Jökull - 01.12.1975, Blaðsíða 17

Several Problems in Radiometric Dating
TRAUSTI EINARSSON,
DEPARTMENT OF ENGINEERING AND SCIENCE,
AND SCIENCE INSTITUTE, UNIVERSITY OF ICELAND
ABSTRACT
In radiometric dating there are several sources
of error xvhich seem to have been given less
attention than is desireable, and which are dis-
cussed in this paper. The main topics are:
Indications of the effects of groundwater circula-
tion on the loss of Ar and Sr, the zero point in
K/Ar-dating and argon excess, the argon level
in groundwater compared with the rate of ac-
cumulation of radiogenic argon in potassium-
bearing minerals, the role of dimension in dif-
fusion losses from minerals. Ways are pointed
out to control diffusion loss in single-crystal dat-
ing by the use of minerals of the most various
sizes. For Tertiary rocks the main phases of
groundwater circulation may be reconstructed
from geomorphological history and phases of
argon loss thus inferred. In this way one may
distinguish between the radiometric age of geo-
morphological phases and that of the modelled
rocks. Here, Iceland is used as an example. In
an earlier high-temperature stage in the British
Tertiary basalts, the argon is similarly thought
to have been lost; the fact that most dykes give
ages in the same range as the lavas, is thought
to support this conclusion, because the study of
argon retention suggests that dykes possibly con-
tain originally various amounts of excess argon.
This leads to a general test of phases of argon
loss by way of dyke ages.
CONTENTS
1. lndications of the effects of groundwater circulation on
the loss of Ar and Sr.
2. The zero point in K jAr-dating, and argon excess.
3. The argon level in groundwater compared with the rate
of accumulation of radiogenic argon in potassium-bear-
ing minerals.
4. The role of dimension in diffusion losses from minerals.
a) The sphere.
b) The plate.
c) Whole-rock dating of basalts, and the sites of the pot-
assium.
d) Equilibrium of a daughter element in a mineral as-
semblage.
e) Geological considerations.
5. Non-radiometric, semi-absolute time scales for the Terti-
ary. Distinction between the radiometric age of rocks and
geomorphological phases by consideration of ground-
water changes.
1. INDICATIONS OF THE EFFECTS
OF GROUNDWATER CIRCULATION
ON THE LOSS OF Ar AND Sr.
Groundwater circulation is most commonly
the logical result oí a hydrostatic head, which
is intimately connected with topography. In
addition, structure plays a decisive role in
groundwater flow. It is the rule rather than
the exception that normal groundwater circula-
tion in mountainous regions goes down to such
depths that the emerging water has a tempera-
ture of 50°C or more. These thermal waters
have quite generally dissolved various sub-
stances, and these must have come from the
minerals along the course of the flowing
groundwater. Radon is a normal dissolved gas
in thermal waters. In spite of some complica-
tions as to the locus of the last absorption of
the radon contained in thermal waters, it can
hardly be doubted that radon was also absorb-
ed by the water along its course of the highest
temperature and pressure. Besides absorption
of radon, the many effects of groundwater
reaching depths with temperatures of 100—150
°C include at least partial dissolution of such
minerals as feldspars, to restore mineralogical
equilibríum by the formation of zeolites, cal-
cite, chlorite, and quartz. It seems hardly pos-
sible to assume that rocks of an age above 50—
100 million years have escaped such effects of
groundwater, because their present existence at
the surface implies that they have been shelter-
ed from denudation by a cover of younger rocks,
for at least tens of millions of years. The radio-
JÖKULL 25. ÁR 1 5