanent magnetization (DRM)

  rock particles align with Earth field as they settle
• iron-titanium oxides as remanence carriers

  the most common magnetic minerals form in igneous rocks along the solid solution lines. Magnetite and titanomagnetite eroded from igneous rocks make up a major component of the magnetic minerals in sedimentary rocks
• titanomagnetites

  magnetization becomes smaller and less stable as temperature increases

  magnetization and stability falls to zero at the Curie temperature Tc
• for magnetite Tc = 580 degrees C and decreases with increasing Ti content
• a TRM is acquired during cooling of volcanic rocks as atomic moments start interacting below the Curie temperature (blocking temperature)
• as a rock cools the magnetization in the minerals align with the external field. magnetization becomes fixed in the direction of the external field at the blocking temperature. the strength of the acquired magnetization is linearly proportional to the external field strength
• Néel Theory

  the strength of a remanent magnetization will decay over time, obeying an exponential law with a characteristic decay time $Mt\ =$$\ M0\ e^{\left(-\frac{t}{τ}\right)}$. the larger the relaxation time, the more slowly the magnetization will decay

  $τ\ =$$\ τ0\ e^{\frac{Eb}{kT}}$
• a DRM is acquired due to magnetic grains aligning with the ambient field during deposition of a sediment. lithification preserves this alignment
• post-depositional remanent magnetization (pDRM)

  a pure DRM is unlikely to survive physical disturbance due to e.g. bioturbation during deposition. exoeriments suggest that in the 'soupy' unconsolidated uppermost sediments, the geomagnetic field can still exert a torque on magnetic particles and realign them. most sedimentary magnetisations are probably pDRMs rather than DRMs
• detrital magnetic particles

  titanomagnetites
• authigenic magnetic minerals

  commonly greigite and pyrrhotite (both ferrimagnetic, both iron sulphates)
• biogenic magnetic minerals

  fine grains and chains of magnetite or greigite
• primary vs secondary magnetisations

  the natural remanent magnetization (NRM) is vectorial addition of the original (primary) and secondary magnetizations that rocks have acquired over time. in order to isolate the primary magnetisation direction, the less stable secondary components must be removed
• primary magnetisation processes

  • thermo remanent magnetisation (TRM)
  • detrital remanent magnetisation (DRM)
  • post depositional remanent magnetisations (pDRM)
• secondary magnetisation processes

  • chemical remanent magnetisation
  • viscous remanent magnetisation
  • isothermal remanent magnetisation
• chemical remanent magnetisation (CRM)

  weathering and diagenesis can lead to growth of magnetic minerals at any time after formation or deposition
• viscous remanent magnetisation (VRM)

  magnetisation changes over time. the primary remanence may decay, and realign in the direction of the present field
• isothermal remanent magnetisaton (IRM)

  lightning will completely destroy primary magnetisation. instantly recognizable by an unusually high magnetisation
• sample collection

  basic sample collection strategies
• sample measurement - detection of secondary magnetizations

  stepwise thermal and alternating field demagnetization methods to detect secondary components
• paleomagnetic sampling

  requirement: have to be sampling GAD field

  this requires measurements averaged over a time interval of >20,000 years to negate the effects of secular variation. typically 10-20 sampling sites are distributed over a continuous stratigraphic section
• establish that the most stable component is primary

  to determine whether a measured magnetisation is ancient or due to later remagnetisation, the use of field tests is essential - if you don't know the age of the magnetisation, you can't use it
• fold test

  if a geological unit was magnetized prior to folding, unfolding will produce greater clustering of paleomagnetic direction -> magnetisation is ancient (pre-folding). if a unit was magnetised dueing or after folding, greater clustering will occur before unfolding
• conglomerate test

  if a geological unit contains an intra-formational conglomerate, the conglomerate should have a random magnetisation if the magnetisation is primary
• baked contact test

  when an igneous body intrudes the country rock, heating will impart a TRM in surrounding rocks, which should align with the magnetisation of the intrusion itself. these should differ from the remanence of the unheated country rock