Jökull - 01.12.1987, Page 48

The permeability in the upflow is provided by near
vertical tectonic fractures. It may be this control of
permeability that forces the water and steam phases to
ascend together, thus causing extensive boiling of the
reservoir water.
The low N2/Ar ratios (<37) in some of the steam
samples from the Sveifluháls area can be explained by
boiling of the reservoir water in two steps. The first step
involves limited boiling at high temperatures (300°C),
steam formation amounting to up to 0.5% by weight.
The second step involves extensive boiling generating
the fumarole steam. Boiling of the reservoir water in two
steps also explains the relatively low H, concentrations
in the fumaroles in the Sveifluháls area. The interpreta-
tion for the N2/Ar ratios and the H2 concentrations imply
that there is no equilibrium steam present in the reser-
voir.
The conceptual model of the evolution of volcanic
systems in Iceland (Steinthórsson et al., 1986) indicates
that the Krísuvík field is a youthful system, probably
devoid of large high level intrusions except for feeder
dykes. The temperature reversals observed in wells in
the field (Arnórsson et al., 1975a) may be due to the
shape of the intrusive heat source and the permeable
fractures; hot water and steam rise along these fractures
and at a shallow level the boiled water spreads laterally.
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