Jökull - 01.12.1972, Side 76

MAGNETIC PROPERTIES
OF PILOT SPECIMENS
a) Description. Various magnetic properties
of four randomly selected Stardalur drill core
specimens were investigated at the Geomagne-
tic Research Laboratory of Memorial Univer-
sity. The equipment used has been described
by Deutsch et al. (1971) except for a ballistic
magnetometer constructed by Kristjánsson
(1973). Measurements were aimed at finding
the causes of the high remanence in the Star-
dalur tholeiite lavas; its average is of the order
of ten tirnes higher than those iri other Ice-
landic Lower Quaternary basalts (Kristjánsson
1970). Data are listed in Table 2, along with
some results for a very small specimen from 60
m depth.
b) Magnetic mineral. Thermomagnetic cur-
ves in air for two specimens are shown in Fig.
3. Curie points for all pilot specimens (Table
2) are typical for high-temperature oxidized
titanomagnetites, where the magnetic constitu-
ent is magnetite (Tc = 580 °C), possibly some-
what oxidized towards maghemite composition.
On heating, some of this mineral becomes furth-
er oxidized to weakly magnetic hematite, caus-
ing a drop in the strong-field magnetization
(upper part of Fig. 3) and in susceptibility
(Table 3).
The conclusion that the Stardalur magnetic
mineral is magnetite (as is the case in most
oxidized basalts) is supported by microscope
observation of polished sections and by hyste-
resis curve shapes (Kristjánsson 1973) as well
as x-ray and other evidence (Steinthórsson and
Sigvaldason 1971).
c) Alternating field (AF) demagnetization.
One possible cause of the high remanence in-
tensity is that the NRM resides in magnetite
grains of single-domain size. A good criterion
for indicating this is the Koenigsberger ratio of
thermal remanence (Jt/ktF, see legend to Table
3) after cooling the rock frorn a high tempera-
ture in an external field F. However, Krist-
jánsson (1972) finds no significant difference
between the Koenigsberger ratio in the Stardal-
ur pilot specimens and in specimens from other
Icelandic basalts.
Fig. 4 and Table 3 show the median destruc-
tive alternating field strength for four pilot
specimens to be about 200 Oe. This value is
fairly low for a basalt and excludes the pos-
sibility that the remanence resides in highly
anisotropic needles of magnetite. Only in the
specimen from 60 m depth, which is both un-
TABLE2 - TAFLA 2
Some magnetic properties of pilot specimens from Starclalur
Nokkrir seguleiginleikar firnm sýna úr borholu 1
Depth, m M.D.F. in Oe Dir. ch. Tc, in degrees C Magnetite % Com-
Dýpi, m NRM TRM NRM-250 Oe heating cooling by volume ment
60 700 N.A. about 1° 620° 615° N.A.
73 220 220 1-2° 560° 560° 2.4 Z
96.5 200 180 about 1° 585° 585° 2.3 z
105.4 180 250 < 1° 615° -610° N.A. c
119.6 180 180 < 1° 570° 570° 1.45 c
M.D.F.: Median destructive field, i.e. the demagnetizing fielcl at which a half of the NRM or
artificial TRM intensity has been eliminated
Dir. ch.: Directional change of natural remanence during demagnetization
Tc: Strong-field Curie point, using the definition of Deutsch et al. (1971)
Magnetite % by volume: as measured through saturation magnetization, using a ballistic. magn-
etometer (N.A.: not available)
Comments: Z— contains zeolites; C— cracked, breccia-like
74 JÖKULL 22. ÁR