Jökull - 01.12.1972, Page 79

all froni Quaternary deep-ocean samples dredg-
ed at 45° N.
A value of 0.93 Oe for the palaeo-field
strength at Stardalur during the short time of
emplacement of the flows between 73 and 120
m depth is thus quite plausible, when it is also
kept in mind that some of this field is due to
the strong positive anomaly created by the
underlying flows.
CONCLUSIONS
Geophysical anomalies previously found in
the vicinity o£ Stardalur, SW-Iceland, are as-
sociated with a central volcanic complex. This
complex includes a caldera of 6.5 km diameter;
a positive magnetic anomaly, essentially co-
incident witli the caldera, is most probably due
to the presence of normally magnetized intru-
sives emplaced during the Olduvai geomagnetic
event.
A sharp maximum in the magnetic anomaly
at Stardalur farm is due to the presence o£
highly magnetic tholeiite lava flows, penetrated
by the core drill between 41 and 143 in depths.
Measurements on drill chips from greater
depths show this rock formation to continue
to at least 200 m. The carrier of remanence is
magnetite, occurring in high-temperature oxid-
ized titanomagnetites. The combined effects of
a high magnetite content (2—3 times the aver-
age in Icelandic basalts) and a high palaeo-
field strength (2.5—3 times average) may cause
a remanence intensity of 6—9 times the Ice-
landic Upper Tertiary—Lower Quaternary bas-
alt average. Tliis is a major part of the factor
of ten or twenty by which the NRM intensity
of the highly magnetic Stardalur flows actually
exceeds tlxat of the average Icelandic basalts.
The remaining factor can be explained by a
relatively small eífective grain size (Steinthórs-
son and Sigvaldason 1971) caused by a consist-
ently high oxidation state of the former, or by
their possible rapid cooling. In Greenland
basalts, Kristjánsson (1973) has shown that
either effect may increase the average reman-
ence intensity by a factor of at least two.
ACKNOWLEDGEMENTS
The authors wish to thank Prof. Thorbjörn
Sigurgeirsson, Dr. E. R. Deutsch, Dr. Gudmund-
ur Pálmason, Mr. Jens Tómasson and Mr. Thor-
valdur Búason for much help and information
during this ivork. The authors also acknowledge
financial assistance from tlie Science Fund of
Iceland (Vísindasjódur).
REFEREN CES
Ade-Hall, J. M., P. J. C. Ryall and R. E. Ger-
stein. 1972: Magnetic results from basalt
drill cores from the M.A.R. median valley
at 45° N. EOS 53, 355.
Búason, Th. 1971: Unpublished data, University
of Iceland.
Carmichael, C. M. 1970: The Mid-Atlantic ridge
near 45° N. VII. Magnetic properties and
mineralogy of dredged samples. Can. Jour.
Earth Sci. 7, 239-256.
Deutsch, E. R., L. G. Kristjánsson and, B. T.
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