Jökull - 01.12.1988, Blaðsíða 59

Leg end
tS3 1 □ 5
m 2 H 6
L aJ 3 U 4 7 8
Flatey
T j ö r n e s
Age.Ma Thickness.m
--- 600-
MATERIALS AND METHODS
The lava samples were ground to grain sizes in
the range 180-250 |im. Five aliquots of each sample
were analyzed for potassium on a Beckman 495
atomic absorption spectrophotometer. The samples
were dissolved in H2S04, HF and HN03 and an
intemal buffer of 200 ppm Na was added to each
sample in order to eliminate enhancement effects
from sodium. The precision of measurements was
typically within 1%.
Argon isotopic dilution measurements were made
on an omegatron mass spectrometer at the Bullard
Laboratories in Cambridge. Both the equipment and
the argon extraction procedure have been described
elsewhere (Grasty and Miller, 1965; Albertsson et
Fig. 2. A correlation diagram showing a geomag-
netic time scale (Mankinen and Dalrymple 1979) to
the left, a cross section of the Tjömes rocks in the
centre and the borehole section from Flatey to the
right. Legend: 1) Lava flows; 2) Hyaloclastites;
3) Tephra; 4) Tillites; 5) Other sediments; 6) Norm-
ally/Reversely magnetized rocks; 7) Fossils;
8) Unconformities. The radiometric dates shown for
the Tjömes rocks are from Albertsson (1978), recal-
culated in accordance with Steiger and Jáger
(1977). (Redrawn from Eiríksson et al., 1987).
Mynd 2. Líklegar tengingar. Vinstra megin er segul-
tímakvarði frá Mankinen og Dalrymple (1979),
Tjörnessnið er í miðju og borholusniðið frá Flatey
er hægra megin. Skýringar: 1) Hraunlög; 2) Mó-
berg; 3) Öskulög; 4)Jökulberg; 5) Annað setberg;
6) Rétt-Öfugt segulmagnað berg; 7) Steingervingar;
8) Roffletir. K-Ar aldursákvarðanir við Tjörnes-
sniðið (Albertsson, 1978) eru endurreiknaðar til
samrœmis við Steiger og Jáger (1977). (Eiríksson
o.fl., 1987, endurteiknað).
al., 1982). The analytical errors associated with a
radiometric age determination have been described
elsewhere (Evans et al., 1973). The statistical error
reported here with each age is 2 standard deviations.
Sample weights of 10-15 g were used for the argon
extraction. The samples were degassed and baked
for 12 hours prior to extraction. The bake-out tem-
perature was kept at or below 100°C in order to
minimize the possibility of fractionating some of the
loosely held argon in the sample (cf. Grasty, 1964;
Baksi, 1974; McDougall et al., 1976).
Petrographical work shows the lava flows to be
olivine-normative tholeiites (Gunnarsson et al.,
1984). The core study reported smectites in cracks
in the middle and lowermost lava flows and some
JÖKULL, No. 38, 1988 57