Jökull - 01.01.2014, Blaðsíða 48

J. Helgason and R. Duncan
Figure 6. Lavas on top of the lowest
sedimentary horizon have lenticular units
that interfinger with underlying conglom-
erate/breccias. – Hraunlög ofan á neðstu
setmynduninni hafa linsulaga einingar sem
fléttast saman við hnullungaberg/brekksíu
fyrir neðan.
rections in samples after peak field demagnetization
steps of respectively 0, 10, 15 and 20 mT and best re-
sults chosen. Results were corrected for lava tilt of 1◦
toward N35◦E.
Sampling for age determinations
Although the lowest exposed strata in Hafrafell are in-
tensely cut by dikes, and within the mesolite-scolecite
zeolite zone, we were able to find portions of flow
units with low vesicle contents and surprisingly fresh
interiors. In the upper sequence, lower grade zeolites
are still abundant, but here we also collected relatively
fresh samples, covering units ranging from oldest to
youngest strata.
K-Ar analytical methods
We prepared groundmass separates from whole rocks
for age determinations by crushing, sieving to ob-
tain the 0.1–0.5 mm size fraction, ultrasonic wash-
ing in distilled water, drying and hand-picking un-
der a binocular microscope to obtain the freshest,
phenocryst-free samples possible, from 5 units with
wide stratigraphic distribution. We loaded approxi-
mately 5 g of each sample into a Mo-crucible and a
1×10−8 vacuum was achieved by overnight baking
(at 175◦C) of the glass extraction line. Samples were
then fused by radio-frequency induction heating of the
crucible, and active gases gettered over a hot Ti–TiO2
metal sponge. We then measured the isotopic compo-
sition of Ar mass spectrometrically using an AEI MS-
10 instrument at Oregon State University, connected
on-line with the gas extraction system. A known
amount of 38Ar (by calibration against standard FCT-
3 biotite, 28.201±0.046 (Kuiper et al., 2008) intro-
duced during fusion to determine the concentration of
radiogenic 40Ar (Dalrymple and Lanphere, 1969). We
made corrections for mass discrimination based on
frequent measurements of atmospheric Ar from an on-
line reservoir. K-contents were determined by atomic
absorption spectrophotometry.
RESULTS
Paleomagnetic results
The magnetic polarity patterns for sections HL, HP1,
U and T, together with information on polarity direc-
tion and VGP values are presented in Appendix 1.
Magnetic polarity results are illustrated on the strati-
graphic columns for Hafrafell (Figure 4). The results
will be discussed below together with results of age
determinations and stratigraphy.
Results of age determinations
Results of the K-Ar dating are given in Table 2, ar-
ranged in stratigraphic order. Stratigraphic height
refers to position within the master section for
Hafrafell. Formation number and coordinates further
locate the stratigraphic position of each unit. Ages of
dated units are also inserted on Figures 3 and 4. Ana-
lytical errors on total fusion ages (Ma) are reported
48 JÖKULL No. 64, 2014