Jökull - 01.12.1975, Síða 26

Interstitial glass as carrier of the argon can
probably be excluded, as it is a slowly cooled
glass, cf. Section 2.
d) Equilibrium of a daughter element
in a mineral assemblage.
In relation to single-crystal dating, we have
sofar only considered single minerals, of the
shape of a sphere or a plate, containing a
radioactive mother element. The calculation of
the loss of a daughter element woulcl naturally
be complicated very much by going over to the
shapes of real rock minerals.
When D is not zero, a daughter element will
ooze out especially at sharp corners and edges.
But the importance of the size remains, the loss
must be taken to be in the first instance in-
verse to the square of the diameter of the
crystal.
We shall now take into account that a vol-
canic or metamorphic rock is, as a rule, an as-
semblage of minerals, some species (the “pro-
ductive” ones) containing a radioactive mother
element, the others (the “receptive” ones) not.
But each species has its own value of diffusivity
for the particular daughter e'ement, ancl each
species occurs in various shapes and sizes. The
daughter element under consideration will then
ooze out of the productive species, most rapidly
from the smallest crystals, and it diffuses into
the receptive species, the concentration here
increasing most rapidly in the smallest crystals.
We shall here still assunie that interstitial
groundwater does not exist. (Stagnant ground-
water rnight also be included in the considera-
tion, but we prefer to leave out the accompany-
ing complications here).
During the slow rise of concentration in the
productive minerals — reminding in principle of
Fig. 2 — a kind of equilibrium with the con-
centration in the surrounding receptive species
is established. In detail this equilibrium distribu-
tion of the element will depend on the various
diffusivity coefficients, and the sizes and shapes
of the crystals.
If the rock uncler consideration is relatively
deeply buried, or if it forms a large rnass, rela-
tively little loss from its centre will occur. This
corresponds to a rise of C0 and Cmax for the
individual productive minerals. If by drilling,
samples for dating are obtained ftom a central
24 JÖKULL 25. ÁR
part of such a mass, almost the total quantity of
the respective daughter element, produced dur-
ing the lifetime of the rock, might still be pre-
sent. But it is distributed among all the minerals
of the rock. Then, it is not sufficient to measure
the ratio of mother- to daughter-element in a
productive mineral, the whole content of both
elements in a representative sample of the rock
must be measured and then, by consideration of
volumes, concentrated in the productive miner-
als.
Another way of getting samples from within
rock masses, is to sample from walls, formed by
heavy glacial erosion during the Pleistocene.
This has also application in connection with
whole-rock dating. When groundwater circula-
tion is left out of consicleration, as we have clone
sofar in d), deep burial (apart frorn temperature
effects) aids the retention of argon within the
rocks, and in areas where deep sections have
recently been formed by gfacial work, higher
ages than elsewhere may be expected. The ages
of about 12 My in the basalts of the Eastern
Fjords in Iceland, in contrast to 1—3 My in the
inland flat area of the Fljótsdalsheidi (Einars-
son, 1971), may largely be due to this pheno-
menon, because the Eastern Fjords are just such
an area in which heavy dissection and removal
of rocks took place in Pleistocene time. An age
difference of 10 My between the two areas is
highly questionable, as they belong to a lava
sequence in which there are no erosional uncon-
formities (cf. 1. c.).
e) Geological considerations.
We have considered two extremes: 1) separate
productive minerals, surrouncled by a medium
(flowing groundwater) which ensures constancy
of Cn at the surface of the minerals, and 2) a
mineral assemblage without an interstitial
medium. In the latter case it was seen to depend
on the size of the rock mass itself, or the depth
of its burial, how far the daughter element
will be retained within the rock, although
distributed unevenly among the minerals.
The geological reality must be taken to lie
between these extremes. Groundwater is gener-
ally present below a shallow depth, and above
an uncertain level where pressure has made the
rocks almost or entirely impervious to water.
The volume of groundwater in such a dense