|
general ideas:
The sequence of measurements outlined below starts
with low-field measurements and works its way up to higher fields or
measurements that are more time consuming and should be conducted once
you have a general idea about the magnetic properties of your core. It
also includes a few steps that allow you to check the quality of your
data.
- Low-field
susceptibility (χlf): fast, easy and non-destructive,
gives you a first idea about the abundance of magnetic minerals in
your samples. If you have a dual-frequency sensor you can also
measure frequency-dependent susceptibility (χFD) for
information of SP grains.
- if you want to
do a paleomagnetic analysis: now is the time.
- Anhysteretic
Remanent Magnetization (ARM): can yield information about the
presence of small (SD) grains. I prefer a peak AF-field of 100mT and
a bias field of 50μT. ARM can also be expressed as χARM.
STOP - now it's a good time to plot your data: (χlf, χARM) as well as the ratio
of ARM/χlf. Plotting your data now
allows you to get an idea about the magnetic variations occurring in
your core and gives you a first check on whether you did everything
correctly. Does everything make sense? Are there any unexpected outliers
that might be caused by a measurement error? It's still time to go back
and repeat either of the measurements. ARM/χlf also
gives you a first idea about changes in magnetic grain size. |
- Isothermal
Remanent Magnetization (IRM): gives you more information on the
remanence-carrying fraction and the ARM-ratio is useful for magnetic
granulometry. I prefer an acquisition field of 100mT (same as peak
AF field during ARM acquisition). In my opinion that makes the
ARM-ratio a more useful granulometric tool as both ARM and IRM
affect the same coercivity ranges. Check your data by plotting IRM
and ARM-ratios.
- Saturation
Isothermal Remanent Magnetization (SIRM): same as IRM but
measured at higher acquisition field (1000mT +).
- S-ratios:
are a rapid way to assess magnetic coercivity and hence
mineralogy. Backfields can range from 100mT (then probably more a
granulometric tool) to 300mT and above.
By now you have
pretty much ruined the initial magnetic signal of your sample, so the
remaining measurements are all conducted at high fields or
susceptibility measurements. This pretty much concludes the list of
"quick-and-dirty" analyses. So you might want to sit back, plot your
data, think about it for a while and figure out where to invest your
time and efforts next. |