Jökull - 01.12.1975, Blaðsíða 27

rock as granite is about 1% at a shallow depth.
On the whole one must, therefore, consider
interstitial groundwater to have been present
in rocks. Its role in connection with radiometric
dating is a very complicated question, as we
have pointed out in sections 1 and 3. But
generally, one should think that it is of import-
ance, whether or not the groundwater has flow-
ed through the rock, and even through the
minerals. The flow would be largely determin-
ed by groundwater pressure gradients, which
in turn depend on the landscape at each time.
A dip of a flat land, corresponding to a surface
gradient of 200 m per 100 km, would suffice
to cause a groundwater flow through granite,
which could be serious from the point of view
of radioactive dating. But also the geology of
the area as a whole, of which the rock under
consideration is a part, influences groundwater
flow through the rock.
One cannot possibly reconstruct the history
of groundwater flow for rocks of some consicler-
able age. One could try to test the influence
of the geological factors in the following way:
1) The age for each productive mineral type
is measured, using a variety of sizes and shapes,
as said before, to test losses of a daughter ele-
ment. 2) Whether loss is indicated or not in
this way, the ages of the receptive minerals are
also tested in the same way. This treatment
might enable the analyst to make necessary cor-
rections to lead him at least closer to the true
age of the rock.
In practice the grouping of Precambrian ages,
for instance, is interpreted as metamorphic
phases which are dated in this way, and beyond
which a correction is not thought to be possible.
The possibility of reconstructing the history
of groundwater flow is, however, considerably
greater in connection with the dating of such
young rocks as Neogene or even Paleogene
ones. In this case the tectonic history, in parti-
cular uplifts and the following dissection of
such areas may be known in the main outlines,
and these factors have increased the flow of
cold groundwater down to some depth and
lowered the temperature within rocks which
are now exposed. The “age” of such rocks may
date this cooling phase. We shall come back
to this topic in the next section.
As to still younger rocks, such as those of
1—4 My age (cf. the geomagnetic time scale),
we must keep in mind that they have been
soaked with water for much of their life time,
and that even cold groundwater dissolves sub-
stances frorn such igneous rocks as fresh basalts.
One way of testing loss of 40Ar into ground-
water is to test 40Ar/38Ar in water emerging
from different depths, as indicated by the temp-
erature. Another way is to grind a sample down
to the coarseness of its grains, and test experi-
mentally the rate of loss of 40Ar into water at
various temperatures ánd pressures. If such
tests indicate the possibility of significant loss
of 40Ar in 1 to 4 My old rocks, a correction of
the measured ages must take into consideration
the history of the grounclwater flow, as far as
it could be reconstructed from the geomorpho-
logic and climatic history.
The present author has not seen a single
report on radiometric dating, or text books, in
which the physical principles statecl in section
4, i. e. the importance of grain size, have been
mentioned, although the tests of reliability
following from these principles seem most es-
sential.
Discordant ages for any rock mass are very
common, sometimes to such a degree that the
analyst has the only choice to postulate some
unknown geological events, to explain the gross
discordance in the ages given by various dating
methods. In such cases radiometric dating is
just no independent method of dating. The
omission of some of the physico-geological prin-
ciples in radiometric dating which we have
discussed leaves one in doubt about the extent
of the necessary revision of some of the radio-
metric material sofar collected.
5. NON-RADIOMETRIC, SEMI-ABSOLUTE
TIME SCALES FOR TERTIARY ROCKS
Distinction between the radiometric age of
rocks and of geomorphological phases, by
consideration of groundwater changes.
When it is realized how often it is difficult to
measure geological time with certainty by radio-
metric methods, it seems worthwhile to recon-
sider those older methods which were abandon-
ed when radiometry in its early clays seemed
to be the only sensible geological clock.
We shall not enter upon the possibility,
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