International Journal of Geosciences, 2012, 3, 891-898
http://dx.doi.org/10.4236/ijg.2012.325091 Published Online October 2012 (http://www.SciRP.org/journal/ijg)
Helium and Argon Isotopic Studies of Fossil Material and
the Theoretical Evolution of He and Ar in Earth’s
Atmosphere through Time
Yuko Arakawa, Jun-Ichi Matsuda*
Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Japan
Email: *matsuda@ess.sci.osaka-u.ac.jp
Received July 31, 2012; revised August 30, 2012; accepted September 28, 2012
ABSTRACT
We analyzed the elemental concentrations and the isotopic compositions of helium and argon in Cambrian to Jurrassic
aged Gastropod, Ammonite and Trilobite fossils in order to examine variation in these gases th rough time. Fossil sam-
ples yielded He and Ar isotopic ratios close to the present day atmospheric values, but also indicated some addition of a
radiogenic component. We compared the results to theoretical values calculated from a mathematical model of Earth’s
atmosphere assuming mantle degassing. Results from our mathmatical models showed that the 40Ar/36Ar ratio of Earth’s
atmosphere increased rapidly after the formation of the Earth, but has been almost identical to the present day va lue for
the last 1 Ga. For atmospheric helium, model results were consistent with present day atmospheric values, assuming
complete helium degassing from the continental crust into the atmosphere. The model suggests that the atmospheric
3He/4He ratio has remained relatively constant for the last 0.1 Ga. Given the similarity between present day and ancient
He and Ar isotopic ratios, we conclude that the corresponding ratios measured in ancient fossil material may partially
reflect composition of the ancient atmosphere and are not necessarily due to contamination by the present day atmos-
phere.
Keywords: Fossils; Argon; Helium; Isotopes; Atmospheric Evolution
1. Introduction
The Earth’s atmosphere is believed to be secondary in
origin, having been produced by degassing from the
Earth’s interior. This understanding is evident from the
lower atmospheric abundances of noble gases relative to
the abundances of active gases with similar mass num-
bers [1]. Noble gas isotopes are often used to trace the
degassing history of the Earth’s mantle, and various
models have been proposed based on their abundances
and isotopic ratios (e.g. [2]). We previously proposed a
model for noble gas transfer from Earth’s interior [3,4]
based on the finding that the lower mantle has experi-
enced an extensive degassing history [5].
Most mantle degassing models calculate the hypo-
thetical isotopic traj ectories for noble gases in the mantle
(e.g. [6]), but few have addressed the isotopic evolution
of atmospheric heliu m and argon through Ear th’s histor y.
Noble gas models, including the one described here, as-
sume that the Earth began its existence with the planetary
helium isotopic ratio (3He/4He = ~10–4), which differs
from the 3He/4He ratio of the present mantle (~10–5) by
an order of magnitude, and from that of the present at-
mosphere (1.4 10–6) by two orders of magnitude. If
ancient terrestrial samples show 3He/4He and 40Ar/36Ar
ratios similar to modern atmospheric ratios, it is very
likely that this finding is attributed to present day at-
mospheric contamination. This interpretation assumes
that ancient He and Ar isotopic ratios were markedly
different than present day values. In the absence of viable
gas inclusions from which pristine He and Ar isotopic
values could be measured, the precise value of these iso-
topic ratios in the ancient atmosphere remains unknown.
This report examines the potential use of fossil mate-
rial as a record for atmospheric He and Ar evolution
through time. We begin with the hypothesis that fossil
material may contain ancient gas signatures within their
internal void spaces. To test this hypothesis, we meas-
ured the He and Ar isotopic ratios of crushed samples of
varying age and compared our results to those estimated
from a model of atmospheric He and Ar isotopic evolu-
tion through time.
2. Noble Gas Analysis of Fossils
2.1. Samples and Experimental Methods
*Corresponding author. Samples used in this study included Gastropod, Ammon-
C
opyright © 2012 SciRes. IJG
Y. ARAKAWA, J. MATSUDA
892
ite, and Trilobite fossils of Cambrian to Jurassic age.
Samples were obtained through commercial sample col-
lections. Sample names, weights and the localities are
given in Table 1. Samples weighed between 5 to 38 g,
and had an average size of about 3 × 5 cm. To capture
noble gases potentially encapsulated in the internal void
space of these samples, we crushed the samples under
vacuum using a closed system crushing device specifi-
cally built in our laboratory to process large tektite sam-
ples [7]. The sample chamber of the crushing device is a
stainless steel cylinder with a diameter of 10 cm and a
height of 10 cm. An airtight piston is inserted into the
chamber by turning a handle that slowly crushes the
sample. This device makes it possible to reduce a large
sample into fine scale fragments (Figure 1), extract no-
ble gases and analyze them within a closed system.
The crusher directly connects to a purification system
and a VG5400 noble gas mass spectrometer. The purifi-
cation system consists of two Ti-Zr getters held at 700˚C,
and a cryogenic cold trap which separates He and Ar
fractions. Further details concerning the purification sys-
tem and the noble gas measurements can be found in
Matsuda et al. [8,9]. Elemental concentrations and iso-
topic ratios for He and Ar were corrected to procedural
blanks analyzed before each sample measurement. The
measured concentrations of procedural cold blanks for
4He (and 3He) were effectively zero, and those for 36Ar
were 0.6 to 4.2 × 10–11 cm3 STP. The noble gas concen-
trations of the samples were normalized to the original
sample weights. The uncertainty for the noble gas con-
centration was about 10%. Helium isotopic ratios were
calibrated using the HESJ internal He Standard [10] and
Ar isotopic ratios were calibrated using a pipetted air
(atmospheric v a lu es) .
2.2. Results
The results for sample He and Ar concentrations and
isotopic ratios are given in Table 1.
We plotted the 4He and 40Ar concentrations in Figure
2 along with theoretical and atmospheric envelopes for
40Ar and 4He values. The theoretical 40Ar-4He space was
constructed according to assumptions and calculations
described below. The 4He isotope is produced by radio-
genic decay of 238U, 235U and 232Th. The 40Ar isotope
forms from 40K (electron capture). The amounts of ra-
diogenic 4He and 40Ar ([4He]rad and [40Ar]rad) are written
as follows:
4238 235232
rad
He8U7U6Th t
 




, (1)
40 40
rad
Ar K
et


, (2)
Table 1. Concentrations and isotopic ratios of He and Ar in fossils.
Sample [4He] 3He/4He [36Ar] 38Ar/36Ar 40Ar/36Ar 40Ar/36Arcorrected*Note
×10–11cm3STP/g ×10–6 ×10–11 cm3 STP/g
Gastropod
(9.68618 g) 35
<6.1
110
0.1882
±0.0052 299.0
±6.0 298.4
±17.4
Turbo sp.
Albian, Fredericksburg F m.,
Early Cretaceous period
Ammonite
(32.73537 g) 870
0.170
±0.049 1000
0.1836
±0.0019 280.6
±1.7 294.4
±6.2
Perisphinctes sp.
Tulear, Madagascar,
Jurassic period
Ammonite
(19.39876 g) 540000
0.361
±0.106 420
0.1829
±0.0018 315.9
±2.0 331.9
±6.0
Goniatite sp.
Middle Devonian-
Permian period,
Trilobita
(5.49984 g) 4500
0.212
±0.058 48
0.1922
±0.0054 380.2
±7.6 367.0
±18.6
Flexicalymene retrosa
Cincinnatian, Arnheim Fm.,
Late Ordovician period
Trilobita
(18.09444 g) 220
2.7
±1.8 7.6
0.1901
±0.0052 368.9
±7.2 362.3
±17.9
Homotelus bromidensis ESKER
Mohawkian, Bromide Fm.,
Middle Ordovician period
Trilobita
(9.10144 g) 9500 0.158
±0.029 150
0.1879
±0.0052 320.4
±6.3 320.7
±17.5
Asaphiscus wheeleri (MEEK)
Wheeler Shale Fm.,
Middle Cambr ian period
Air 1.399 0.188 295.5
*
We corrected for the mass fractionation effect by calculating a 40Ar/36Arcorrected ratio when the 38Ar/36Ar ratio is 0. 188 (see te xt).
Copyright © 2012 SciRes. IJG
Y. ARAKAWA, J. MATSUDA 893
Figure 1. A trilobite sample before and after the crushing.
The sample was an Asaphitscus wheeleri specimen from the
Middle Cambrian period.
Figure 2. The measured 4He and 40Ar contents of crushed
fossil samples shown in 4He-40Ar space. The line labeled
“present day atmosphere” shows present day atmospheric
ratios for 4He and 40Ar. The upper line gives these ratios
calculated from K/U=10,000 and Th/U = 3.8 (see text).
where bracketed terms refer to absolute amounts of the
individual nuclide, t is time, and
,
and e
are decay constants for 238U, 235U, 232Th, and 40K (elec-
tron capture), respectively. The upper continental crust
and Archean bulk crust have a Th/U ratio of about 3.8
[11]. The amount of [4He]rad can thus be represented only
by [238U] as
42 238
rad
He85.1 1023U t

 
 
 
. (3)
The K/U ratio for the upper continental crust is 10,000,
and is assumed to be th e same for the Archean bulk cru st
[11]. As the mantle K/U ratio is estimated to be 12,700
[12], the elemental fractionation between K and U is
similar under different conditions. Thus we get the fol-
lowing equation by eliminating time t and using K/U =
10,000 from the two Equations (2) and (3),
440
rad rad
He35 Ar
 
 
. (4)
The new expression for 4He does not depend on time,
and is represented by the upper line plotted in Figure 2.
Assuming closed systems for radiogenic 4He and 40Ar for
the crust (and the whole Earth), the encapsulated 4He and
40Ar contents should lie on this line. Present day atmos-
pheric compositions would be located below this line
reflecting He loss from the atmosphere to space. The data
points of fossils in this study are situated between the
upper line and the present day atmospheric line below (Fig-
ure 2). We interpret the position of the empirical data
(between the upper and lower 4He-40Ar envelopes) as
evidence that the He and Ar contents of the fossil sam-
ples reflect ancient atmospheric compositions, assuming
some degree of 4He loss from the ancient atmosphere.
All samples gave reasonable He and Ar isotopic ratios
except for the Gastropod sample, which yielded only a
trivial amount of 3He. The 3He/4He ratio of this sample
could therefore be given only as an upper limit. Meas-
ured values for the stable Ar isotopes (38Ar/36Ar ratio)
varied relative to the expected value of 0.188 (Table 1).
We interpret variation relative to the 38Ar/36Ar constant
as a mass fractionation effect that occurred during the
trapping stage of the gas at the void space in fossil mate-
rial. We corrected for the mass fractionation effect by
calculating a 40Ar/36Arcorrected ratio based on the 38Ar/36Ar
ratio in Table 1. The obtained values are close to the
present day 40Ar/36Ar ratio of the atmosphere (295.5)
with some variation. Certain higher values are in terpreted
to reflect the addition of radiogenic 40Ar.
Similarly, the measured 3He/4He ratios are lower than
the present day atmospheric value due to the addition of
radiogenic 4He. The crushing apparatus and procedures
used here were intended to exclude radiogenic material
issuing from solid parts of the fossil but previou s studies
using the same technique have demonstrated that some
inclusion of a radiogenic component in inevitable (e.g.
[13]). The effect of the addition of radiogenic component
is larger for He than for Ar. At any rate, considering that
He isotopic ratios generally change in orders, the meas-
ured 3He/4He ratios were close to the present day atmos-
pheric value with some addition of a radiogenic compo-
nent. Ancient terrestrial samples often exhibit He and Ar
isotopic ratios similar to those of the present day atmos-
phere, which are generally interpreted to reflect the pre-
sent day atmospheric contamination. Although th is inter-
pretation fits the criterion of simplicity, it assumes that
ancient atmospheric He and Ar isotopic ratios were mar-
kedly different than present day values. Below, we will
assess this assumption through mathe matical modeling of
atmospheric He and Ar through time.
3. Mathematical Modeling of He and Ar
Isotopic Ratios in t he A tm os ph ere
3.1. Model Assumptions and Construction
The model used here is based on our previously pub-
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Y. ARAKAWA, J. MATSUDA
894
lished work concerning He and Ar dynamics in the man-
tle [3,4]. The model assumes that the Earth was initially
uniform in composition but was subsequently divided
into three separate reservoirs: the degassed or depleted
mantle (Mid-Oceanic Ridge Basalt; MORB source), the
less-depleted mantle (Oceanic Island Basalt; OIB source),
and the surface reservoir (crust and atmosphere). Indi-
vidual elements are exchanged through mass flow be-
tween reservoirs, and velocities decrease exponentially as
a function of time. We treated the decrement factor as a
variable parameter, and used the steady-state flow model
proposed by Porcelli and Wasserburg [14,15].
Mass flow from the depleted and less-depleted mantle
reservoirs to the surface reservoir (MDout and MLout, re-
spectively) decreases as a function of time t with the
decrement factors D
and L
, respectively, as fol-
lows:
 
Dout DoutD
0exp
M
tM t

, (5)
 
Lout LoutL
0exp
M
tM t
. (6)
Only solid elements were assumed to be recycled into
the mantle with an extraction factor, r. Noble gases were
not recycled in the mantle because of the subduction bar-
rier for noble gases [16,17]. Continental crust was as-
sumed to be produced at a constant rate throughout the
Earth’s history, and radioactive elements such as U, Th,
and K were transferred from the depleted reservoir to the
continental crust with an extraction fact or, x.
We estimated values for D
, L
, and x based on
representative 3He/4He and 40Ar/36Ar ratios from MORB
and OIB source reservoirs: 3He/4He = 1.1 10–5 and
40Ar/36Ar = 6 104 in MORB; and 3He/4He = 4.7 10–5
and 40Ar /36Ar = 3 103 in OIB [5,18-20]. The planetary
3He/4He ratio (1.41 10–4) was used as the initial
3He/4He ratio. The extraction factor x was estimated fro m
the present 40K concentration in bulk continental crust
(3.6 1016 atoms/g [21]) and in MORB (1.8 1014 at-
oms/g [12]). The restricted values for L
, D
, and x
are 1.32 to 1.34 10–9, 2.60 to 3.32 10–10, and 40 to
274, respectively [4]. Further details of the model pa-
rameters are given in Kamijo et al. [3] and Seta et al. [4].
3.2. Atmospheric Argon Isotopic Ratios
The total amounts of 36Ar and 40Ar in the atmosphere at
time t are described by the following equations:
 

36
36
Ar
36 AD Dout
Ar L Lout
Ar
,
dtdtCMt
CM t

 (7)
 

40
40
Ar
40 AD Dout
Ar L Lout
Ar
,
dtdtCMt
CM t

 (8)
where C is the concentration of species represented by
the associated superscript, and the subscripts A, D, and L
represent the atmosphere, depleted mantle, and less-de-
pleted mantle reservoirs, respectively.
From these equations we can calculate 36Ar and 40Ar
concentrations and the 40Ar/36Ar ratio of the atmosphere
through time. The results are shown in Figure 3. The
observed 40Ar/36Ar ratio for the present day atmosphere
is identical to the calculated model valu e due to its use as
a boundary condition input to th e model. Th e two lin es in
Figure 3 are derived from present-day concentrations of
40K in the continental crust and in the depleted mantle,
corresponding to the upper and lower limits for this pa-
rameter [4]. As seen in Figure 3(a), 36Ar abund ance rap-
idly increases during the first 1 billion years and is al-
most constant thereafter. The increase in 40Ar abundance
through time is more steady due to the addition of radio-
genic 40Ar from 40K. The 40Ar/36Ar ratio in the atmos-
phere increased very rapidly since the formation of the
Earth and has been almost constant for the last 1 billion
years (within 4% of the present day atmospheric value).
3.3. Atmospheric Helium Isotopic Ratio
Calculating the atmospheric He isotope ratio through
time requires compensation for He escape into space. To
incorporate these conditions, we added an escap e term in
the form of a simple first-order rate equation. The total
amounts of 3He and 4He in the atmosphere at time t are
described by the following equations:
 
 
3
33
He
3AD Dout
He L Lout1A
He
He ,
dtdtCMt
CMtkt

  (9)
 
 
4
44
He
4AD Dout
He L Lout2A
He
He ,
dtdtCMt
CMtkt

  (10)
where k1 and k2 are the escape coefficients for 3He and
4He, respectively. In addition to Jeans thermal escape,
non-thermal escape may occur due to the photoionization
of He and the interaction of He with the magnetic field
[22]. Ozima and Podosek [23] summarized the major
fluxes of the total escape for 3He and 4He as 10.1 atoms
cm–1·s–1 and 3.06 10–6 atoms cm–1·s–1, respectively.
The values for k1 and k2 were calculated from these
fluxes as 1.20 10–6 year–1 and 8.64 10–7 year–1, re-
spectively. Note that these values were independently
obtained from the present atmospheric helium budget. As
the nonthermal escape flux is considerably larger than
that of the thermal escape (especially for 4He) and does
not depend on the temperature of the Earth, we assumed
that k1 and k2 are constant through the time.
Using the above values, we obtain the results shown in
Figure 4. Both 3He and 4He abundance rapidly increased
during the first several million years and then decreased
Copyright © 2012 SciRes. IJG
Y. ARAKAWA, J. MATSUDA 895
0 1×10
9
2×10
9
3×10
9
4×10
9
5×10
9
4.00×10
39
3.50×10
39
3.00×10
39
2.50×10
39
2.00×10
39
1.50×10
39
1.00×10
39
5.00×10
38
0.00
0 1×10
9
2×10
9
3×10
9
4×10
9
5×10
9
1.20×10
42
1.00×10
41
8.00×10
41
6.00×10
41
4.00×10
41
2.00×10
41
0.00
0 1×10
9
2×10
9
3×10
9
4×10
9
5×10
9
Figure 3. Time versus (a) 36Ar abundance (atoms); (b) 40Ar
abundance (atoms); and (c) 40Ar/36Ar ratio in the atmos-
phere. The two lines correspond to the upper and lower
limits derived from the present day concentration of 40K in
the continental crust and in the depleted mantle (for details,
see [4]). The closed circle represents the observed present
day atmospheric values.
0 1×10
9 2×10
9 3×10
9 4×10
9 5×10
9
0 1×10
9 2×10
9 3×10
9 4×10
9 5×10
9
Figure 4. Time versus (a) 3He abundance (atoms) and (b)
4He abundance (atoms) in the atmosphere, with no helium
degassing from the continental crust. The two lines cor-
respond to the upper and lower limits derived from the
present day concentration of 40K in the continental crust
and in the depleted mantle (for details, see [4]). The closed
circle represents the observed present day atmospheric
values.
slowly due to thermal and non-thermal escape. The es-
cape term describes a relatively efficient process for He,
and a less efficient process for the more refractory Ar
(Figures 3(a) and (b)). The calculated present day at-
mospheric 3He value is in good agreement with the ob-
served atmospheric 3He abundance (Figure 4(a)). The
calculated present day atmospheric 4He value however is
about an order of magnitude lower than the correspond-
ing observed 4He value (Figure 4(b)). Our k1/k2 ratio is
1.39, which is slightly higher than the simple mass
Copyright © 2012 SciRes. IJG
Y. ARAKAWA, J. MATSUDA
896
square root ratio ((4/3)1/2= 1.15). In the current model,
the only way to reconcile the calculated and observed
present day atmospheric 4He values, would be to lower k2
by an order of magnitude. Given that helium escape from
the atmosphere is not isotope–specific, both k1 and k2
must be treated the same and thus cannot be used to re-
solve discrepancies in observed and calculated 4He val-
ues.
In previous models (i.e. [3,4]), we assumed that noble
gases transported into the continental crust remained
there. However, helium is a relatively labile and tiny
element and may degas from the continental crust into
the atmosphere during weathering etc. We accounted for
these degassing mechanisms in the present model as fol-
lows:
 
 
3
33
3He
ADDout
He 3
L Lout1Acc
He
HeHe ,
dtdtCMt
CMtkt dtdt

  (11)
 
 
4
4
4He
AD Dout
He4 4
L Lout2Acc
He
HeHe,
dtdtCMt
CMtktdtdt

  (12)
where the subscript cc represents continental crust. We
assume that all the helium in the continental crust de-
gasses to the atmosphere.
Under these conditions, we ob tain the results shown in
Figure 5. Values for 3He (Figure 5(a)) calculated from
this crust degassing model are almost the same as those
of the previous model (shown in Figure 4(a)), indicating
that the amount of 3He in the continental crust is neglig i-
ble relative to 3He contents of the atmosphere. Values for
4He calculated in the crust-degassing model (Figure 5(b))
however are markedly different from the non-degassing
scenario shown in Figure 4(b). This suggests that the
radiogenic 4He produ ced from U and Th in th e contin ent-
tal crust is relatively large. The calculated value for pre-
sent day atmospheric 4He (Figure 5(b)) is about an order
of magnitude higher than that of the non-degassing sce-
nario shown in Figure 4(b), and is close to the observed
present day atmospheric value. The corresponding
3He/4He ratio calculated in the crust-degassing model is
also consistent with observed atmospheric values (Fig-
ure 5(c)). This ratio decreases rapidly during the first
several million years of Earth history, then changes
slowly, and shows little change over the last 0.1 billion
years. The 3He/4He ratio at 0.1 Ga is only 10% - 12%
higher than th e present day value.
3.4. Comparison of Measured Fossil He and Ar
Values with Model He and Ar Trajectories
The Ar and He isotopic ratios obtain ed from fossil mate-
rial were fairly consistent with present day atmospheric
values although there is some addition of radiogenic
component. We plotted the isotopic ratios versus age
Time (year)
0 1×10
9 2×10
9 3×10
9 4×10
9 5×10
9
Time (year)
Time (year)
0 1×10
9 2×10
9 3×10
9 4×10
9 5×10
9
0 1×10
9 2×10
9 3×10
9 4×10
9 5×10
9
Figure 5. Time versus (a) 3He abundance (atoms); (b) 4He
abundance (atoms); and (c) 3He/4He ratio in the atmosphere,
assuming complete helium degassing from the continental
crust to the atmosphere. The two lines correspond to the
upper and lower limits derived from the present day con-
centration of 40K in the continental crust and in the de-
pleted mantle (for details, see [4]). The closed circle repre-
sents the observed present day atmospheric values.
Copyright © 2012 SciRes. IJG
Y. ARAKAWA, J. MATSUDA 897
along with isotopic trajectories from the mathematical
models in Figure 6. The age is given as “B. P.” (pr esent
day at time zero) and represented on a logarithmic scale.
For Ar, we plotted 40Ar/36Arcorrected ratios after making the
correction for the mass fractionation effect. Figure 6
clearly indicates that Ar and He isotopic ratios have been
nearly constant for the last 1 and 0.1 billion years, re-
spectively. The ages of fossils analyzed in this study fell
Figure 6. Comparison of (a) 40Ar/36Ar ratios (corrected for
the mass fractionation effect) and (b) 3He/4He ratios meas-
ured from fossil material with corresponding trajectories of
Ar and He isotopic evolution through time from mathe-
matical models. The two trajectories correspond to the up-
per and lower limits derived from the present day concen-
tration of 40K in the continental crust and in the depleted
mantle (see the captions of Figures 4 and 5). Here, age is
represented on a logarithmic scale in years before present
(B. P.).
between 108 and 109 years, a range consistent with the
range of their know n stratigraphic ages. Considering that
the isotopic ratios are represented on a linear scale, the
measured 40Ar/36Ar ratios in fossils were the values
slightly higher than those given by the mathematical
model. We attribute the variation relative to the model
trajectory to a radiogenic component within the fossil
material (Figure 6(a)). Similarly, a plot of fossil 3He/4He
ratios versus age relative to the predicted isotopic trajec-
tory (model) indicates the addition of radiogenic 4He. We
interpret the similarity of these ratios to the present day
atmosphere that the measured isotopic ratios are not sim-
ply the result of sample contamin ation by the p resent day
atmosphere. Our models suggest that 40Ar/36Ar and
3He/4He ratios in the ages of fossils also indicate the
similar values to the present day atmosphere. The rela-
tively high 40Ar/36Ar ratios and low 3He/4He ratios meas-
ured from fossil material are likely to reflect the addition
of radiogenic component from gas-mineral exchange
within the fossil matrix to the trapped ancient atmos-
phere.
4. Conclusion
We measured elemental abundances and isotopic ratios
of He and Ar in fossil material of varying age, using a
large, closed system crushing apparatus. The elemental
abundances of radiogenic component of He and Ar are
between the present day atmospheric value and those
estimated from the radiogenic decay product from the
continental crust. Th e measured 3H/4He ratios were lower
than the corresponding present day atmospheric value,
whereas measured 40Ar/36Ar ratios were higher than the
present day atmospheric value. These isotopic ratios in
fossils therefore indicate the addition of a radiogenic
component to He and Ar contents that are similar to pre-
sent day atmospheric values. These results may indicate
that the fossil’s void space has exchanged gas with the
present atmosphere. However, results from a math-
ematiccal model of atmospheric He and Ar isotopic ra-
tios through time offer an additional explanation. The
model results specifically suggest that He and Ar isotopic
ratios have been nearly constant for the last 0.1 and 1
billion years, respectiv ely. It is therefore possible that the
fossils retained ancient atmospheric signatures in their
void space, along with a radiogenic component incorpo-
rated from solids and released during the crushing pro-
cedure.
1.00×10-5
8.00×10-6
6.00×10-6
4.00×10-6
2.00×10-6
0.00
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