Environmental Mineral Magnetism
Introduction
Most sediments have some magnetic material in them. The type of magnetic material, its size and concentration in the sediment all affect the
type of magnetic signal it gives off. Magnetic minerals include magnetite
and hematite, both iron oxides with different magnetic properties.
Magnetite is often found in granitic rock and coarse-grained sedimentary deposits like sandstone. Hematite forms as iron-rich rocks decompose,
turning bright red when exposed to air (like rust).
Environmental mineral magnetism is the study of magnetic characteristics determined by the chemical structure of the mineral types within the
sample. Unlike paleomagnetism, which measures the sample's polarity relative to the earth's magnetic poles, mineral magnetism samples do not
need to be measured in situ, or have their original orientation noted.
Magnetism measurements are relatively fast, inexpensive and easy to make. Perhaps the greatest advantage of this analysis, however, is that
it's non-destructive, which means the sample can be used for another analysis. In multi-disciplinary studies, where one 5" diameter core is
being sampled for pollen, diatom, ostracode, ICP, XRD and isotope geochemistry, mineral magnetism, carbon, radiocarbon, lead isotope and amino
acid dating analyses, we tend to run out of sediment before we run out of ways to examine it. Any non-destructive method is a great asset.
Techniques:
When samples are taken from cores, the mud is dried, then packed into small, 1x1x1 cm cubes and weighed. Magnetic
susceptibility measures the ability for samples to become magnetic. Our machines measure susceptibility while spinning the sample around
perpendicular axes (x, y and z). Below is a list of measurements we typically take for mineral magnetic analyses:
- We measure the natural susceptibility of the sediment at two different frequencies. This gives us information about the amount of
magnetite in the sample and its grain size (ultrafine magnetite is formed in soils, larger magnetite is usually eroded out of rocks).
- We apply a small magnetic field to each sample and measure susceptibility again. This gives us ARM (anhysteretic remanent
magnetization), which is a measure of the amount of magnetite with grain sizes larger than ultrafine.
- Finally, we apply a strong magnetic field along one axis and measure susceptibility, then reduce the strength of the field and
reverse it, measuring susceptibility one last time. This IRM (isothermal remanent magnetization) measures how much of the sample
contains magnetite versus hematite or other magnetic minerals.
Environmental Magnetism Sites
Palaeomagnetism and Rock Magnetism
Iron Filings
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U.S. Department of the Interior | U.S. Geological Survey