Jökull - 01.01.2013, Page 5

The Stardalur magnetic anomaly, SW-Iceland
around Stardalur and in other volcanic centers in
search of strongly magnetized materials. Basaltic
andesites and dikes yielded rather high magnetiza-
tion values (Table 1 of Friðleifsson and Kristjánsson,
1972), but not sufficiently high to explain the local
anomaly peak. Andesites later sampled elsewhere
have also given some high values (Kristjánsson et al.,
1977) but dikes in general seem not to be very differ-
ent from lava flows in this respect (Kristjánsson, 1970
and other data).
Initial studies on petrology and magnetization
Steinþórsson and Sigvaldason (1971) reported on the
content of iron and some trace elements in 17 sam-
ples from the first Stardalur core. The iron averaged
11% by weight (15.6% Fe2O3) which is rather high
for basalts but not exceptional; extrusives in central-
volcano calderas tend to be more iron-rich than oth-
ers (K. Grönvold, pers. comm., 2013). An unusu-
ally high percentage of this iron appeared to reside in
opaque grains which included crystals of exsolved ti-
tanomagnetite (50–100 µm), slender ilmenite crystals
of up to 25 µm length, small clustered euhedral crys-
tals of magnetite, 3–4 µm, and rare pyrite. Steinþórs-
son and Sigvaldason (1971, p. 9) concluded that iron
in the magma crystallized in titanomagnetite (at the
expense of FeMg-silicates) because of high oxygen
fugacity. The lavas had suffered zeolitization and also
contained mixed-layer clay minerals, but epidote was
not found.
Búason (1971) obtained 102 core specimens of
3.0–3.1 cm diameter and 3.2–3.4 cm length from the
6-cm core for magnetic studies. Some additional
magnetic measurements on selected pieces of the 6-
cm core were made by Friðleifsson and Kristjánsson
(1972), see below. The lavas appeared at first sight
to be fairly solid, but their average density was only
around 2500 kg m−3 due to small-scale porosity. Us-
ing an astatic magnetometer, Búason found an average
NRM intensity Mr of 61 Am−1; this property var-
ied rather erratically with depth (upper dots of Fig-
ure 2) and did not show any correlation with the iron
content of the lavas, or with their boundaries. We
have checked Búason’s intensity measurements with a
MEDA fluxgate magnetometer, and they are very sat-
Figure 2. Natural remanence intensities (upper dots) and volume susceptibilities for samples from 41–143 m
depth in Stardalur drill hole 1. – Styrkur varanlegrar segulmögnunar (efri punktar) og segulhrifastuðull (segul-
viðtak) fyrir sýni af óvenju sterkt segulmögnuðu bergi úr kjarnaholu við Stardalsbæinn.
JÖKULL No. 63, 2013 5