Jökull - 01.12.1975, Síða 19

fused much deeper into the basaltic glass in
the Icelandic rocks, mentioned above, than in-
dicated by palagonite rims. In fact, an analysis
of quite fresh and young basaltic glass (from
Valahnúkur) gave 3.10 weight percent H2O+
and 3.60% H2O— (Einarsson, 1950), which can-
not be anything but absorbed water that en-
tirely permeated the glass fragments without
causing any signs in thin section of palagonit-
ization.
As K/Ar is the only radiometric metliod sofar
used on Icelandic rocks, mainly basalts, and as
the potassium in basalts may reside largely in
interstitial glass, cf. section 4, one may wonder
how much of the argon has been washed out
by permeating groundwater. A further reason
for doubting seriously the retention of argon in
interstitial glass, is given at the end of the
next section.
In the lower parts of the Icelandic Tertiary
basalt plateau, the basaltic glass is practically
entirely palagonitized, and zeolite zones indi-
cate such a deep burial that the temperature
must regionally have reached 100—200 °C. Very
serious loss of argon from such rocks woulcl
not be unexpected. (Cf. addition in Proof).
2. THE ZERO POINT IN K/Ar-DATING,
AND ARGON EXCESS.
About 80—85% of the terrestrial heat flow is
estimated to be radioactive heat, and of this
some 12—18% come from 40K, with correspond-
ing production of 40Ar. It is a complicated
question, what has become of the 40Ar that has
been produced in this way within the earth
through geological ages, especially in the upper-
most 50—100 km or so. But at any rate it would
be expected that magma contains some con-
siderable amount of radiogenic argon when it
is erupted. How is this argon lost, so that
historic or postglacial extrusive basalts, say,
seem not to contain it?
For holocrystalline basalt lavas, the question
does not seem to be particularly difficult. Dur-
ing the crystallization, the argon will on the
whole be left in interstitial spaces, and it is
more than doubtful that it will go into the
interstitial minute potassium feldspar (Dal-
rymple and Lanphere, 1969, p. 181). But even
if it did so, it would leak out very rapidly
from such micron-size grains or lamellae, or
from interstitial glass while this matter was still
at a high temperature, cf. also end of this sec-
tion. The argon must thus be expected to go
as a free gas into the interstitial spaces, from
which it will be washed by percolating water
after the cooling of the lava. The washing out
of the argon in this way, might take some years
to hundreds of years at the outmost. An initial
diffusion of the gas may also cause much of
the loss, to establish the zero point of the radio-
metric clock in this case.
What would happen in a subaquatic extru-
sion, can also be answerecl on the basis of data
on retention of excess argon in deep to shallow
sea extrusions. We refer in particular to Dal-
rymple and Moore (1968), which we shall discuss
in some detail.
Pillows were obtained from depths down to
5000 m. There was a certain relation to water
depth. Pillows which had formed at depths
more than about 2000 m contained excess argon
in a glassy crust of about 2 cm thickness, while
deeper (in the pillows) the retained argon fell
rather rapidly to zero. The retention in this
crust appears not to be dependent on depth
within the interval 5000 to about 2000 m. The
retention in the outermost shell, 0—1 cm, was
found to be (measured in age): 42.9 My (2590
m depth), 14.1 My (3420 m), 30.3 My (4680 m),
and 19.5 My (5000 m). These four pillows come,
naturally, from as many different eruptions
ancl are, therefore, not easy to compare. But as
far as the material goes, it does not indicate
dependence of argon content on water depth
in the deep sea, and this was to be expected,
as will be made clear.
On the other hand, there is tliis depth limit
between retention and no (detected) retention
at about 2000 m. This limit is easy to under-
stand. It is due to the fact that the critical
pressure for water is 222 bars, corresponding
to the depth 2160 m in sea water.
Above this depth limit, quenching is due to
boiling, and especially in shallow water this
leads to an extremely rapid formation of a very
thin layer of glass, the temperature of which
falls to and below the boiling point within a
fraction of a second.*) This cool surface means
that the further heat exchange between the pil-
low and the water is relatively slow.
As we go deeper into the sea, the boiling
*) See footnote next page.
JÖKULL 25. ÁR 1 7