Geology Reference
In-Depth Information
2.5.2
Ratios of Environmental Magnetic Parameters
2.5.2.1 χ
ARM
/χ
The χ
ARM
/χ ratio was first proposed as a magnetic grain size measurement by
King et al. (1982). Since single domain magnetite grains typically acquire a
strong χ
ARM
and susceptibility normalizes for magnetic mineral concentration,
this ratio depends on the domain state of low coercivity ferrimagnetic grains
(magnetite) to measure grain size. High ratios (~5-15) indicate small, SD
grains; low ratios (<1-2) are coarser MD grains (Peters & Dekkers 2003). The
caveat for using this ratio is that both parameters (χ
ARM
and χ) must be
measuring the magnetization of the same magnetic mineral in the sample.
Paramagnetic and diamagnetic mineral contributions to susceptibility will
reduce the effectiveness of this ratio as a grain size measure.
Peters and Dekkers's (2003) compilation also shows that SIRM/χ can be
used as magnetic grain size measure, but the relationship between grain
size and SIRM/χ is not as well defined as the χ
ARM
/χ ratio, particularly for
magnetite, in the critical 0.1-10 µm grain size range.
2.5.2.2 ARM/SIRM (χ
ARM
/M
rs
)
The ratio of ARM (or ARM susceptibility) to saturation remanence (designated
as either SIRM or M
rs
) is also used to detect magnetic mineral grain size. The
advantage this ratio has over the χ
ARM
/χ ratio is that both ARM and SIRM are
remanence measurements, so there is no concern about the contribution of
paramagnetic or diamagnetic minerals to the measurements. However, there
is the possibility that the ARM and SIRM measurements may activate differ-
ent magnetic mineralogies in a sample. ARM is preferentially carried by the
low coercivity ferrimagnetic minerals (e.g., magnetite and greigite) and SIRM
could also be carried by higher coercivity minerals (e.g., hematite and pyrrho-
tite). The ratio is meaningless as a grain size measure if ARM and SIRM are
carried by two different magnetic minerals in the sample, so the magnetic
mineralogy should be carefully determined before interpreting ARM/SIRM
variations.
2.5.2.3 Goethite to Hematite Ratio
Both goethite (FeOOH) and hematite (Fe
2
O
3
) can be formed pedogenically
and their relative abundance is controlled by moisture availability during
soil formation (Yapp 2001; Harris & Mix 2002; Abrajevitch et al. 2009). If
both goethite and hematite are detrital minerals and haven't been affected
postdepositionally by heating or diagenesis, their ratio could be a powerful
proxy for moisture availability (precipitation vs. evaporation) in the source
area of a sedimentary rock. Although there are geochemical techniques for
measuring the goethite to hematite ratio in a sedimentary rock, magnetic
techniques are relatively fast, cheap, and nondestructive. A magnetic ratio
can be constructed in order to detect variations in the goethite to hematite
ratio in a rock. Goethite is distinguished magnetically by having very high
coercivities (1000s of mT) but a Neel temperature of only about 125°C
