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1971
Mass fraction and the isotopic anomalies of xenon
and krypton in ordinary chondrites
Edward W. Hennecke
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Hennecke, Edward W., "Mass fraction and the isotopic anomalies of xenon and krypton in ordinary chondrites" (1971). Masters Theses.
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MASS FRACTIONATION AND THE ISOTOPIC ANOMALIES
OF XENON AND KRYPTON IN ORDINARY CHONDRITES
BY
EDWARD WILLIAM HENNECKE, 1945-
A THESIS
Presented to the Faculty of the Graduate School of the
UNIVERSITY OF MISSOURI-ROLLA
In Partial Fulfillment of the Requirements for the Degree
MASTER OF SCIENCE IN CHEMISTRY
1971
T2572
Approved by
51 pages
~
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1.94250
ii
ABSTRACT
The abundance and isotopic composition of all noble gases are
reported in the Wellman chondrite, and the abundance and isotopic
composition of xenon and krypton are reported in the gases released by
stepwise heating of the Tell and Scurry chondrites.
Major changes in
the isotopic composition of xenon result from the presence of radiogenic Xel29 and from isotopic mass fractionation.
The isotopic com-
position of trapped krypton in the different temperature fractions of the
Tell and Scurry chondrites also shows the effect of isotopic fractionation, and there is a covariance in the isotopic composition of xenon
with krypton in the manner expected from mass dependent fractionation.
The results from this study indicate that simple isotopic frac-
tionation is responsible for many isotopic anomalies which have
previously been attributed to nuclear reactions.
iii
ACKNOWLEDGMENTS
I am thankful to Dr. 0. K. Manuel for his guidance and support.
I am likewise sincerely appreciative to Dr. William H. Webb for his
patience and support.
My thanks go to Mr. B. Srinivasan for assist-
ance in helping me learn the operating procedure for the mass spectrometer.
I am grateful to Mr. John V. Albrecht who did the data
reduction.
I would also like to thank Mr. David E. Sinclair and
Mr. Victor J. Becker for assisting me in the laboratory and chart
reading.
iv
TABLE OF CONTENTS
Page
ABSTRACT
11
ACKNOWLEDGMENTS . . • . . . . . . • . . . . . . . . • . . • . . . . . . . . . . . . . . . . .
111
LIS T 0 F FIGURES . . • . • . • . . . . . . . • . . . . . . . . . . . . . . • . . • • . . . . . . . • .
v
LIST OF TABLES . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Vl
I.
INTRODUCTION . • . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . .
1
II.
LITERATURE REVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
III.
EXPERIMENTAL PROCEDURES
. . .. . . . .. . . . . . . . . . . . .
~
IV.
RESULTS AND DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . .
15
A.
NOBLE GASES IN WELLMAN
. . •. . . . . . . . .. . . . . . . .
lS
B.
KRYPTON AND XENON IN TELL AND
SCURRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
V.
CONCLUSIONS
. . . . . . . . . . . . . . . . . .•. . . . . . . . . .. . . . . . . .
22
BIBLIOGRAPHY........... . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . 40
VITA
. . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
v
LIST OF FIGURES
Page
1.
2.
3.
4.
5.
6.
7.
Abundance pattern of noble gases in the Wellman Chondrite
relative to the cosmic abundances of these gases
..... .
24
Abundance pattern of krypton isotopes in the Wellman
chondrite and in average carbonaceous chondrites relative to solar krypton in lunar breccia # 10021 . . . . . . . . . . . .
26
Abundance pattern of xenon isotopes in the Wellman chondrite and in average carbonaceous chondrites relative to
solar xenon released at 800°C from lunar fines # 10084
28
Abundance pattern of krypton isotopes in the different temperature fractions of the Tell chondrite relative to solar
krypton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
Abundance pattern of xenon isotopes in the different temperature fractions of the Tell chondrite relative to solar
xenon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
Abundance pattern of krypton isotopes observed by stepwise
heating of the Scurry chondrite normalized to solar
krypton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
Abundance pattern of xenon isotopes observed by stepwise
heating of the Scurry chondrite norn1alized to solar
xenon • . . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
Vl
LIST OF TABLES
Page
I.
II.
III.
Concentration and isotopic composition of noble gases in
the Wellman chondr ite . . . . . . . . . . . . . . . . • . . . • . . . . . . . . . . . . . .
37
Concentration and isotopic composition of krypton and
xenon in the Tell chondrite . . . • . • • . . . . . • • • . . . . . . . . . . . . . . • .
38
Concentration and isotopic composition of krypton and
xenon in the Scurry chondrite . • . . . . . . . • . • . . . . . . . • • . . . . . . .
39
1
I.
INTRODUCTION
This investigation of the abundance and isotopic composition of
the noble gases, helium, neon, argon, krypton and xenon, in ordinary
chondrites was undertaken to obtain additional data with which to examine the different models proposed to explain isotopic anomalies of
these gases 1n nature.
It has been known for several years (Suess, 1949; Brown, 1949)
that the abundance of noble gases in the earth's atmosphere relative to
their cosmic abundance shows a preferential depletion of the lightweight noble gases.
Suess (1949) noted that the mass fractionation
pattern seen across the abundances of the noble gases might be accompanied by fractionation across the isotopes of the individual gases.
Early studies of noble gases in meteorites also showed a preferential depletion of the light-weight noble gases.
These studies reveal-
ed isotopic anomalies of all five noble gases due to nuclear reactions
and to radioactive decay.
For exan1ple, spallation reactions induced
by cosmic rays have produced isotopic anomalies of all five noble
gases.
Radiogenic He4 and Ar40 have been generated by the decay of
uranium, thorium and potassium, and excess Xel29 and Xel31-136
occur in meteorites due to the decay of now-extinct Il29 and Pu244,
respectively.
These processes arc well-documented, and it is gen-
erally accepted that natural radioactive decay or induced nuclear
reactions have altered the isotopic composition of all noble gases in
2
·meteorites.
However, in addition to the effects oi well-docun1ented
nuclear reactions, there are also isotopic anomalies of all five noble
gases in meteorites, and there is now a divergence of opinion on the
role of nuclear reactions and isotopic mass fractionation in generating
these anomalies.
Manuel (1967) noted a variation in the isotopic composition of
neon in the Fayetteville meteorite which could not be produced by
known spallation reactions.
He suggested that these variations were
the result of isotopic fractionation.
Pepin (1968) suggested that anum-
ber of different con1ponents of trapped neon exist in meteorites and
argued that there is no sign that diffusive fractionation is responsible
for any of the isotopic variations.
Anders et al. (1970) noted a vari-
ation in the isotopic composition of helium and neon in carbonaceous
chondrites and concluded that these isotopic variations could result
from nuclear reactions.
But Srinivasan and Manuel (1970) have shown
that the He3/He4, Ne20fNe22 and Ar36JAr38 ratios in these carbonaceous chondrites covary in the manner expected from isotopic fractionation.
Kuroda and Manuel (1970) suggested that an enrichment of
the heavy isotopes of xenon in carbonaceous and gas-rich meteorites is
due to isotopic fractionation, but Anders and Heymann (1969) attribute
this enrichment of heavy xenon isotopes to the fission decay of a volatile, superheavy element (Z = ll2 to ll9).
Pepin et al. (1970) reported
a new type oJ cosmogenic krypton in the lunar surface.
Manuel (1970)
suggested that this was not cosmogenic krypton, but an enrichment of
the light isotopes due to simple isotopic mass fractionation.
Thus the
controversy over the role of mass fractionation versus the role of
nuclear reactions and radioactive decay includes anomalies observed
in all five noble gases.
This study was initiated to obtain reliable data with which to examine the merits of these two views of isotopic anomalies of noble
gases in meteorites.
Evidence for the origin of the isotopic anomalies
of xenon and krypton was the primary concern of this investigation,
since recent analysis of noble gases in lunar material reveals unambiguous evidence for isotopic fractionation in the three light-weight
noble gases (Hohenberg et al., 1970}.
Furthermore, one of the earlier
proponents for the generation of isotopic anomalies of helium, neon and
argon by nuclear reactions {Black, 1969} has recently admitted that
additional data showing the covariance of the isotopies of these three
gases in meteorites place severe constraints on the irradiation model
{Black, 1971).
II.
LITERATURE REVIEW
Many clues of early geologic events have been recorded in the
stable noble gases, helium, neon, argon, krypton and xenon.
The
chemical inertness and volatile nature, even at relatively low temperatures, resulted in almost complete loss of these elements from
more condensable material when solid planetary matter formed in our
solar system.
Due to this early depletion of noble gases from more
condensable matter, many isotopic anomalies of these elements have
been generated in solid planetary n1aterial subsequent to this separation by induced nuclear reactions and by natural radioactive decay of
more abundant elements.
The early studies of terrestrial noble gases emphasized the role
of natural radioactive decay in producing noble gas isotopes.
The
U-He and Th-He dating methods were developed over 60 years ago by
Strutt (1908a, l908b, 1909), and Weizsacker (1937) suggested the K-Ar
dating method about 11 years prior to experimental verification {Suess,
1948) for the decay of K40 to Ar40.
Studies of noble gases in meteorites were originally confined to
age determinations of iron meteorites by the U -He method (Arrol et
al., 1942).
The ages estimated from these early studies were later
found to be in error due to the production of helium isotopes within
iron meteorites by nuclear reaction induced by cosmic rays (Paneth et
al., 1952). Gerling and Pavlova {1951) discovered radiogenic Ar40 in
5
stone meteorites and first estimated their K-Ar age.
Stonner and
Zahringer (1958) attempted to date iron meteorites by the K-Ar method,
but here, too, interference from cosmic-ray induced reactions caused
errors in the original K-Ar ages estimated for iron meteorites
(Anders, 1962).
Other investigations have shown that bombardment by cosmic
rays produces isotopic anomalies of neon (Reasbeck and Mayne, 1955),
krypton (Clarke and Thode, 1961) and xenon (Rowe, et al., 1965) by
spallation reactions, that the cosmic rays produce secondary neutrons
which are captured to produce excess He3, Ar36, Ar38, Kr80, Kr82,
Kr83, Xel28, Xel29, and Xel31 (Clarke and Thode, 1961; Fireman and
Schwarzer, 1957; Goel, 1962; Marti et al., 1966; Alexander et al., 1968)
and that when the meteorites formed they contained two now-extinct
nuclides (Reynolds, 1960a; Kuroda, 1960) which decayed to produce
excess Xel29 from 1129 (tl/2 = 17 x 106 yrs.) and excess Xel31-136 from
Pu24 4 (tl/2 = 82 x 106 yrs. ).
Although the generation of isotopic anomalies of all the stable
noble gases by nuclear processes is well docurnented, there is also
evidence for the production of isotopic anomalies of noble gases by
simple isotopic mass fractionation at the t1me of the separation of
these from more condensable elements.
Prior to the detection of iso-
topes with a mass spectrometer, Aston (1913) diffused neon through
clay pipes and noted that the existence of neon atoms of different mass
6
could account for changes he observed in the density of neon.
Suess
(1949) and Brown (1949) independently noted that a comparison of the
earth's inventory of noble gases with the solar abundances of these elements showed preferential loss from terrestrial material of the gases
lighter than krypton.
Subsequent work on noble gases in terrestrial
sediments (Canalas et al., 1968) found high concentrations of xenon
which, when added to the atmospheric inventory of noble gases, show
that the earth's fractionation of noble gases extends to the heaviest
noble gas, xenon.
The trapped noble gases 1n stone meteorites display a similar
fractionation pattern (Reynolds, 1960b; Manuel and Rowe, 1964), with
the xenon preferentially retained over the neon by a factor ""'104 -105.
Signer and Suess (1963) discussed the role of mass fractionation
in generating isotopic anomalies of neon and argon in planetary material.
They noted a covariance of Ar36 I Ar38, Ne20 /Ne22 and
Ne20j Ar36 ratios in atmospheric and meteoritic gases.
Manuel (1967)
reported variations in the trapped Ne20fNe22 and Ne2lfNe22 ratios in
the different temperature fractions of the Fayetteville meteorite and
suggested that these variations result from isotopic mass fractionation.
However Pepin and coworkers (Pepin, 1967; Pepin, 1968; Black, 1969;
Black and Pepin, 1969) reported that there is no sign that diffusive
fractionation is responsible for any of the isotopic variations of meteoritic neon.
Recent results from analyses on the isotopic composition
of trapped meteoritic helium, neon and argon at the University of
7
Minnesota, the University of Chicago and Rice University have been
attributed to nuclear reactions (Black and Pepin, 1969; Black, 1969;
Black, 1970; Anders et al., 1970; Mazor et al., 1970), but other authors (Kuroda and Manuel, 1970; Manuel, 1970; Srinivasan and Manuel,
1970) have maintained that the observed variations result from simple
isotopic fractionation.
For the heavy noble gases a similar divergence of opinion has
developed.
Srinivasan et al. (1969) first suggested that the spontane-
ous fission of superheavy elements may be responsible for the enrichment of heavy xenon isotopes in carbonaceous chondrites.
Anders and
Heymann (1969) have arrived at a similar conclusion by comparing the
excess heavy xenon isotopes with the content of volatile elements in
carbonaceous chondrites, and Eberhardt et al. (1970) have interpreted
the isotopic composition of solar-type xenon as evidence for large
amounts of fissiogenic xenon in carbonaceous chondrites.
However,
Kuroda and Manuel (1970) recently pointed out that the enrichment of
heavy xenon isotopes in carbonaceous and gas-rich meteorites is
paralleled by an enrichment of heavy neon isotopes.
These authors
concluded that the enrichment of the heavier isotopes of both gases result from isotopic mass fractionation.
Additional evidence for the pro-
duction of isotopic anomalies of meteoritic xenon and krypton by fractionation have been reported from noble gas analyses of the Leoville
(Manuel et al., 1970) and Potter (Manuel et al., 1971) chondrites.
8
III.
EXPERIMENTAL PROCEDURES
A detailed description of the mass spectrometer was given by
Reynolds (1956) and the general operational procedures used in this
laboratory were given by Canalas (1967).
The specific procedure for
sample, air spike and blank analyses used in this study will be outlined
below.
Identical procedures were carefully followed in sample, air
spike and blank analyses.
This reduced the possibility of error due to
variations in the procedure used for gas analysis.
The sample system consisted of a combined extraction and preliminary cleaning system and a secondary cleaning and gas extraction
system.
These two systems were separated by a metal bellows valve.
The secondary cleaning and gas extraction system was also connected
separately through metal bellows valves to vacuum pumps and to the
mass spectrometer.
The extraction and preliminary cleaning system consisted of a
titanium furnace, a copper-copper oxide furnace, a quartz extraction
bottle, air spikes, and a cold trap.
The titanium furnace and copper-
copper oxide furnace acted as getters to remove the more reactive
gases in the preliminary cleaning system.
The cold trap was employ-
ed to prevent any residual material with a relatively low vapor pressure from entering the secondary clean-up system.
The components of the secondary cleaning and separation system were a charcoal finger, a titanium furnace, a cold trap and an
9
ion gauge.
The charcoal finger was used as an adsorption surface for
separation of the noble gases prior to analysis in the mass s pectrometer.
The ion gauge was used to detect leaks and to check the pres-
sure of both cleaning systems before each analysis.
The titanium
furnace of the secondary system was used for a final clean-up prior to
gas analysis in the spectrometer.
The cold trap of the secondary
cleaning system was used to prevent any residual contaminats with low
vapor pressure from entering the mass spectrometer.
The samples were mounted on a slide wire assembly attached to
the molybdenum crucible within the water -jacketed, quartz extraction
bottle.
A 10 kilowatt Lepel radio-frequency induction heater was em-
ployed to heat the crucible, and an optical pyrometer was used to
estimate the temperature of the crucible.
Prior to sample analysis the titanium furnace and copper -copper
oxide furnace were degassed at elevated temperatures.
Then the
molybdenum crucible was degassed at 1800°C for two hours and then
cooled.
The valve separating the preliminary system from the secon-
dary system was then closed and the titanium and copper-copper oxide
furnaces were brought to equilibrium temperatures of 850°C and
ssooc, respectively.
After these steps had been completed, a magnet
was used to pull the pen holding the sample on the slide wire assembly.
This released the sample which fell into the previously degassed
crucible.
The crucible was heated to the desired temperature by the
10
radiofrequency generator and that temperature was maintained for 3 0
minutes.
Then the induction heater was turned off, and the titanium
and copper-copper oxide furnaces were cooled with compressed air
for 30 minutes.
After the furnaces were cooled, solid COz was placed on the cold
trap of the preliminary system.
The valve connecting the secondary
cleaning system to the vacuum pumps was closed, and liquid nitrogen
was placed on the charcoal finger.
After five minutes, the valve sepa-
rating the preliminary from the secondary system was opened and the
gases were pumped from the preliminary to the secondary cleaning
system for 15 minutes by the liquid nitrogen cooled charcoal.
The two
cleaning systems were then isolated and the secondary titanium furnace was heated to 850°C.
The liquid nitrogen was removed from the
charcoal and replaced with a heater (T == 200°C) to drive the gases off
the charcoal.
After 15 minutes of scrubbing on the hot titanium, this
furnace was cooled for 30 minutes with compressed air.
After the titanium furnace had been cooled, the charcoal finger
was cooled with liquid nitrogen,
The gases were allowed to equilibrate
on the charcoal for 30 minutes at the liquid nitrogen temperature.
At
this time solid COz was placed on the cold trap of the secondary cleaning system.
After ten minutes the valve leading to the mass spectro-
meter was opened for two minutes to allow helium and neon to enter
the mass spectrometer.
While helium and neon were analyzed, solid
11
C02 was placed on the charcoal finger to release the argon.
Due to the
large amount of radiogenic Ar40, the argon was analyzed in two fractions.
The first fraction, consisting of about one-tenth of the total
argon, was used to measure the ratios of the argon isotopes and the
second fraction was used to determine the total amount of argon released from the sample.
While the second fraction of argon was being
analyzed the remaining argon was pumped from the secondary cleanup system to prevent interference with subsequent krypton analysis.
Then a bath, maintained at a constant temperature by the equilibrium
between solid and liquid mercury, was placed on the charcoal finger to
release the krypton.
After 30 minutes the valve to the mass spectro-
meter was opened for three minutes allowing the krypton to enter the
spectrometer for analysis.
After the valve was closed, a heater was
placed on the charcoal finger to release the xenon.
After analysis of
the krypton was completed the valve was again opened for three minutes and xenon was admitted to the mass spectrometer for analysis.
For all of these analyses the noble gases previously in the spectrometer were pumped from the tube prior to admitting the next noble
gas.
The procedure outlined above was employed for the Wellman
chondrite, for its hot blanks and for those air spikes which were run in
conjunction with this sample.
The only exception to this procedure was
for the air spikes, where the molybdenum crucible was not heated.
12
In the case of Scurry and Tell meteorites the above procedure
was slightly altered.
The operational procedure in the preliminary
clean-up system was not changed.
Since only krypton and xenon were
to be analyzed in these two samples, these gases were pumped from
the preliminary system onto charcoal at the sublimination temperature
of solid COz.
Then the secondary system was isolated from the pre-
liminary system and the He, Ne and Ar were pumped from the secondary system prior to the final scrubbing on hot titanium.
After the
valve to the pumps was closed, the titanium furnace was heated to
850°C, a heater was placed on the charcoal driving off krypton and
xenon, and the gases were scrubbed a second time for 15 minutes.
The furnace was cooled for 30 minutes with compressed air and then
the mercury bath (-39°C) was placed on the charcoal separating krypton from xenon.
The krypton and xenon were then analyzed separately
in the manner described earlier.
The mass spectrometer was calibrated before and after each
sample by analyzing small volumes of air (z 0.1 cc STP).
The air
spikes were used to determine the isotopic n1ass discrimination and
the sensitivity of the mass spectrometer for each noble gas.
The
mass discrimination across the isotopes of each gas was calculated
by comparing the observed peak ratios with the isotopic compos it ion
of atmospheric neon (Eberhardt et al., 1965), argon (Nier, l950a),
krypton (Nief, 1960) and xenon (Nier, 1950b).
The sensitivity of the
13
spectrometer was obtained by comparing the peak height (ie., the
spectrometer signal) with the volume of the air spike and the concentrations of noble gases in the atmosphere {Verniani, 1966).
Blank analyses were run before and after each sample.
The
spectrometer signal from the blank was always less than O.lo/o of the
sample signal except at mass 78.
this study.
No values for Kr 78 are reported in
The errors reported in the isotopic ratios are one standard
deviation (cr) from the least squares line through the observed ratios as
a function of residence time in the mass spectrometer.
From varia-
tions in the mass discrimination observed in the air spikes, it is estimated that the correction for mass discrimination introduced a maximum error of about ± 0. 5o/o, however, the concentration reported for
gas has a n1uch larger error of about ± ZOo/o, due to variations in the
signal per unit volume of a given gas,
Three meteorites were used for this investigation.
The gases
were extracted fron1 the Wellman chondrite by a single melt experiment at 1800°C.
Stepwise heating experiments were perforn1ed on the
Scurry and Tell chondrites.
The extraction temperatures were 600°C,
900°C and at 1800°C, successively.
Relatively large meteorite samples, weighing about 10 grams
each, were used in this study.
These were purchased frorn the An1cr-
ican Meteorite Laboratory (AML).
The nan1c and type of each mete-
orite, the sa rnple weight and the AML catalogue number of the
14
specimen from which the samples were taken are as follows:
The
w-ellman, Texas olivine bronzite chondrite, 10.888 grams from specimen# Hl2. 56; the Tell, Texas olivine hyperthene chondrite, 8. 474
grams, from specimen# H24. 67 and the Scurry, Texas olivine bronzite chondrite, 7. 909 grams from specimen # H65. 25.
15
IV.
A.
RESULTS AND DISCUSSION
NOBLE GASES IN WELLMAN
The abundances and isotopic composition of the noble gases in
Wellman are shown in Table I together with the isotopic composition of
trapped noble gases in carbonaceous chondrites and the solar-type
noble gases observed in gas-rich meteorites and in the moon.
The iso-
topic composition of trapped He, Ne and Ar in carbonaceous chondrites
are the range of values reported in a comprehensive study (Anders et
al., 1970; Mazor et al., 1970) and the trapped Kr and Xe are the average values (Eugster et al., 1967; Marti, 1967) observed in carbonaceous chondrites.
The isotopic composition of solar-type He, Ne and
Ar are from the Fayetteville gas-rich meteorite (Manuel, 1967).
The
Kr is from lunar breccia# 10021 (Funkhouser et al., 1971), and the
solar Xe is that released from lunar fines at 800°C (Marti et al.,
1970).
The solar abundance of each rare gas is from the abundance
tables of Suess and Urey (1956).
The isotopic composition of xenon and krypton in Wellman is intermediate between the isotopic composition of these gases in carbonaceous chondrites and in the moon.
Except for the obvious excess of
xel29. we interpret these two gases to represent the isotopic composition trapped in Wellman.
The isotopic composition of the trapped com-
ponent of the three light-weight noble gases in Wellman is masked by a
cosmogenic component.
For helium and argon it is not possible to
16
unambiguously separate the trapped and the cosmogenic components.
However, these two components of neon can be separated by the method
of Manuel (1967).
The concentration of trapped neon and its isotopic
composition as calculated by this method are shown in Table I.
Since
Manuel (1970) and Srinivasan and Manuel (1970) have shown that the
He3 /He4, Ne20 /Ne22 and Ar36 / Ar38 ratios in carbonaceous chondrites
covary in the manner expected from a common mass dependent fractionation process, we have shown in Table I the isotopic composition of
trapped He and Ar obtained by assuming that the trapped He, Ne and Ar
in Wellman have undergone a common fractionation process.
To compare the abundance of trapped noble gases in Wellman
with the cosmic abundance of noble gases, we employ the equation of
Canalas et al. (1968).
where xm is any trapped noble gas isotope with mass number m.
By
using the abundances of trapped gases in Wellman from Table I and the
cosmic abundances from Suess and Urey (1956), the fractionation pattern is obtained for trapped noble gases in Wellman as shown in Fig. 1.
Although the fractionation across the isotopes of a given noble gas cannot be quantitatively related to the separation of one gas from another
since the latter may not depend on atomic mass alone (Signer and
Suess, 1963), we note that the abundances of noble gases fit a smooth
17
fractionation pattern and that this correlates with the difference between the isotopic composition of solar gases and the isotopic composition calculated for the trapped He, Ne and Ar in Wellman.
The fractionation pattern across the isotopes of trapped Kr and
Xe in Wellman are shown in Fig. 2 and Fig. 3.
The isotopic composi-
tion of Kr and Xe from average carbonaceous chondrites (Eugster et
al., 1967; Marti, 1967) is shown for comparison.
For these two plots,
Eq. 1 was modified to show the fractionation across trapped Kr and Xe
by normalizing the mass spectrum to the heaviest isotope.
and
=
In these equations the krypton from lunar breccia # 10021 (Funkhouser
et al., 1967) and the xenon released at 800°C from lunar fines # 10084
(Marti et al., 1967) were used to represent solar gases.
It has been
noted earlier (Manuel et al., 1971) that the possibility of gas loss and
isotopic fractionation effects of the solar-type gases implanted on the
lunar surface may correlate with long exposure ages and with a depletion of the light-weight gases (He, Ne and Ar) relative to the heavier
gases (Kr and Xe).
These two analyses of lunar material were chosen
to minimize these effects.
18
From Fig. 2 and Fig. 3 it can be seen that the preferential depletion of the lighter weight noble gases from Wellman, as shown in Fig. 1,
has caused a selective depletion of the light isotopes of the two heavy
noble gases, krypton and xenon.
Futhermore, there is a correlation
between the fractionation observed across the isotopes of these two
noble gases:
For both Kr and Xe, the selective loss of the light iso-
topes is greater in average carbonaceous chondrites (A VCC) (Eugster
et al., 1967; Marti, 1967) than in Wellman.
The results of this study
on the isotopic composition of trapped xenon and krypton in Wellman
therefore show evidence of a common fractionation process as has
been reported earlier from a stepwise heating experiment on the
Leoville carbonaceous chondrite (Manuel et al., 1970).
The fine structure features of the fractionation curve shown in
Fig. 3 are intriguing.
Kuroda (1971) has considered this problem in de-
tail and concluded that nuclear reactions which have occurred in the
sun over geologic time may be responsible for some of these effects.
Relative to solar xenon the minimum at Xel26 and the excess at Xel24
over that expected from fractionation could arise from a deficiency of
Xel24 and an excess of Xel26 in the xenon currently implanted on the
lunar surface from the solar wind.
It is not unlikely that the abun-
dance of these two xenon isotopes have been altered by nuclear reactions in the sun.
Xel24 has the largest thermal neutron-capture cross
section of any stable xenon isotope and the production of Xel26 from
19
from rlZ7 via the (y, n) reaction is ::::: 0. 5 barns for high energy photons
(Nascimento et al., 1961).
B.
KRYPTON AND XENON IN TELL AND SCURRY
The abundance and isotopic composition of xenon and krypton
from the stepwise heating of Tell and Scurry are shown in Table II and
Table Ill, respectively.
In Tell the atomic weight of both xenon and krypton is lightest in
the gases released at the highest extraction temperature.
Although
there is evidence for smq.ll excesses of Kr80, Xel24 and Xel26 from
spallation in the melt fraction of Tell, isotopic fractionation plus the
in situ decay of rl29 seem to be the two dominant effects in generating
variations in the isotopic composition of Kr and Xe.
This is shown in
Fig. 4 and Fig. 5 where the Kr and Xe isotope ratios from the stepwise heating of Tell are shown in the manner employed for Wellman.
The isotopic composition of Kr and Xe released by stepwise heating of Scurry shows the same general trend as the gases in Tell.
This
is shown in Fig. 6 and Fig. 7 where again we observe a covariance in
the fractionation across the isotopes of Kr and Xe.
In both Tell and
Scurry there is a small excess of Kr80 which we attribute to spallation reactions (Marti et al., 1966) induced by cosmic rays.
The less
pronounced minimum at Xel26 in Tell and Scurry (Fig. 5 and Fig. 7)
than in Wellman and AVCC (Fig. 3) is also suggestive of a small
20
component of spallation-produced Xel26 (Marti et al., 1966) in the
xenon of Tell and Scurry.
Since the xenon and krypton released in different temperature
fractions of the Scurry and Tell chondrites show variations due to mass
fractionation, it seems likely that mass fractionation has also played a
role in generating some of the isotopic variations of noble gases reported in the different temperature fractions of carbonaceous and gasrich chondrites (Manuel, 1967; Black and Pepin, 1967; Black, 1969;
Manuel et al., 1971; Rowe, 1968).
Evidence for this process is com-
pelling in view of (a) the covariance of the isotopic composition of xenon
with krypton in the ordinary chondrites studied here and in carbonaceous chondrites (Manuel et al., 1970), (b) the covariance of trapped
xenon with trapped neon in the different temperature fractions of carbonaceous and gas-rich meteorites (Kuroda and Manuel, 1970), {c) the
covariance of He3 /He4, Ne20 /Ne22 and A1·36 I Ar38 for the total trapped
gas in carbonaceous chondrites (Srinivasan and Manuel, 1970) and
(d) the general agreement of the isotopic fractionation pattern observed
across each noble gas with the fractionation pattern observed across
the abundances of all the trapped noble gases.
Thus, there is cogent evidence of a mass dependent fractionating
process which altered both the abundance pattern of noble gases and
their isotopic composition in plan(•tary n1aterial.
It should be empha-
sized that the isotopic fractionation of noble gases has been known for
21
years.
In 1913 Aston (1913) obtained evidence for isotopes by altering
the density of neon by diffusion through clay pipes.
It appears that
solid planetary material has behaved as the clay stems in selectively
retaining the heavier isotopes of the noble gases.
22
V.
1)
CONCLUSIONS
The abundance pattern shown in Fig. 1 for the Wellman chon-
drite is cogent evidence that mass fractionation has altered the abundances of all trapped noble gases (helium, neon, argon, krypton and
xenon) in this meteorite.
2) A comparison of the isotopic composition of each trapped
noble gas in the Wellman chondrite with the isotopic composition of
solar noble gases (see Table I) shows that mass fractionation has also
altered the isotopic composition of each noble gas.
3) The excess Xel29 in all three meteorites is due to the in situ
decay of the now-extinct radioactive Il29.
4) There is a variation in the degree of fractionation across the
isotopes of krypton and xenon in the different temperature fractions of
the Tell and Scurry chondrites.
The isotopic compositions of these
two gases covary due to a common fractionating process which has
altered both xenon and krypton.
5) In addition to the effects of mass fractionation the xenon in
chondrites shows a selective depletion of Xel26 relative to solar xenon.
It is suggested that this is due to excess Xel26 which has been produced
in the sun via the ('Y, n) reaction on Il2 7.
2..3
Figure l
Abundance pattern of noble gases in the Wellman chondrite relative to
the cosmic abundances of these gases.
0
-2
E
u..
(!)
0
-4
...J
L
Ytf:LLMAN
Q
COSMIC
-6
He
Ne
20
Xe
Kr
Ar
40
60
MASS
80
100
120
NUMBER
Figure 1
N
~
25
Figure 2
Abundance pattern of krypton isotopes in the Wellman chondrite and in
average carbonaceous chondrites relative to solar krypton in lunar breccia # 10021.
1.04
1.00
E~
0.96
lL.
0.92
I
0.88r
/
u
80
82
0
SOLAR KRYPTON
6
0
WELLMAN
AVCC KRYPTON
83
MASS
84
86
NUMBER
Figure 2
N
0'
27
Figure 3
Abundance pattern of xenon isotopes in the Wellman chondrite and in
average carbonaceous chondrites relative to solar xenon released at
800°C from lunar fines # 10084.
1.20
I. 10
1.00
E~
u..
0.90
0
SOLAR
XENON
6_ WELLMAN
0.80
D
124
126
AVCC
XENON
128 129 130 131 132
MASS
134
136
NUMBER
Figure 3
N
00
29
Figure 4
Abundance pattern of krypton isotopes in the different temperature fractions of the Tell chondrite relative to solar krypton.
1.04
1.00
0.96
E~
lL
0.92r
0.88J-
~
/
A/'
80
82
MASS
6 soo•c
e soo· c
TELL
0
MELT
TELL
0
SOLAR
KRYPTON
83
84
TELL
86
NUMBER
Figure 4
1./.l
0
31
Figure 5
Abundance pattern of xenon isotopes in the different temperature fractions of the Tell chondritc relative to solar xenon.
I. 10
1.00
0.90
E~
0.80
"'0.70
6_
600 • C TELL
•
900• C TELL
D
0
MELT
TELL
SOLAR XENON
0.6
124
126
128 129 130 131 132
MASS
134
136
NUMBER
Figure 5
'""'
N
33
Figure 6
Abundance pattern of krypton isotopes observed by stepwise heating of
the Scurry chondrite normalized to solar krypton.
1.04
1.00
e~
LL
I
•
/-
/~
0.94f-
0.88~
80
82
MASS
83
6
•
soo• C
SCURRY
goo• C SCURRY
0
MELT SCURRY
0
SOLAR KRYPTON
84
86
NUMBER
Figure 6
VJ
.::.
35
Figure 7
Abundance pattern of xenon isotopes observed by stepwise heating of the
Scurry chondrite normalized to solar xenon.
1.10
1.00
0.90
E~ 0.80
~
I
o. 1or-
/
~~
6.
//
e
M'/
D
6oo•
goo•
MELT
c
c
SCURRY
SCURRY
SCURRY
Q SOLAR XENON
I
0.60
124
126
128 129 130 131 132
MASS
NUMBER
Figure 7
134
136
VJ
0'-
Table I
Concentration and Isotopic Composition of Noble Gases in the Wellman Chondrite
Wellman
Noble
Gases
Trapped Gas in Carbonaceous Chondrites
observed
trapped
He31He4
He4
o. 0253 ± 0. 0004
6. 04 x l0-7 cc STPigm
0, 88 X 10-4
5. 25 x l0-7 cc STPI gm
(1. 25-4. 20) x 10-4
Ne20 /Ne22
Ne21JNe22
Ne22
l. 4 15 ± 0. 003
± 0. 003
5. 39 X 10-8 cc STPI gm
7.3
0.025
4. 35 x 10-9 cc STP/ gm
7. 77-13.10
0.025-0.036
0.23
0.172-0. 200
o. 876
o. 450
Ar381 Ar36
Ar40 I Ar36
Ar36
± 0. 001
3143 .± 26
3. 96 x 10-9 cc STPigm
Kr80JKr84
Kr821Kr84
Kr831Kr84
Kr861Kr84
Kr84
0. 0423 ± 0. 002
0. 2048 ± o. 0007
0. 2045 ± o. 0006
0. 3058 ± o. 0007
l. 41 X 10-10 cc STP/ gm
< 0.0423
Xe124 I xel3 o
Xe126 I Xel3 0
xel28 I xel3 0
xe1291xel30
xel31;xe13°
xel32fxel30
xe134;xel30
xel36;xel30
xel30
0. 0296 ± 0. 0002
0. 0268 ± o. 0002
0. 508 ± 0. 003
8.18±0.05
4. 97 ± 0. 03
6. 09 ± 0. 03
2.36 ± 0.01
l. 95 ± 0. 01
1.17 x 10-ll cc STPI gm
0.0296
0.0268
0.508
(6.37)
4.97
6. 09
2.36
l. 95
1.17 x 10-ll cc STPI gm
< 3.1 x 10-4
3. 08
x 106 per 106 atoms Si
> 12. 6
> 0. 033
8. 36
3. 27 x l0-9 cc STP/ gm
0. 2048
0.2045
0.3058
1.41 x 10-lO cc STP/gm
Solar
Gas
x 105 per 106 atoms Si
< 0.177
l. 26 x 105 per 106 atoms Si
0.0392
0. 20 12
o. 20 16
0.3097
0.0427
0.2065
0.2055
0.297
2. 93 x 10 1 per 106 atoms Si
CJ. 0283
0.0253
0.506
(6. 35)
5.07
6. 21
2.37
l. 99
0. 0299
0.0288
0. 512
6.38
5.00
6. 05
2.23
l. 81
l. 62 x 10-1 per 106 atoms Si
VJ
-..j
Table II
Concentration and Isotopic Composition of Krypton and Xenon in the Tell Chondrite
Noble Gas
Kr801Kr84
600°
o. 0384
±
o. 0004
Extraction Temperature
900°
o. 0400 ±
0. 0009
Melt
o. 0452
±
o. 0006
Solar Gas
o. 0427
o. 202 ± o. 002
o. 200 ± o. 002
o. 204 ± o. 001
0.2065
Kr8 3 1Kr84
o. 1 99 ± o. 001
o. 199 ± o. 001
o. 205
0.2055
Kr861Kr84
0. 306 ±
o. 001
o. 306 ± o. 002
o. 307 ± o. 001
Kr84 x lQ-10 cc STPigm
0.80
0.35
0.33
Kr821Kr84
±
o. 001
0.297
xe124 I xel3 o
o. 0242
±
o. 0004
o. 0229
±
o. 0006
o. 0292
±
o. 0003
0.0299
Xe126 1Xel30
o. 0227
±
o. 0004
0. 0236 ±
o. 0005
o. 0275
±
o. 0004
0.0288
0.473 ±
o. 006
o. 509
±
o. 005
o. 512
6. 62 ±
o. 04
7. 68 ± 0. 07
6.38
5.21±0.04
5. 03 ± 0. 03
5.00
o. 03
6.16 ± 0. 03
6.05
o. 02
2. 00 ± o. 02
2.23
xe128 1xel3 0
0.477±0.004
o. 06
Xe129 1Xel30
6. 50±
xe1311xel30
5.19±0.04
xel32;xel30
6. 57 ±
o. 05
6. 62 ±
xel34 I xel3 o
2. 53 ±
o. 01
2.57 ± 0.01
xel36 I xel3°
2. 16 ±
o. 01
2.19 ±
Xel30 x 10-12 cc STPigm
5.3
1.2
o. 02
2. 38 ±
l. 81
7.8
I.JV
00
Table III
Concentration and Isotopic Composition of Krypton and Xenon in the Scurry Chondrites
Noble Gas
600°
o. 0005
Kr801Kr84
0. 0396 ±
Kr82/Kr84
o. 20 l
±
Kr83 1Kr84
o. zoo
± 0. 002
Kr861Kr84
o. 306
±
Kr84 x 10-lO sec STP I gm
0.48
o. 001
o. 001
Extraction Temperature
900°
o. 0003
Melt
o. 0003
0,0427
0. 204 ±
o. 001
0.2065
0.198 ± 0. 001
o. 204
±
o. 001
0.2055
o. 001
o. 306
± 0. 001
0. 0397 ±
0.198 ±
0. 306 ±
o. 001
o. 79
o. 0420
Solar Gas
±
0.297
Zd!
xel241xe130
0. 0256 ±
o. 0005
o. 0244
± 0. 0002
o. 0251 ±
xel26 1 xeBO
o. 0226
±
o. 0005
o. 0234
± 0. 0004
o. 0242
xel281xel30
o. 4 79
±
o. 003
o. 488
± 0. 003
0. 0002
o. 0299
± 0. 0003
0.0288
0. 493 ± 0. 004
0. 512
Xe129 1Xel30
6. 58 ± 0. 04
6. 69 ± 0. 03
6. 69 ± 0. 03
6.38
xe 131 1 xeB O
5.17 ± 0. 04
5.19±0.03
5. 11 ±
o. 02
5.00
Xel3 2 I xeB 0
6. 53 ±
o. 05
6. 59 ± 0. 05
6. 40 ±
o. 02
6.05
xe134 I Xel3 0
2. 51 ± 0. 01
2. 55 ± 0. 02
2. 46 ±
o. 01
2.23
xe136;xel30
2.14 ± 0.01
2. 14 ±
2.07 ± 0.01
1. 81
Xel30 x 10-12 cc STP/ gm
2.4
3.6
o. 01
7.6
VJ
"-'!
40
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45
VITA
Edward William Hennecke was born on September 28, 1945 in
Cape Girardeau, Missouri.
He received his primary and secondary
education in Jackson, Missouri.
He attended the University of Mis-
souri at Rolla, Missouri where he obtained his B.S. in chemistry in
January 1968.
He has been enrolled in the Graduate School of the University of
Missouri-Rolla since September 1968.
He had held teaching assistant-
ships continuously from September 1968 to January 1971 and received a
research assistantship from the National Science Foundation, NSF-GA16618 in January 1971.