Jökull - 01.12.1972, Side 78

J/Jo
Fig. 4. Alternating-field (AF) demagnetization
curves for Stardalur pilot samples, normalized
to unit intensity before treatment. The TRM
is acquired on cooling in air from 610° C.
Mynd 4. Afsegulmögnunarferlar í breytisviði,
fyrir upprunalega segulmögnun (NRM) og seg-
ulmöpnun, sem sýnin draea i sie við kcelingu
frá 610° C (TRM).
The last column of Table 3 shows palaeo-
íield strengths computed by this formula. The
agreement in these independent determinations
of Fp is very good, indicating a mean palaeo-
field strength of 0.93 ± 0.06 Oe. This standard
deviation and the experimental uncertainty of
± 1% are, however, quite small in comparison
to the uncertainty in the underlying assump-
tions. These include (a) that no viscous decay
of the NRM has occurred in situ; (b) that the
NRM is a primary TRM or CRM whose pro-
portionality to the applied field is the same
as that of the artificial TRM; and most im-
portant (c) that secondary alteration has not
changed the relevant properties of the magne-
tite in the basalt.
These assumptions are obviously commort to
all work on palaeo-field strength work, but the
above method has been tested on the 1961 Askja
lava flow and yields an intensity of 0.55 Oe
(the regional field being 0.515 Oe) in one
specimen. Smith (1967) has by similar met’nods
found that the magnetic field strength in Ice-
land in Upper Cenozoic times averaged 0.34 ±
0.03 Oe (standard error), a result strongly sup-
ported by work in other parts of the world.
One of Smith’s 31 samples has originally been
cooled in an external field of about 0.9 Oe,
ancl one in 0.64 Oe. Carmichael (1970) similar-
ly obtained an Fp value of 1.2 Oe, and Ade-
Hall et al. (1972) six values of 0.64— 1.07 Oe.
GA8BR0
DIABASE
BASALT
SAND
CORES Stardalur
CHIPS Reykjavik
CHIPS Reykjanes
A A 0
ICELANDIC ROCKS
% MAGNETITE
0
□
0 12 3 4 5 6 7
SUSCEPTIBILITY 10'3 Gauss/Oe
Fig. 5. Volume content of magnetite, comput-
ed from saturation magnetization measurements
at room temperature with a ballistic magneto-
meter, plotted against initial volume suscepti-
bili-ty for various rock types from Iceland. The
chips used are from deep drilling for geo-
thermal heat. See text.
Mynd 5. Tengsl milli seguljárnsinnihalds og
ségulhrifstuðuls i ýmsum bergsýnum frá IslaJidi,
þ. á m. Stardalsblágrýti og borsvarfi úr djúþ-
holum.
76 JÖKULL 22. ÁR