Geology Reference
In-Depth Information
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If magnetite or magnetite plus iron sulfides are found in the sam-
ples, then ARM is the best choice for measuring magnetic min-
eral concentration variations throughout the section. Usually, a
peak field close to 100 mT is the limit for ARM application in
most paleomagnetic laboratories and will efficiently activate
magnetite and greigite (Fe
3
S
4
) grains in the sample. The bias field
chosen will vary with the equipment used, but is usually a low
field close to the geomagnetic field in strength (~50 μT). The
results should be normalized by the bias field strength for
comparison to ARMs applied with different bias fields. ARMs can
be applied by modified alternating field demagnetizers either
home-made or commercially available.
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If hematite is the dominant ferromagnetic mineral in the samples,
then an IRM can be used to measure its concentration variations.
If both magnetite and hematite are present, first an ARM can be
measured followed by application of an IRM. Alternating field
demagnetization of the IRM at ~100 mT can remove the effects of
magnetite activated by the application of the IRM to isolate the
concentration variations of the hematite. An impulse magnetizer
which discharges a capacitor through a coil surrounding the
sample is used to apply IRMs in fields as high as 1 T for a standard
sized paleomagnetic sample (2 × 2 × 2 cm plastic box) and 5 T for
the small 11 mm diameter cores or small plastic cylindrical
sample holders.
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If ancient goethite and hematite are both present in the samples,
the goethite to hematite ratio can be measured using Equation
(2.5) or Equation (6.1). The sequence of measurements for the
G/H ratio is as follows: IRM applied in a high field, usually bet-
ween 3.5 and 5 T to activate all the ferromagnetic minerals in the
sample, thermal demagnetization of the IRM at about 130°C to
remove any goethite magnetization, and then alternating field
demagnetization at about 100 mT to remove the magnetization of
any low coercivity magnetic minerals, like magnetite.
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Magnetic parameter ratios can be measured to detect slight varia-
tions in the amount of magnetite relative to hematite (S-ratio) or the
magnetic particle grain size of magnetite (ARM/χ or ARM/SIRM).
For magnetic particle size variations, it is important that all the
magnetic parameters are carried by the same magnetic mineral.
7.5
Time Series Analysis: Summary of Procedures
Once the rock magnetic measurements have been made and the data series
have been constructed, time series analysis is used to detect periodicities in
the data. Chapter 4 provides the theoretical background for the analyses
used to detect cycles in a data time series; this chapter outlines the practical
