Geology Reference
In-Depth Information
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Figure 12.1.4. Four parallel profiles at 20m intervals over an active coal-seam fire at the Cummings A,
North Dakota, area after Sternberg and Lippincott, 2004. The red-dashed lines designate the zone of active
burning.
Magnetic Properties
H
igh-temperature, low-pressure, pyrometamorphic rocks develop unusual mineral assemblages and lithologies
(Foit et al., 1987; Cosca et al., 1989; Clark and Peacor, 1992; Stracher et al., 2005; Masalehdani et al., 2007; Sokol
and Volkova, 2007; Vapnik et al., 2007). These rocks acquire a TRM in much the same way as igneous rocks,
although the nature of the magnetic minerals may be somewhat different. In addition, the alterations tend to
enhance the magnetic susceptibility of these minerals. Either the TRM or the enhanced susceptibility can cause
magnetic anomalies. These magnetic properties can be measured in paleomagnetism laboratories. The field of
paleomagnetism is concerned with the history of the Earth
s magnetic field, the application of these changes to
geologic problems, and the magnetic properties of geologic materials (Butler, 1992).
'
Cisowski and Fuller (1987) note that the natural remanent magnetization (NRM; the resultant of all naturally
acquired permanent magnetization) of CM samples from Southern California can be increased up to 4 orders of
magnitude by thermal metamorphism, far above the normal for sedimentary rocks and comparable to extrusive
igneous rocks. Glassy samples are weak due to iron residing in paramagnetic (grains that have magnetic
susceptibility but do not acquire a permanent magnetization) or superparamagnetic (grains that are effectively
paramagnetic by virtue of being very fine grained) phases rather than ferromagnetic (a special case of ferromag-
netism, characteristic of minerals that can acquire a permanent magnetization).
Rock magnetic analyses suggest that white-to-pink pyrometamorphic rocks are magnetite-rich and red samples are
hematite-rich. Susceptibilities of these rocks also tend to increase 1 or 2 orders of magnitude. Ron and Kolodny
(1992) also find that samples of chalks and marls in the Mottle Zone of Israel increase their NRM intensity due to
oxidation of iron sulfides into maghemite and hematite (and some magnesioferrite). Khesin and Itkis (2002) find
that altered rocks in the Hatrurim Basin of Israel have susceptibilities about an order of magnitude higher on the
average than slightly altered rocks, although for particular samples this correlation does not always hold. Further
magnetic studies of rocks from this region (Khesin et al., 2005) reveal the complexity of their magnetization,
including heterogeneity over distances of millimeters to centimeters. Some of the highest susceptibilities in the
Hatrurim basin were for brick-like rocks and porcellanites (Khesin et al., 2005). There is also a general correlation
between NRM and susceptibility. Magnetite, hematite, and goethite seem to be important in different samples.
De Boer et al. (2001) explicitly looked at the TRM properties of pyrometamorphic rocks from CM of coal seams in
China, using a variety of techniques of rock magnetism. The mineralogical change caused by the heating
enhanced susceptibilities and remanences 2
3 orders of magnitude above characteristic values for the parent
sedimentary rocks. Magnetite, maghemite, and hematite were all involved in bearing the remanence, mainly as
fine-grained pseudo-single domain grains (with just a few domain walls, nearly as small as single domain grains).
Some samples even contained pure metallic iron, a rare occurrence in terrestrial rocks. Thorpe et al. (1998) also
found a highly magnetic coked zone containing native iron due to the intrusion of magmatic fluids into a coal seam
and the consequent reduction of pyrite. As noted by Cosca et al. (1989) and reiterated by de Boer et al (2001), the
magnetic minerals found in pyrometamorphic rocks depend in a complex way on chemical and physical variables
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