Jökull - 01.12.1984, Blaðsíða 76

generally above 400 Oe, but for the others gen-
erally under 300 Oe.
When the same samples are subjected to anh-
ysteretic remanence (A.R.M.) acquisition in a
0.5 Oe steady field, the a.c. fields needed for
acquisition of 50% of saturation A.R.M. seem
for the former to be above 300 Oe (generally
about 100 Oe higher than the m.d.f. of T.R.M.
for the same sample), and for the latter they are
below 200 Oe. This is illustrated in Fig. 6.
In stable samples, the saturation A.R.M. is of
the order of 0.5—1.5 times the intensity of the
original T.R.M. left in the sample, but in poorly
stable to unstable samples the ratio of A.R.M. to
original T.R.M. is of the order of 4—5. This is
clearly the result of viscous decay of the T.R.M.
in the latter with time since emplacement.
As regards strong-field thermomagnetic curves
in air, Tertiary lavas with low stability of primary
remanence (and/or relatively strong V.R.M.)
have a greater tendency than others to exhibit
double Curie points (Fig. 7a). The lower Curie
point is then in the range 250—400°C and the
upper one is above 500°C. Unstable samples may
also have one Curie point in the range 300° or less
ARM ACQUISITION
-/»« m in
05 OE
5■
3-
I /
I ' '
1 /
i,'^ •' AC. PEAK FIELD, OE
—i-----1---1----1—■— ♦ i----1 i---1-
100 300 500 700 900
Fig. 6. Anhysteretic remanence (A. R. M.)
acquisition curves in fields of increasing strength,
for three Icelandic basalt samples. Two (circles)
are magnetically stable, one (squares) is un-
stable.
6. mynd. Uppbygging riðstraums-segulmögnunar
í tveim sýnum með áreiðanlega segulstefnu og einu
með óstöðuga segulstefnu.
to 530°C, but the behaviour of magnetically
stable samples is most commonly dominated by a
component having a Curie point above 540°C
(Fig. 7b). These differences are probably due to
the magnetic minerals in the unstable samples
being less oxidized than those in stable samples,
but other factors such as grain size and alteration
thermomagnetic curve in air, from a Tertiary lava
sample whose primary remanence is poorly
stable. b) Same for a stably magnetized sample.
7. mynd. Segulmögnun tveggja sýna úr íslenskum
hraunum semfall afhitastigi ísterku sviði. Varan-
lega segulmögnunin í a) var óstöðug, en í b) stöð-
ug-
74 JÖKULL 34. ÁR