Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 178

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The temperature in the lower crust heneath the crest of the
Mid-Ocean Ridge may reach about 350°C (Miyashiro, 1973). The
temperature reached by the deepest drillholes in Icelandic geo-
thermal areas sofar is just below 300°C.
The composition of the original rocks is considered to be rather
similar to that of the Icelandic geothermal areas. The main dif-
ference should be more common occurrences of pillow-lavas in the
ridge rocks.
The water in the Reykjanes area and other geothermal systems
on the Reykjanes peninsula is seawater in origin whereas the water
in other geothermal systems in Iceland is meteoric. The chemical
composition of the Reykjanes thermal waters and those of some of
the inland high-temperature areas are compared in Table 2. The
great difference in composition between the thermal waters is not
seen to influence the overall course of alteration to a marked degree.
Data of low-temperature metamorphism active on the Mid-
Atlantic Ridge are rather scarce. Referring to two examples, Mel-
son et al. (12) describe how hydrothermal metamorphism has
transformed oceanic tholeiites into low grade metamorphized rocks.
The grade of alteration of the rocks is from zeolite/saponite bear-
ing basalts to greenscist facies rocks. The variation in clay mineral
formation with metamorphic grade in metabasalts from the Mid-
Atlantic ridge described by Miyashiro et al. (13) shows a similar
pattem as found in the Icelandic geothermal fields. The meta-
basalts were dredged from a transverse fracture zone and were
probably recrystallized at some depth. The mafic minerals in the
zeolite facies metabasalts were found to be smectite and mixed-
layer minerals made up of chlorite, smectite and vermiculite com-
ponents. Chlorite proper does not appear. The zeolites identified
were natrolite, thomsonite, chabazite, stilbite and laumontite. Anal-
cime was also found and was the most widespread zeolite species.
These examples show close similarities to the results of hydrother-
mal alteration of basaltic rocks in Icelandic geothermal fields. Also
the clay minerals formed in both are of similar types.