Jökull - 01.12.1978, Blaðsíða 39

Fig. 6. Magnetization variation along
sampling profiles D and E, southern Geitafell
gabbro. Jnrm: natural remanent magneti-
zation, K: susceptibility, Q,,: Q_ ratio of
Koenigsberger, S200: “stability”, = intensity
after AF demagnetization in 200 Oe field,
divided by Jnrm, INC: “stable” inclination of
remanence, DEC: “stable” declination of
remanence.
Mynd 6. Breytingar í segulsvibi eftir mœlilínum D
og E í Geitafelli. Jnrm: upprunaleg segulmögnun, K:
segulhrifstuðull, Q^: Königsbergerhlutfall, S200:
stöðugleiki segulmögnunarinnar eftir afsegulmögnun
með riðstraumi í 200 Ö sviði deilt með styrk upp-
runalegrar segulmögnunar, INC: halli segulstefnu
sýnis, DEC: misvísun segulstefnu sýnis.
reverse magnetizations, even those from the
southeasterly zone where positive anomaly
values exist over the gabbro outcrop. Inten-
sities of the natural remanent magnetization
reach a maximum of 95 X 10-3emu/cc over
the area of maximum magnetic anomaly.
Much lower values are recorded on both sides
of the magnetization maximum averaging
about 6X 10-3emu/cc. This conforms with
the distribution of the AZ magnetic anomaly.
The zone of strong magnetization
represents about 15% of the volume of the
investigated gabbro and indicates special
emplacement or cooling conditions. Suscep-
tibilities reflect the intensity variations to a
minor degree. The Q ratio (between the
natural remanence and the induced magneti-
zation) is always larger than unity and has a
mean value of 4.5.
Stable remanence directions were almost
always obtained after AF-demagnetization
procedure, using the demagnetization equip-
ment and the P.A.R. spinner magnetometer at
the Geophysics Institute, Copenhagen. Usu-
ally 75 Oe were sufficient to cancel a viscous
normal component and to obtain stable
reverse directions. For fields larger than 600
Oe directions of magnetization of most samp-
les became unstable, resulting in irregular
fluctuations of directions. Therefore directions
for 200 Oe field amplitude have been plotted
in Fig. 6.
Mean values for the inclination and declin-
ation of the gabbro from profile D are —62°
and 247° respectively (Fisher’s precision
parameter k = 55, and cone of confidence
a95 = 6.8°). This deviation from the values for
a reversed central axial dipole can partly be
explained by a tilting of the gabbro after
cooling by some 35° towards NW which con-
forms with the dip of the plateau basalts
around the gabbro (Annels 1967).
A mean declination of 188° and a mean
inclination of —60° were obtained after cor-
rection for a 35° northwesterly tilt. The value
of the inclination is significantly too shallow
for a central axial dipole field. The effects of
the demagnetizing field of the gabbro on the
thermoremanent magnetization and the
secular variation of the earth’s magnetic field
must be considered as well. Cooling of the
gabbro at temperatures around 500° to 540 °C
(the interval between blocking temperatures
and Curie temperatures of magnetite) proce-
eded so rapidly (timespan probably below a
few hundred years) that averaging out of the
secular variation could not take place. It is
noteworthy, however, that both too shallow
inclinations and easterly declinations have
been found to be quite common for plateau
basalts within zeolite zones in E-Iceland
(Watkins and Walker, 1977).
Some declinations and inclinations, parti-
JÖKULL 28. ÁR 37