10.2 The
40Ar)39Ar dating
technique
The very different chemical affinities of
potassium and argon are responsible for the major limitations of the K–Ar dating method. However, these limitations can be
overcome by converting 39K to 39Ar in a nuclear reactor,
by irradiation with fast neutrons. This causes an n,p (neutron capture, proton emission) reaction:
3919K + n 6 3918Ar + p
This reaction permits the potassium
determination for a K)Ar age to be made as part of the argon
isotope analysis, thus opening up many new opportunities for the K–Ar dating technique.
10.2.1 40Ar)39Ar measurement
The comparatively long half-life of 39Ar
(t1/2 = 269 yr) means that
it can be regarded as a stable isotope for mass spectrometric analysis, which
was first applied to 40Ar)39Ar dating by Merrihue
and Turner (1966). It is interesting to note, however, that the concept of
combined irradiation and mass spectrometric analysis was applied to the I)Xe system in meteorite studies five
years earlier (section 15.3).
The production of 39Ar from 39K
during the irradiation is expressed as:
/
|max e
39Ar =
39K )t |
Ne
Fe de [10.7]
|min e
/
where )t is the irradiation time, Ne is the flux density of neutrons with energy e, and Fe is the capture cross-section of 39K
for neutrons of energy e. The
production must be integrated over the total range of neutron energies, which
is a very difficult calculation in practice. Therefore, the normal procedure is
to use a sample of known age as a flux monitor.
Taking the K)Ar decay equation [10.2], which is
reproduced here:
40Ar*
= (8EC/8total) @
40K (e8total t ! 1)
and dividing through on both sides by equation
[10.7], yields:
40Ar* | 8EC 40K |
))) = | ))))
@ )))))))))) | (e8total t ! 1) [10.8]
39Ar |
8total 39K )t
I Ne Fe de
|
However, the term in the large parentheses is
the same for sample and standard. Therefore it is customary to refer to it as a
single quantity, whose reciprocal J
can be evaluated as a constant (Mitchell, 1968). Hence, for the standard:
e8t ! 1
J
= ))))))))) [10.9]
40Ar*
/ 39Ar
where t is
known. Rearranging equation [10.8] for samples of unknown age yields:
1 | (40Ar*) |
t =
)) . ln | J ()))))
+ 1 | [10.10]
8 | (39Ar ) |
In order to obtain an accurate value of J for each unknown sample, several
standards need to be run, representing known spatial positions relative to the
unknown samples within the reactor core (Mitchell, 1968). Hence J values for each of the samples can be
interpolated.
10.2.2 Irradiation corrections
During the irradiation of 39K,
interfering Ar isotopes are generated from calcium
and other potassium isotopes by neutron reactions (Fig. 10.8). Brereton (1970)
and Dalrymple and Lanphere
(1971) made detailed studies of the magnitude of these effects and their
correction. However, it appears in practice that many workers have simply
ignored the interferences.
Fig. 10.8. Part of the chart
of the nuclides in the region of potassium showing the production reaction
(heavy arrow) and major interfering reactions (solid) during neutron
activation. Dashed reaction to 37Ar is the interference
monitor. Data from Mitchell (1968).
Mitchell
(1968) suggested that acceptable results could be obtained without interference
correction on minerals over 1 Myr old, provided that
K/Ca was greater than 1. In such circumstances, a simple atmospheric correction
may be considered adequate:
40Ar* (40Ar) (36Ar)
))) = ()))) !
295.5 ()))) [10.11]
39Ar (39Ar
)meas (39Ar)meas
Turner
(1971a) showed that Ar interferences could be kept to
a minimum by variation of certain irradiation parameters. The principal
interferences which must be considered (Fig. 10.8) are:
40K n , p 6 40Ar
40Ca n , n" 6 36Ar
42Ca n, " 6 39Ar
Other interferences occur but may be omitted as
insignificant.
The approaches suggested by Turner were as
follows:
1)
Optimisation of neutron dose according to age (Fig. 10.9a) to maximise 39Ar
production, without generating significant artificial 40Ar from 40K.
(K content is not considered as a factor because the intended target and
interfering one are both K isotopes).
2)
Optimisation of sample size according to age and K content in order to obtain
the total 40Ar and 39Ar yields necessary to achieve the
desired counting statistics during mass spectrometric analysis.
3) The
K/Ca ratio in the sample also dictates an optimum neutron dose to generate
enough 39Ar without significant interfering 36Ar (Fig.
10.9b). However, the optimum values largely overlap with those prescribed by
criterion 1.
Fig. 10.9. Optimisation of
neutron dose for (a) K content, and (b) K/Ca ratio. Hatched area in (a)
and bold lines in (b) indicate regions of acceptable compromise between
sufficient 39Ar production and minimal 40Ar or 36Ar
interference in typical rocks. After Turner (1971a).
Theoretically,
very young rocks can be activated with less than 1% interferences by following
these rules. However, this may require an immense sample size. In practice, a
better alternative may be to use more irradiation but apply corrections. The
complete correction formula (in terms of Ar isotope
ratios) is:
[10.12]
40Ar*
))) =
39Ar
40Ar 36Ar 37Ar (36Ar) (40Ar)
))) ! 295.5 @ )))
+ 295.5 @ ))) @ ()))) ! ())))
39Ar 39Ar 39Ar (37Ar)Ca
(39Ar)K
)))))))))))))))))))))))))))))))))))))))))))))))))))))))
37Ar (39Ar)
1 ! )))
@ ())))
39Ar (37Ar)Ca
where 37Ar/39Ar is the
interference monitor ratio measured for the unknown, which must be corrected
for 37Ar decay from the time of irradiation until analysis (t1/2 = 35 days); and where (36Ar/37Ar)Ca
, (39Ar/37Ar)Ca and (40Ar/39Ar)K
are production ratios of Ar isotopes from the
subscripted elements. These production ratios are determined by irradiating
pure salts of Ca and K respectively in the reactor of interest, and are
characteristic of the neutron flux of that reactor. Values for these production
ratios measured by various authors for different reactors have typical ranges
of 2.1 )
2.7, 6.3 ) 30 and 0.006 ) 0.031, respectively (Dalrymple and Lanphere, 1971).
10.2.3 Step heating
Because the potassium signature of a sample is
converted in situ to an argon
signature by the 40)39 technique, it is possible to liberate argon
in stages from different domains of the sample and still recover full age
information from each step. Merrihue and Turner
(1966) demonstrated the effectiveness of this ‘step heating’ technique in their
original Ar)Ar dating study of meteorites, adapting the
method from its previous application to I)Xe analysis of meteorites (section
15.3.1).
The
great advantage of the step heating technique over the conventional ‘total
fusion’ technique is that progressive outgassing
allows the possibility that anomalous sub-systems within a sample may be
identified, and, ideally, excluded from an analysis of the ‘properly behaved’
parts of the sample. This can apply to both separated minerals and whole-rock
samples. Most commonly the technique is used to understand samples which have
suffered argon loss, but it may also be a help in interpreting samples with
inherited argon.
In
the case of partially disturbed systems, the domains of a sample which are most
susceptible to diffusional argon loss (such as the
rim of a crystal) should be outgassed at relatively
low temperatures, whereas domains with tightly bound argon (which are more
resistant to disturbance) should release argon at higher temperature. In order
to understand the history of disturbed samples, results of the step heating
analysis are normally presented in one of two ways: as a K)Ar
isochron diagram, analogous to a suite of samples
analysed by conventional K)Ar; or as an age spectrum plot.
Step
heating results from the meteorite Bjurbole (Merrihue and Turner, 1966) are plotted on an isochron diagram in Fig. 10.10. The straight-line array
indicates a simple one-stage closed-system history for the meteorite. However,
the initial 40Ar/36Ar ratio may be only partially
meaningful, since it is a mixture of initial Ar and
atmospheric contamination. The isochron plot can be
useful to see the relative amounts of radiogenic argon and
atmospheric/inherited argon in the sample. However, it conceals one of the most
useful pieces of information about a step heating analysis, which is the
position of each argon release step in the overall heating experiment. This
information is displayed in the argon spectrum plot.
Fig. 10.10. Step heating data for the Bjurbole meteorite presented on the Ar)Ar
isochron diagram. Numbers by data points signify
temperatures of each release step in oC. After Merrihue and Turner (1966).
To
construct a spectrum plot, the size of each gas release at successively higher
temperature is measured in terms of the magnitude of the 39Ar ion beam
produced. Each gas release can then be plotted as a bar, whose length
represents its volume as a fraction of the total 39Ar released from
the sample, and whose value on the y
axis is the corrected 40Ar/39Ar ratio from equation
[10.12]. The latter is proportional to age, which is sometimes plotted on a log
scale, and sometimes linear. Determination of a reliable crystallisation age
from the spectrum plot depends on the identification of an age ‘plateau’. A
rigorous criterion for a plateau age is the identification of a series of
adjacent steps which together comprise more than 50% of the total argon
release, each of which yields an age within 2 standard deviations of the mean (Dalrymple and Lanphere, 1974; Lee
et al. 1991). However, plateaus have
been ‘identified’ in many instances on the basis of weaker evidence.
The
age spectrum plot displays the ideal behaviour of the K)Ar
system in tektite glasses. These are objects which were completely melted
during flight through the atmosphere and then rapidly quenched on landing.
Thus, young tektites which have not been affected by weathering yield perfect
plateaus (Fig. 10.11). However, the most useful application of the 40)39 method is on samples with a
complex geological history that does
involve secondary argon loss.
Fig. 10.11. Ideal 40Ar/39Ar
age spectra for two
10.2.4 Argon loss events
In order to assess the usefulness of 40)39 dating on disturbed systems,
Turner et al. (1966) applied the
method to chondritic meteorites. Many of these
objects yield conventional K)Ar ages below 4.5 Byr,
with U)He ages
clustering around 500 Myr (Anders, 1964). For
example, step heating results from a whole-rock sample of the Colby meteorite
generate a complex age spectrum (Fig. 10.12), which was attributed to argon
loss at ca. 500 Myr (Turner et al., 1966; Turner, 1968).
Fig. 10.12. 40)39 argon release pattern for the Colby
meteorite, showing evidence for disturbance after formation. The best fit curve
is consistent with a model in which 40% of argon was lost during a thermal
event (see below). After Turner (1968).
Turner
et al. suggested that when these
meteorites were subjected to an ancient heating event (possibly in a collision
between planetessimals), Ar
loss occurred from the surface of mineral grains, and its transport within
grains was by volume diffusion. This argon loss model is illustrated
schematically in Fig. 10.13. Turner et
al. argued that step heating analysis of the sample in the vacuum system
would mimic the natural thermal event, so that domains near the surface of
minerals, which had suffered geological disturbance, would outgas first in the
experiment. In contrast, domains near the cores of grains would be resistant to
geological disturbance, and would also outgas at the highest temperatures in
the laboratory. (Although these meteorites are chondrites,
it is the minerals in the chondrules that retain
argon, rather than the chondrules themselves).
Fig. 10.13. Schematic illustration of the
geological history of a mineral grain in a partially disturbed meteorite. 1) At
4500 Myr; 2) 500 Myr ago,
before thermal event; 3) immediately after the event; 4) present day. After
Turner (1968).
In
order to test their diffusion model, Turner et
al. calculated theoretical age spectrum plots, assuming that the meteorites
were formed at 4500 Myr and metamorphosed in a single
event at 500 Myr. Argon loss was modelled by assuming
volume diffusion from spherical K-bearing mineral grains (probably mainly
feldspar). The results of two different models are compared with the measured
argon release profile of the Bruderheim meteorite in
Fig. 10.14. The first model assumed argon loss from spherical grains of uniform
size, but this gave a bad fit to the measured profile (curve a). However, thin
section analysis of the meteorite revealed the existence of variably sized
feldspar grains. Therefore, Turner et al.
calculated the result of argon diffusion from grains with a log-normal size
distribution. This model generates a family of solutions according to the
amount of grain-size variation allowed. Curve b in Fig. 10.14 shows the best
result, assuming a standard deviation (F[log radius])
of 0.20, equivalent to four fifths of grains falling within a factor of two
from the mean radius.
Fig. 10.14. Argon release profile of the Bruderheim meteorite, compared with calculated argon loss
profiles from spherical mineral grains formed at 4.5 Byr
and disturbed at 0.5 Byr. a) assuming uniform size
distribution; b) log-normal size distribution with F = 0.20. After Turner (1968).
The
shape of best-fitting curve in Fig. 10.14 is also dependent on the fraction of
total Ar that was lost in the heating event. Thus,
the concave upwards shape of curve b indicates that Bruderheim
suffered more than 80% argon loss. In this case the low temperature part of the
profile yields the age of metamorphism (0.5 Byr) but
the formation age is lost because even the highest temperature release steps
give ages well below 4.5 Byr. On the other hand, the
shape of the Colby release profile (Fig. 10.12) approximates to ca. 40% argon
loss, so that the high-temperature argon release steps still preserve an
indication of the age of formation. For samples which lost less than 20% (not
shown), the high temperature plateau still records a good crystallisation age
but the metamorphic age is badly constrained.
The
relatively good agreement between the analysed and the log-normal model pattern
in Fig. 10.14 suggests that the diffusional loss
mechanism is a good description of the thermal disturbance of meteorites. This
might be expected, since both the geological event and laboratory measurements
were based on heating of anhydrous phases in a vacuum. Turner (1972)
demonstrated a similar good fit to the diffusional
model for experimental data from a lunar anorthosite.
In contrast, terrestrial 40)39 dating generally involves hydrated minerals such as biotite and hornblende. In this case, the diffusional argon loss mechanism may not provide such an
accurate model. An assessment can be made by examining 40)39 data for the Eldora stock
(Berger, 1975), for which conventional K)Ar data were described above. Fig.
10.15 shows age spectrum plots for hornblendes, biotites
and feldspars in the vicinity of the stock.
Of
the three minerals studied, hornblende (Figs. 10.15 a and
b) displays the type of pattern most similar to Turner’s thermal diffusion
degassing model, although this resemblance may be misleading. The most distant
sample (not shown) yields an excellent plateau age of ca. 1400 Myr. Samples at 1130, 950 and 248 ft (ca. 350, 290 and 75
m) display serious Ar loss from the outside of
grains, but approach the ‘true’ age in the highest-temperature fractions.
However, Berger recognised that the pattern of Ar
loss might reflect alteration to biotite, rather than
diffusional Ar loss from
hornblende. This interpretation is supported by dating experiments on synthetic
hornblende)biotite mixtures (Rex et al., 1993). Another problematical observation is that the sample
11 ft (3.5 m) from the contact displays an intermediate ‘false’ plateau of high
quality. Finally, the sample 2 ft (0.6 m) from the contact displays a
saddle-shaped pattern, in which the lowest-age fraction approaches the age of
metamorphism.
Coarse
biotite (Fig. 10.15c) behaves somewhat differently.
Its maximum age at infinite distance from the stock (1250 Myr)
is lower than the hornblende age. At intermediate distances the spectra are
irregular, but exhibit a general decrease in ‘plateau’ age as the stock is
approached. Hence, it appears that biotites can be
partially but uniformly outgassed, possibly because
of enhanced diffusion parallel to the cleavage. Finally, K-feldspar suffers
irregular and disastrous Ar losses, as is known from
conventional K)Ar analysis (Fig. 10.15d).
Berger
concluded that hornblendes were able to generate plateaus of high quality which
were nevertheless meaningless. This may make hornblende a dangerous material on
which to base geological interpretations of age, in the absence of independent
confirmatory evidence. On the other hand, partially re-set biotites
were always identifiable by their irregular patterns, making biotite ages a more reliable tool for age interpretation.
The exact meaning of the plateaus in the biotite and
hornblende samples distant from the stock is equivocal, since the country-rocks
are paragneisses with a long history of thermal
events. Subsequent studies have indeed generated many examples of meaningless
plateaus in hornblende, and more rarely, in biotite.
Fig. 10.15. Ar)Ar age spectrum plots for mineral
phases at different distances from the Eldora stock. Figures beside age spectra
indicate distances in m. a), b) hornblende, c) biotite,
d) K-feldspar. Release steps with identical ages are separated by slashes.
After Berger (1975).
10.2.5 Excess argon
As well as detecting argon loss, step heating
analysis can also be used to evaluate cases where excess argon is present in a 40Ar–39Ar
analysis. Following early work by Lanphere and Dalrymple (1971), a more detailed examination of this
problem was made by Lanphere and Dalrymple
(1976). In this study, step heating analyses were made on separated mineral
phases from several rocks that were known from earlier work to contain excess
(inherited) argon. The data were presented on K/Ar isochron diagrams and age spectrum plots for comparison.
The example shown in Fig. 10.16 (a and b) comes from a
sample of Mg-rich biotite separated from a kimberlite dyke that intrudes Devonian sediments in
On
the K/Ar isochron diagram,
it can be seen that the data scatter badly above a 150 Myr
reference line. This line was based on the estimated age of the kimberlite, with an intercept equal to the atmosphere
point. The scatter of the data provides evidence of excess argon, but is not
further diagnostic. However, when the data are plotted on the spectrum plot,
they form a ‘saddle-shaped’ pattern that was found to be characteristic of all
of the samples with excess argon analysed by Lanphere
and Dalrymple (1976). Unfortunately, the minimum age
from the saddle does not give the age of intrusion, since it is still above this
estimated age. Therefore, such minima must be regarded only as maximum ages for
the rock, as argued by Kaneoka (1974).
Fig. 10.16. Comparison between the K/Ar isochron plot and age spectrum
plot for a biotite grain with excess argon. Note the
characteristic ‘saddle shaped’ profile. Numbers indicate the temperature of
each heating step in oC. F = fusion step.
After Lanphere and Dalrymple
(1976).
In
seeking an explanation for the saddle-shaped age spectrum associated with
excess (inherited) argon, Kelley (2002) suggested that this feature was caused
by inclusions of various types. For example, fluid inclusions in mineral grains
are expected to release argon at low temperature, whereas mineral inclusions
may release argon at high temperature. A special case of the former type is
exhibited by anorthoclase grains in a lava from Mt Erebus,
10.2.6 Dating paleomagnetism:
a case study
Paleomagnetic measurements are a vital tool in
the reconstruction of ancient plate tectonic motions, by comparison of
‘apparent polar wander paths’ (APWPs) for various
continental fragments. One essential step in the construction of an APWP
‘track’ for a given terrane is to date the time when
magnetic remanence was inherited by the rock.
However, the magnetic remanence is relatively easily
overprinted because it has a comparatively low blocking temperature.
The
dating of magnetic remanence took a major step
forwards when
A
good example of such work is provided by the oldest reliable Ar)Ar age for terrestrial rocks (Lopez
Martinez et al., 1984), on the
Fig. 10.17. Age spectrum and Ca/K spectrum from
Barberton komatiite sample B40A. Mineral phases
responsible for gas releases are identified. Error boxes where visible are 1F. After Lopez
Martinez et al. (1984).
The
top half of Fig. 10.17 reports Ca/K ratios, calculated from the measured 37Ar/39Ar
ratio (section 10.2.2), which help characterise the mineral phases in the
sample which gave rise to different parts of the age spectrum. By microprobe
analysis, the authors were able to deduce that the mineral giving rise to the
age plateau was metamorphic tremolite, while the
low-temperature, low- Ca/K phase was stilpnomelane.
The high- Ca/K phase may represent pyroxene relics of the original igneous
mineralogy.
A
K)Ar isochron diagram was plotted (Fig. 10.18) in order to
examine the composition of the non-radiogenic end-member, and test for
inherited argon. In this case the isochron diagram
was plotted in the alternative form 36Ar/40Ar versus 39Ar/40Ar
(Turner, 1971b). This representation helps to curtail the strong correlation
between the two ordinates which occurs with the conventional K)Ar isochron
diagram, making error estimates easier. An initial 40Ar/36Ar
ratio of 281 " 18 (2F) is calculated from the inverse of
the y axis intercept, after expansion
of analytical errors to absorb a small amount of geological scatter. This is
within error of the atmospheric value of 295.5, so an insignificant amount of initial
argon was probably present. These data are from a sample which was stored under
vacuum between irradiation and analysis. This was found to be necessary in
order to prevent a strong absorption of atmospheric argon by the sample. The
reciprocal of the x intercept yields
the radiogenic 40Ar/39Ar ratio, equivalent to an age of
3489 " 68 Myr (2F). This is almost identical to the plateau age.
Fig. 10.18. Inverse argon)argon isochron
plot for two Ar)Ar runs ( ! , > ) on the Barberton komatiite B40A. The age is determined from the intersection
on the x axis. After
Lopez Martinez et al. (1984).
Age
spectrum results from the better of two basaltic komatiite
samples are shown in Fig. 10.19. In contrast to the komatiites,
these samples display significant excess argon in the low- and high-temperature
gas releases, with an integrated age of 3778 Myr.
Nevertheless, the best plateau age of 3447 Myr is in
close agreement with the best komatiite results. The
saddle-shaped form is well known for samples containing excess argon.
The
Ca/K plot for the basaltic komatiite suggests that
the plateau is due to hornblende, while the disturbed parts of the spectrum are
again related to stilpnomelane. Lopez Martinez et al. speculated that K)Ar systematics
in this mineral might have been disturbed during oxidation from ferro- to ferric-stilpnomelane.
Since the plateau ages are in all cases identified with metamorphic minerals,
they must be dating a thermal event which occurred less than 100 Myr after eruption. Hale (1987) tentatively identified this
event as the intrusion of the nearby Threespruit granitoid pluton.
Fig. 10.19. Age and Ca/K spectrum for two runs
on a basaltic komatiite showing evidence of inherited
Ar during low- and high-temperature emission from
high Ca/K domains. Arrows separate successive gas releases with identical ages.
After Lopez Martinez et al. (1984).
10.2.7 39Ar recoil
The Ar)Ar dating technique was found to be
particularly useful for dating small whole-rock samples of lunar material,
especially fine-grained mare basalts. The dashed profile in Fig. 10.20 shows a
typical release pattern (Turner and Cadogan, 1974),
attributed to 8% radiogenic Ar loss from K-rich sites
with low Ar retentivity.
However, other samples showed either a sharp decrease in apparent age in the
high-temperature fractions, or, particularly in fine-grained rocks, a
progressive decrease in apparent age over most of the gas release. The latter
examples led workers to suspect that Ar
redistribution was occurring within the sample, possibly during the irradiation
process.
Fig. 10.20. The effect of fine crushing on a 40)39 age spectrum, due to 39Ar
recoil. Dashed profile = analysed rock chip of a lunar mare basalt. Solid
profile = similar sample activated after fine powdering. After Turner and Cadogan (1974).
It
was proposed by Mitchell (in Turner and Cadogan,
1974) that recoil of 39Ar during the n,p reaction from 39K could cause small-scale
re-distribution of this nuclide. Turner and Cadogan
calculated that this effect could deplete argon from the surface of a K-bearing
mineral to a mean depth of 0.08 :m (Fig. 10.21). In order to test the practical
effects of this process on fine-grained material, they powdered a sample of
medium-grained ferrobasalt to a grain size of 1 ) 10 :m before irradiation. This was expected
to bring ca. 10% of K-bearing lattice sites to within 0.1 :m of a grain boundary, whereupon 39Ar
could recoil out of the lattice. It was anticipated that the 39Ar
released would enter low-K minerals such as plagioclase, pyroxene and ilmenite, leading to an old apparent age during
low-temperature release (K-bearing minerals) and a young apparent age during
high-temperature release.
Fig. 10.21. Plot showing calculated drop in 39Ar
concentration at the surface of a K-bearing mineral due to recoil, in response
to bombardment with an isotropic neutron flux. After Turner and Cadogan (1974).
The
results from this experiment (Fig. 10.20) showed that while abnormally old ages
were produced at low temperature, the data approached the ‘true’ plateau age at
intermediate temperatures. Therefore, Turner and Cadogan
argued that 39Ar released by recoil must have been lost from the
sample altogether, rather than absorbed by low-K phases. This is probably due
to the fact that adjacent grains are in less intimate contact in a powdered
sample than they are in a fine-grained rock sample. The unusually high ages in
the highest temperature fraction (Fig. 10.20) were tentatively attributed to an
incorrect Ca correction, due to recoil of the monitor isotope 37Ar
during the n," reaction from 40Ca. This transformation should result in
four times more recoil than proton emission from 39K.
Argon
recoil has important implications for minerals whose diffusional
history is explained in terms of micro domains (section 10.5.3). The most
important examples are feldspars, which have exsolution
lamellae about 0.01 – 0.3 :m thick, but show little evidence for recoil effects in their plateau
ages or Arrhenius plots. The lack of any such
evidence led McDougall and Harrison (1988) to speculate that a large fraction
of 39Ar recoils might occur at low energy, with reduced
displacements.
Onstott et al.
(1995) re-examined this question using theoretical calculations and ion
implantation experiments, but these continued to support a mean 39Ar
recoil distance of 0.082 :m. The implications were examined for three minerals showing exsolution of K-rich and K-poor lamellae (amphibole,
plagioclase and K-feldspar). In all three cases, calculations indicated that 39Ar
concentrations would be significantly homogenised and 37Ar almost
totally homogenised between adjacent lamellae (Fig. 10.22). Therefore, Onstott et al.
concluded that the lamellae were too small to be the domains controlling volume
diffusion of argon in the samples of amphibole and plagioclase studied. The
situation for K-feldspar was less clear, but they suggested that some re-interpretation
of results might be necessary for the smallest domain sizes of K-feldspar used
in thermal history analysis.
Fig. 10.22. Predicted argon isotope
distribution after neutron irradiation of a plagioclase grain. The grain has
alternating lamellae of calcic plagiocase
(60 nm wide) and 50% plagioclase and K-feldspar (320 nm wide). After Onstott et al.
(1995).
The
calculations of Onstott et al. were tested by a direct determination of the 39Ar
recoil distance by Villa (1997). A thin slab of KCl
was sandwiched between two sheets of silica, but on one side the silica layer
was shielded by a silicon coating 95 nm thick. The whole assembly was then
irradiated to simulate a 40–39 argon analysis. After irradiation, the 39Ar
concentration on the inner surface of each silica sheet was analysed, and from
the difference between the shielded and unshielded surface, a mean 39Ar
recoil distance of 80 " 20 nm in silicon was calculated. Based on the relative densities of
silicon and K-feldspar, the recoil distance in the mineral was estimated as
about 70 nm. This measurement supported the theoretical calculations of Onstott et al.
(1995), and therefore led Villa (1997, 1998) to question the meaning of
K-feldspar thermochronometry using the Multi
Diffusion Domain (MDD) model (section 10.5.3). This question remains
controversial.
10.2.8 Dating glauconite
and clay minerals
The problem of 39Ar recoil was found
to be particularly severe in attempts to apply 40Ar)39Ar dating to the authigenic
sedimentary mineral glauconite (e.g. Foland et al., 1984).
This is probably due to the very small grain size of the glauconite
crystallites which make up the grains of a pellet. Smith et al. (1993) showed that this problem might be overcome by
encapsulating glauconite grains in small glass
ampoules prior to irradiation. The recoil products can then be collected for
analysis, in order to correct the Ar release from the
rest of the grain. However, this method is only applicable to a whole-sample
degassing analysis (analogous to a conventional K–Ar
age) and cannot be used with the step heating method.
Smith
et al. (1998) applied the method of
micro-encapsulation to the analysis of suites of single glauconite
grains of three different ages. However, when the data were used to construct
age frequency diagrams, Smith et al.
found multiple age peaks, suggesting several episodes of glauconitization.
Nevertheless, it appeared that the oldest of these episodes was in each case
close to the time of sediment deposition, so that the analysis of a large
sample suite may give a reasonable estimate of the time of deposition. It
remains to be seen whether such ages are sufficiently reliable for calibration
of the stratigraphic column.
The
small grain size of clay minerals (typically 5 – 1000 nm thick) makes them also
very susceptible to 39Ar loss by recoil effects. To combat this
process, the technique of encapsulation was also applied by Smith et al. (1993) to clay minerals. 39Ar
which escaped from the sample due to recoil was held in an ampoule so that it
could be collected and analysed with argon released during laser heating.
However, experiments by Dong et al.
(1995) suggested that the escaping argon fraction is not lost from illite and smectite by direct
recoil, but by thermal degassing of low-retentivity
sites which have picked up recoiling nuclides. These low retentivity
sites are actually the two free surfaces of the clay mineral grain, and the
amount of 39Ar loss is inversely proportional to thickness. For
example, a grain of illite 100 nm (1000 A) thick,
made of 100 silicate–cation–silicate composite
layers, will lose about 1% of its 39Ar (2 out of 200 surfaces); on
the other hand, a grain 10 nm thick might lose as much as 10% of its 39Ar
budget during irradiation.
Dong
et al. argued that 39Ar
loss during irradiation may match 40Ar loss during geological
heating (diagenesis). They measured the 39Ar
signal retained by encapsulation, then subtracted this fraction from the total
gas release of the sample (including all encapsulated argon). The resulting
‘argon retention age’ should be the same as a non-encapsulated age, and was
argued to be a better estimate of the time of deposition or early diagenesis than the encapsulated age. However, retention
ages measured on Paleozoic clays were older to
varying degrees than the non-encapsulated ages. This suggests that 39Ar
loss by recoil was indeed causing a bias to the data, even if this was
superimposed on an argon-loss process during diagenesis.
The fact that recoil loss of 39Ar is a problem in the analysis of many clay samples is indicated by
excess ages for the high temperature release fraction, which is not affected by
low-temperature argon loss events (Kapusta et al., 1997).
Kapusta et
al. (1997) proposed a new method whereby step heating experiments could be performed
on fine-grained material such as glauconite or clay.
This approach involves irradiating two aliquots of the sample to be dated. The
first is used for the step heating analysis, whereas the second is encapsulated
and used to determine a total release age (relative to a standard of known
age). The standard of known age is used to determine the J value of the irradiation in the usual way. The total release age
of the encapsulated aliquot is then used in turn to calculate a ‘JC’ correction for the step-heated
aliquot, modified from equation [10.9]:
e8t !
1
JC = ))))))))) [10.13]
(40Ar*
/ 39Ar)total release
The JC
value allows normalisation of both the neutron flux and recoil loss, provided that
the encapsulated sample has the same grain size distribution and crystal
make-up as the step-heated sample. Kapusta et al. (1997) demonstrated the method on
a glauconite standard (Fig. 10.23). However, it
should be applicable to clay minerals that have suffered a combination of
radiogenic 40Ar loss over geological time and 39Ar loss
during irradiation.
Fig. 10.23. Ar
spectrum plot on a 95 Myr old glauconite
standard showing the results of conventional step heating (top two profiles) as
well as a step heating analysis corrected using the JC recoil loss
monitor (hatched). Modified after Kapusta et al. (1997).
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