Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 148

the ðD-value and the content of total dissolved solids. In two cases
it was questionahle whether there existed a slight relation between
the two factors. When such a relation existed it indicated that the
ðD-value increased slightly with increasing amount of total dissolved
solids. This is however quite opposite to the effect we have to ex-
plain here, where high salinity should be associated with low ðD-
value.
Furthennore the öD-value of the thermal brines in the basins
studied by Clayton et al. was about the same as in the local present-
day precipitation. The authors therefore came to the following con-
clusions: “a) The water is predominantly of local meteoric origin,
b) the deuterium content has not been greatly altered by exchange
or by fractionation processes, c) extensive oxygen exchange has
taken place between water and reservoir rocks”.
In another paper, Graf et al. (1965) tried to explain the ðD-varia-
tions that occurred within each basin, as a consequence of micro-
pore fractionation. The authors pointed out, however, that the evi-
dence was tentative and the consequences of micropore fractionation
should be viewed as speculative.
A depletion in ðD due to exchange reactions with hydrated min-
erals also seems rather unlikely. Matsudo et al. (1972) analysed the
ðD-value of borax, gaylussite, nacholite and trona, synthesized in
aqueous solution between 8° and 35 °C, and the ðD of the water
which these minerals were grown from. For borax, no significant
fractionation was found but the other minerals were always depleted
in deuterium relative to the solution from which they were crystal-
lized.
Two deuterium measurements have been made on water from a
core from the well in Vestmannaeyjar. The water was released by
heating the core to approximately 900 °C. In both cases the water
released was about 40%o lower in deuterium than the groundwater.
On the other hand, some laboratory studies which we have carried
out on the zeolite laumontite show that the water in the zeolite is
approximately 30%„ enriched in ðD relative to the water in contact,
but also that equilibriiun is established in a few days.
Deuterium exchange measurements of Moum et al. (1958)
suggest that at 100°C even the least accessible hydrogens, those
of hydroxyl groups, should exchange within 10 years.
144