Jökull - 01.12.1984, Page 34

TABLE 1. Comparison of gas in steam from Grímsfjall. Numbers
show volume %.
TAFLA 1. Samanburður gastegunda í gufufrá Grímsfjalli.
Sample 1 2 3 4 5
co2 21.36 27.31 29.80 7.26 6.82
H2S 0.00 0.00 0.00 0.00 0.00
h2 0.01 0.00 0.00 0.01 0.01
o2 13.59 10.90 10.04 4.26 5.00
ch4 0.01 0.00 0.01 0.01 0.01
n2 63.75 60.66 59.01 87.22 87.54
Ar 1.25 1.11 1.14 1.26 0.61
1. Fumarole on Grímsfjall. Sampled in June 1982 by HB.
2
"• »» »» »» »» »» »» » »»
3. Fumarole on Grímsfjall. Sampled in June 1983 by HK.
4. Drillhole no. 3 on Grímsfjall. Sampled in June 1983 by HK.
»» » » >> »> »» »»
observed on the two nunataks at about 1700 m
a.s.l. (East- and West-Svínahnúkur, Fig. 2). The
exposed rock surface is crossed by SW-NE strik-
ing fractures through which steam emanates.
Fumaroles with temperatures of up to 92.5 °C are
found at Hithóll on the western slopes of
E-Svínahnúkur (Fig. 2). The subglacial steam
outlets have formed extensive ice caves on the
slopes of the mountain. The floor of the caves is
composed of fresh loose lapilli. White calcite
precipitates have heaped up around steam out-
lets. Outside the caves, where the slopes are
exposed to open air, the surface is cemented and
much more altered than the floor of the caves.
The lapilli have become brown to rust coloured
due to oxidation of iron. The evidence of geoth-
ermal activity is less significant on the eastern
part of E-Svínahnúkur. There, a maximum
temperature of 32°C has been measured in a
fracture at a depth of about 1 m. In 1983 two
holes were drilled into the hill, 15 m and 27 m
deep. They yielded saturated steam at low
pressure.
No sulphurous odour can be dected on the
Grímsfjall mountain, neither from the fumaroles
nor in the steam from the drillholes. The low
pressure of the steam and the absence of sulphur-
ous odour indicates repeated boiling and washing
out of the most soluble gases before the steam
leaks out at the surface of the nunatak. There are
probably two to three hundred metres down to
the boiling water surface, from which vapour
seeps upwards and repeatedly condenses and
evaporates on the way to the surface. The mea-
sured ð lsO = -22%o for the condensate fits this
model as the ð lsO values for the water in the
jökulhlaups ranged form -11%0 to -13%o, which
are representative for the precipitation in the
Grímsvötn area. The gas in the steam is mostly
atmospheric with an admixture of carbon dioxide
(see Table 1). The precipitates on the rock sur-
face were found to be carbonates with no traces
of sulphurous materials. But pyrite (FeS2) was
found in varying amounts in drill cuttings from 6
m depth. The drillholes cut through hyaloclastites
to the bottom. The alteration of the basaltic glass
is extensive in certain horizons. The hot acid
condensate of the ascending vapour readily trans-
forms the reactive basaltic glass and a complete
recrystallization has occurred in places.
Surface expressions of geothermal activity
have been less significant on W-Svíahnúkur than
on the eastern nunatak (during the last decade
anyway). Steam emanations are seldom observed
32 JÖKULL 34. ÁR