Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 177

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In quantity the clay minerals are the most important alteration
minerals. At the surface of the geothermal areas and in the upper-
most 50-100 m of drilled sections montmorillonite is formed. The
smectites formed beneath the uppermost tens of meters are triocta-
hedral and iron-rich and are classified as iron-rich saponites. Smec-
tites from all geothermal areas which we have so far investigated
are of this type. Inter-layering of the smectites by “chloritic” in-
ter-layers is found in all but the uppermost rocks. As alteration
progresses this inter-layering gets more prominent. There is found
to be a gradual transformation from inter-layered smectite to
chlorite. The dominant types of chlorite are iron-rich. Intermedi-
ate structures as the “swelling chlorite” and irregularly mixed-
layer minerals of chlorite-smectite are found in the transformation
zone. This gradual transformation of smectites to chlorites does
happen over a rather broad temperature interval. The smectites
are however almost restricted to prevailing temperatures not ex-
ceeding 230°C.
DISCUSSION
The main aspects of the hydrothermal alteration found in active
geothermal areas from other parts of the world are rather similar
to what is found in Iceland (6, 7, 8).
The markedly different composition of the original rocks is how-
ever reflected by some of the types of alteration minerals formed.
This is most significantly reflected by the clay mineral types.
Sub-sea-floor geothermal systems are known to exist (10) and
the bulk of the oceanic crust is subjected to low grade metamorph-
ism (11). The comparison to the hydrothermal metamorphism tak-
ing place in active geothermal fields in Iceland is therefore of in-
terest in the study of the Oceanic-Ridge system. Due to the pres-
sure of the overlying water there will, in a sub-sea-floor geothermal
system, not be any boiling zone with extensive precipitation of
secondary minerals in the upper level. Higher temperatures should
also be found at the sea-floor interface than at atmospheric pres-
sure. At some depth the processes in such systems should however
be comparable to those in inland geothermal systems, if other in-
fluencing factors are similar.
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