Geology Reference
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slurry can either cause electrostratic attraction
between magnetite particles and clay particles (for
pH < 6.5) or repulsion (for pH > 6.5). In fact, Deamer
and Kodama found that pushing the pH up greater
than 6.5 had little effect on the amount of inclination
fl attening in their experiments, so the importance
of the electrostatic sticking mechanism lost some
traction.
Deamer and Kodama did show inclination shallow-
ing due to compaction for a whole range of initial incli-
nations (initial inclinations of 30, 45, 60 and 75°),
distilled and saline pore fl uids, acicular and equi-
dimensional magnetite, natural marine sediments and
four different single-clay synthetic sediments (kaoli-
nite, illite, montmorillonite and chlorite). The volume
loss that occurred in their sediments (typically
c.
55%)
at the low pressures used (0.15 MPa) showed the same
magnitude of inclination fl attening observed in the
Anson and Kodama experiments. The greatest amount
of shallowing occurred at intermediate initial inclina-
tions of 45-60° and was of the order 12°. The tangent-
tangent relationship was not tested by Deamer and
Kodama, but the plot depicted in Fig. 4.1 shows that
the data clearly follow the tangent-tangent relation-
ship of King (1955) .
In Fig. 4.1, Deamer and Kodama's laboratory com-
paction data clearly fi ts the tangent-tangent relation-
ship of King (1955) with the slope of the straight lines
being the best-fi t fl attening factor. The data show that
there is little difference in the fl attening for natural
sediments with saline pore fl uid, single - clay synthetic
sediments with distilled pore fl uid and single-clay syn-
thetic sediments with saline pore fl uid. The
f
factors
only vary over the range 0.68-0.74.
One interesting observation made by Deamer and
Kodama is that the intensity of the sediment's mag-
netization decreased with compaction shallowing
by 25-50%, an important consideration for relative
paleointensity studies using marine sediments (Fig.
4.2). If the amount of intensity decrease is always
roughly the same magnitude then relative paleoin-
tensity records from marine sediments would not
be greatly affected; however, Deamer and Kodama
observed that the greatest amount of intensity decrease
occurred for the steepest initial inclinations and that
there was a strong relationship between intensity
decrease and initial inclination. In fact, Cogne (1987)
saw hints of this effect when he deformed plasticine
embedded with hematite particles with uniaxial com-
pression. When the axis of compression was parallel to
Fig. 4.1
tan
I
f
versus tan
I
0
for single - clay synthetic
sediments (kaolinite, illite, montmorillonite) in saline pore
water (blue), distilled water (green) and for natural marine
sediments (red). The slope of the lines indicates the best-fi t
fl attening factor
f
for each sediment type. The
f
factors range
from 0.68 for distilled pore water to 0.74 for natural
sediments, very similar to those for Anson & Kodama's
(1987) experiments. (See Colour Plate 3)
the magnetization it caused no change in the average
magnetization direction for multiple subsamples, but
caused an increase in scatter of the subsample mag-
netizations. If the same effect occurred at the magnetic
particle scale, then compaction for steep initial inclina-
tions should cause more misalignment of the magnetic
grains and a greater intensity decrease than for samples
with a shallower initial inclination.
A variable amount of intensity decrease with com-
paction could potentially have an effect on the relative
paleointensity measurements made on marine sedi-
ments and would predict that intensity variations
should be correlated to inclination variations for com-
pacted sediments if the effect occurs in nature. Paleo-
secular variation (PSV) of the Earth's magnetic fi eld
causes variations in the local geomagnetic fi eld inclina-
tion and declination on a time scale of thousands of
years. These variations are due both to the variations
in the non-dipole part of the geomagnetic fi eld and to
variations in the main geomagnetic dipole. If compac-
tion causes an NRM intensity decrease that is a strong
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