Jökull - 01.12.1975, Síða 18

metric dating of such rocks must correspond-
ingly be carried out with particular care, cf.
Sections 4 and 5.
At higher temperatures and pressures we meet
regional metamorphism on a higher level. Here
water plays also a decisive role: “a most deci-
sive role in metamorphism must be assigned to
water”. “Where metamorphic reactions involve
liydration or dehydration — as they commonly
do — water must be able to diffuse into or
from the reacting system” (Verhoogen et al;
1970, p. 594). In rocks of apparent ages of
more than 1000 My, indications of metamorphic
events are common.
As an example of the influence of ground-
water circulation on the retention of argon and
strontium, we quote measurements given in a
diagram by Dalrymple and Lanphere, 1969,
see Fig. 1. Dalrymple and Lanphere inter-
pret this diagram as showing the loss of Ar
and Sr as a function of temperature. But this
nrust be rejected on the basis of the great dif-
ference of these two elenrents. Argon loss is
already very marked in laboratory by tempera-
tures above 300 °C, but we know no data in-
dicating a serious loss of the metal Sr at so
low temperatures. When, therefore, the dia-
gram shows that both elements are lost from
muscovite on one hand, and from biotite, on
the other, at very much the same distances
from the intrusive contact, it is not the tempera-
ture which is the primary cause of the loss of
these elements but the groundwater circulation,
which can be said to be an inevitable result of
the heat from the intrusion. The groundwater
has in the present case washed out the radio-
genic elements in accordance with its intensity
of flow, i.e. dynamic pressure gradient and
its diffusion coéfficient in the respective
mineral. The raised temperature is considered
a secondary cause. As a consequence, this case
shows that similar Ar- and Sr-ages based on the
same mineral type, are no guarantee of a reli-
able age. Two or nrore mineral types must be
used, and the sizes of the minerals must be
taken into account, as shown in section 4.
Groundwater diffusion into glass is a well-
known ancl much more conspicuous process
than diffusion in minerals. In the youngest
members of the Icelandic plateau basalts, even
the olivine looks quite unaltered. But in the
glassy members of these series, rims of altera-
Fig. 1. Generalized curves showing the reten-
tion of radiogenic daughter products in mine-
rals from the Snowbank stock as a runction of
distance from the contact with the Duluth
Gabbro. After Dalrymple and Lanphere, 1969,
Fig. 9—12. We consider the loss of Ar and Sr
primarily to be due to groundwater circula-
tion caused by the heat conducted from the
stock, whereas the elevated temperature is a
secondary cause of the loss.
Mynd 1. Eftirstöðvar dótturefnanna Ar og Sr.
i ýmsurn krystöllum og með vaxandi fjarlœgð
frá innskoti í bergið. Aðalorsök Ar- og Sr-taps-
ins er hér talin vera grunnvatnshringrás, sem
hitinn frá innskotinu hefur valdið, en hœkkað
hitastig auliaorsök.
tion (palagonite), usually surround the pores
and the surface of the glass fragments.
The palagonite rims mean gradual diffusion
of water into the glass, until such water con-
centration has been reached that the glass de-
vitrifies into minute zeolites, calcite, and chlo-
rite crystals. But the visible front of palagonit-
ization does not necessarily mean that a lower
concentration of water has not diffused deeper
into the glass. On the contrary, we need only
mention pitchstone to indicate how deeply
water can diffuse into obsidian without causing
obvious devitrification.
Recently, the diffusion of water into obsidian
could be studied by a nuclear reaction techni-
que (Lee et al, 1974) which is based on the
fact that the glass expands when the water
diffuses into it.
These facts indicate that water may have dif-
1 6 JÖKULL 25. ÁR