Jökull - 01.12.1986, Page 7

discharge rates from hot springs are about 15 1/s in
each field and steam discharge is low. High-tempera-
ture activity is the result of the formation of shallow
level magma intrusions. The low surface heat dis-
charge from high-temperature fields in the TL-zone is
indeed suggestive, yet not conclusive, of limited high-
temperature activity in this zone. It seems logic to
relate the apparently limited geothermal activity in
the TL-zone at present to the diminishing tension and
limited volcanic activity.
SUBSURFACE TEMPERATURES
No deep drillholes have been sunk in the Reyk-
holtsdalur and the Upper-Árnessýsla geothermal
systems. It is, therefore, necessary to rely on chemical
geothermometry for evaluation of the subSurface tem-
perature conditions. By the approach of Arnórsson
and Svavarsson (1985) to use overall water compo-
sitions for geothermometry interpretation, it is found
that most hot spring discharges in both areas indicate
subsurface temperatures between 130°C and 150°C.
For most of the waters individual geothermometers
yield temperatures within a narrow range (20°C), thus
giving confidence in the results. Yet, there are some
discrepancies and high H2S-temperatures for the
Laugarvatn springs in Upper-Árnessýsla are particu-
larly noteworthy. The calculated hydrogen sulphide
content in the reservoir fluid from the analysed total
sulphide in the hot spring discharges indicates tem-
peratures in excess of 200°C (fig. 3).
Equilibration between aqueous sulphide and sul-
phide minerals is approached relatively slowly in
geothermal systems, especially at low temperatures.
The strong supersaturation with repsect to sulphide at
the temperature indicated by geothermometers other
than H2S (140°C) at Laugarvatn could hardly be ac-
counted for by excessive leaching. Juvenile contribu-
tion by degassing of magma could, on the other hand,
explain the elevated hydrogen sulphide concentra-
tions. Such contribution could occur by mixing of hot
gaseous fluid phase with overlying cooler geothermal
water. The studies of Gunnlaugsson (1977) demon-
strate that sulphur is transported from igneous intru-
sions into the overlying hydrothermal systems.
In two areas on opposite sides of the Reykjanes-
Langjökull volcanic zone, Klausturhólar and Leirá
(fig. 2), drillhole data have revealed temperatures of
160°C (1100 m depth) and 175°C (2000 m depth),
respectively (Arnórsson et al. 1983, Fridleifsson et al.
1977). Chemical geothermometry indicates even
higher subsurface temperatures. For example, C02
concentrations give temperatures around 200°C in
Fig. 3. C02 content in well waters at Klausturhólar
and Leirá and H2S content in hot spring water at
Laugarvatn. The broken curves indicate C02 and H2S
concentrations in equilibrated geothermal waters
according to Arnórsson et al. (1983). Stars represent
calculated gas concentrations at bottomhole tempera-
tures for Klausturhólar and Na-K geothermometry
temperatures for Leirá and Laugarvatn. The calcu-
lated gas concentrations depend not only on the water
composition but also on the aquifer temperature as
shown by the solid curves. The intercept of the two
curves gives the best estimate for reservoir tempera-
tures. — Styrkur COi í borholum að Klausturhólum
og Leirá og styrkur H2S í hverum á Laugarvatni.
Brotnu ferlarnir sýna styrk C02 og H2S í jarðhita-
vatni, sem náð hefur efnajafnvœgi við ummyndunar-
steindir (skv. Stefáni Arnórssyni o.fl. 1983). Stjörnur
sýna reiknaðan styrk gastegundanna við hitann í
botni holunnar að Klausturhólum og við Na-K hita
fyrir Leirá og Laugarvatn. Reiknaður styrkur gasteg-
undanna er ekki einungis háður efnainnihaldi vatns-
ins, heldur einnig hitastigi þess eins og sýnt er með
heildregnu ferlunum. Skurðpunktur ferla gefur besta
mat á hitastigi i jarðhitakerfunum.
both areas (fig. 3). These data and those from Laugar-
vatn show that there is an overlap in subsurface tem-
peratures in geothermal systems lying within and out-
side the active volcanic zones.
RECHARGE FOR THE REYKHOLTSDALUR
AND UPPER-ÁRNESSÝSLA SYSTEMS
The deuterium content of hot spring discharges
have been used to locate the recharge areas for the
Reykholtsdalur and Upper-Árnessýsla geothermal
systems (Árnason 1976). In Reykholtsdalur the geo-
thermal water represents precipitation that has fallen
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