Jökull - 01.12.1972, Page 77

TABLE 3 - TAFLA 3
Estimated palaeo-field strength at the time of solidification of Stardalur tholeiite lava flows at
70- •120 m depth
Útreiknaður slyrkur fornsegulsviðs
Depth, m Jo Jt k k, F_ = 0.50 Jo K
Dýp i, m 10~3 Gauss 10-3 G/Oe p Jt ' k
73.0 50 27 5.0 4.8 0.89 Oe
96.5 83 30 4.9 3.3 0.93 Oe
105.4 77 14 5.4 2.0 1.02 Oe
119.6 45 20 3.9 3.1 0.89 Oe
Average: 0.93 Oe
JG is the volume intensity of NRM
JJ is the volume intensity of TRM acquired on cooling from 610° C in air in a field of 0.50 Oe
k and kt are volume susceptibilities, measured with a 1000 Hz bridge, respectively before and
after heating to 610° C
Fp is the inferred strength of the local palaeo-field
usually fine-grained ancl highly oxidized, does
this median destructive field exceed 600 Oe.
d) Viscous remanence (VRM). Another pos-
sibility of explaining the large NRM intensity
is that the Stardalur basalts contain unusually
large viscous remanence components acquired
in the present geomagnetic epoch. This possi-
bility, however, is excluded by the good direct-
ional stability of the pilot specimens during
AF treatment (Table 2). Further, the remanence
of two specimens was remeasured six months
after the initial series of measurements, and
the magnitude of the vector change of magnet-
ization in these was found to average less than
0.2% of their mean NRM intensity.
e) Susceplibility and magnetile conlent. Fig. 5
shows the relation between saturation magnet-
ization and initial volume susceptibility in
fresh Stardalur and other Icelandic basic rocks.
All data points correspond to specimens having
a single strong-field Curie point above 500 °C.
In the figure, the magnetization has been con-
verted into equivalent content of pure magne-
tite by volume.
It is seen that the two variables are in direct
proportions to a good approximation, tlie pro-
portionality constant being about 2-2 • 10~3
G/Oe/percent of magnetite. Using this relation
on the susceptibility data by Buason (1971) re-
ferred to above, we estimate the niean magne-
tite content in the Stardalur core to be about
2.5 volume %. Using data from NW- and SW-
Icelancl obtained by Kristjánsson (1970), cor-
rected for the presence of low-Curie-point tit-
anomagnetites, it is likely that the average
magnetite content of Icelandic basalts is 1.0 ±
0.2% by volume.
f) Palaeo-field strength. Another contribut-
ing cause to the high NRM intensity at Star-
dalur may be a high geomagnetic field strength
at the tirne of emplacement. This may be in-
vestigated by comparing the NRM of the pilot
specimens with the TRM acquired in a field
of known strength. These specimens appear to
be good material for sucli a measurement, as
(I) there is very little tendency for VRM build-
up, cf. above; (II) there is no significant differ-
ence between the AF stability of NRM and of
TRM (Fig. 4); (III) although some magnetite
is apparently destroyed on heating, the shape
of the thermomagnetic curves during heating
and cooling is similar (Fig. 3; Table 2).
Using the notation and data of Table 3, we
assume that the ancient field strength Fp is
equal to 0.50 J0/Jt, multiplied by a correction
factor kt/k which according to e) above ex-
presses the partial destruction of magnetite dur-
ing heating.
JÖKULL 22. ÁR 75