Geochlmrco
0016-7037/89/$3.00
er Cosmochrmrco
Acro Vol. 53, pp. 3025-3033
Pergamon press pk.
copyright
0 1989
mnted1”U.S.A.
+ .W
Anomalous Ne enrichment in obsidians and Darwin glass:
Diffusion of noble gases in silica-rich glasses
JUN-ICHI MATSUDA, KAYO MATSUBARA,HARUAKI YAJIMA, and KOSHI YAMAMOTO
Department of Earth Sciences, Faculty of Science, Kobe University, Nada. Kobe 657, Japan
(Received March 22, 1989: accepted in revised form August 14. 1989)
Abstract-We
have determined noble gas concentrations in seven obsidians and a Darwin glass by stepwise
heating. The pattern of noble gases relative to air showed that Ne was enriched in most of the obsidians
and Darwin glass. The release temperatures of Ne were 400-500°C. The occurrence of Ne excess in
Darwin glass and related impact glasses can be explained by diffusion into the glass from the atmosphere.
Diffusion coefficients and activation energies of noble gases in obsidians and a Darwin glass were determined. It is also conceivable that the anomalously high Ne/Ar ratios observed in some submarine glasses
are from sea water or atmosphere rather than derived from the mantle.
INTRODUCTION
the mantle. There have been stimulative discussions about
the noble gas patterns in these samples for the study of the
evolutionary history of the Earth (FISHER, 1970, 1974, 1985;
DYMOND and HOGAN, 1973, 1978; CRAIG and LUPTON,
1976; STAUDACHERand ALLZGRE,1982; OZIMA and ZASHU,
1983; ALL~GREet al., 1983, 1986/87; POREDAand RADICATI
DI BROZOLO, 1984; SARDA et al., 1985, 1988). Obsidians,
also natural glasses, are not quenched in sea water but solidified in air. Noble gas data from obsidians may give us some
information from a different point of view on the study of
noble gases in submarine glasses.
In this paper we report the determinations of major element
and noble gas abundances from seven obsidians. In addition,
we measured noble gases in Darwin glass, which is also silicarich and considered as being produced by impact melting
during shock events. The measurement of Darwin glass is
useful for studying the origin of the Ne excess in obsidians,
that is, whether Ne was derived from the mantle or acquired
on the Earth’s surface when the silica liquid contacted with
the terrestrial atmosphere. The noble gas abundances in these
silica-rich glasses are compared to the chemical composition
and are discussed in relation to the behavior of noble gases
in these materials. A preliminary report on the noble gases
in Darwin glass was given in MATSUDAand YAJIMA(1989).
THE NOBLE GASES are chemically inert and are usually treated
as an ideal monatomic gas. The behavior of noble gases is
dependent on physical conditions such as temperature and
pressure rather than chemical conditions. Studies of noble
gases in the Earth have provided important information on
many geophysical problems like the degassing process in the
Earth-atmosphere system. MATSUDAand NAGAO (1986) reported that heavy noble gases were enriched and positively
correlated with Si02 content in the deep-sea sediments. The
chemical leaching experiment indicated that the amorphous
silica of siliceous microfossils was the main carrier of noble
gases in the deep-sea sediments. MATSUBARAet al. (1988)
showed that amorphous silica of geothermal origin also contained large amounts of noble gases. The heavy noble gases,
especially Xe, are tightly trapped inside the silica, which was
also confirmed in the siliceous microfossils in deepsea sediments (MATSUDA and MATSUBARA, 1989). These studies
indicated that amorphous silica is a good noble gas absorber
on the Earth and led us to the study of noble gas in the silicarich natural glasses. LUX ( 1987) reported that the solubility
of noble gases in silica liquids increased with increasing SiOr
content. The Henry’s constants changed by factors of 4(Ar)8(Xe) from andesite (SiOz 6 1.2%) to ugandite (SiOz 40.5%).
Thus. silica could be a main factor controlling the noble gas
abundances in silicate liquids.
We chose obsidians as natural silica-rich glass for our purpose. When we measured one of our obsidian samples, we
found that Ne was highly enriched compared to the other
noble gases. It is generally believed that the relative abundances of noble gases show a simple variation depending on
the mass number of each noble gas during the physical and
chemical reactions such as adsorption, absorption, diffusion.
and solution, etc. (OZIMA and PODOSEK, 1983). Therefore,
the peculiar behavior of only Ne interested us very much.
LORD RAYLEIGH( 1939) reported a Ne excess in pumice from
the Lipari Islands, of which isotopic compositions showed
that Ne was of atmospheric origin (BOCHSLERand MAZOR,
1975). Similar Ne enrichments have been reported from submarine glasses that are believed to contain noble gases from
SAMPLES
We used the obsidian samples obtained from Turkey, the
U.S., and Japan (Table I). Two samples NE1 and NE2 are
from Mt. Nemrut, and sample ME1 is from Mt. Meydan in
eastern Turkey. Both mountains are young Quarternary volcanoes. One of the authors (J. MATSUDA) collected these
samples when he went to Turkey for a geochemical research
program between Japan and Turkey. Preliminary results show
that the K-Ar ages of NE2 and ME 1 are less than 1 Ma Obsidian samples US1 15, US330a, and US1823 from the U.S.,
and 7473 104 from Japan were given through the courtesy of
Prof. Ui, Kobe University.
The Darwin glass was obtained from Mt. Darwin crater
in Tasmania, Australia. Darwin glass was embedded in a
heavily faulted synclinal series of lightly metamorphosed Si3025
J.-i. Matsuda et al.
3026
Table
1.
samples
Sanple
NWK
Obsidian
NE2
NE3
ME1
us115
US1623
us330a
7473104
Darwin glass
and their
sampling
sites.
Location
Ht. Nemrut, Eastern Anatolia,
Turkey
Ht. Nemrut, Eastern Anatolia,
Turkey
Ht. Meydan, Eastern Anatolia,
Turkey
West of North Sister,
Oregon.
U.S.A.
Glass Mountain,
Hedicine
Lake, California,
U.S.A.
Newberry caldera,
Oregon,
U.S.A.
0goe, Yamagata, Japan
Darwin crater,
Tasmania. Australia
1uroDevonian slates, argillites, and quartzites (FUDALI and
FORD, 1979). FORD ( 1972) and FUDALI and FORD ( 1979)
reported the impact crater as a circular depression, about 1
km in diameter and having walls about 125 m high, suggesting
a meteoritic impact origin of Darwin glass. Major and trace
element geochemistry (TAYLOR and SOLOMON, 1964) and
REE patterns ( KOEBERLet al., 1985) of Darwin glass indicated
that the parent material of the glass might be an argillaceous
sandstone, which is consistent with a terrestrial origin. The
age of Darwin glass by the K-Ar and fission track methods
was reported to be 0.73 Ma (GENTNER et al., 1973).
To get the diffusion coefficients of noble gases from our
stepwise heating experiments, we used the 60- 100 mesh fraction from size separations after crushing samples in an iron
bowl except for the sample NE3. The chipped samples and
60-100 mesh fraction were used to examine the effect of
crushing for Darwin glass.
MAJOR ELEMENTS
Major element com~sitions of all samples were analyzed
by X-ray fluorescence spectrometry, except for Na20 which
was determined by flame photometry. Total iron content was
reported as FezOr. HrO was also measured using the method
given by SUGISAKI(198 I). The results are listed in Table 2.
H20(-) was too low to be determined in many samples and
estimated to be lower than 0.2%. H,O(+) was detected only
for MEl. Impact glass has, in general, low water content
(lower than 0.06%) compared to obsidians which have a water
content of some few tenths of a percent (KOEBERL, 1988).
Our technique of determining water content was not sensitive
enough to recognize such differences. GILCHRISTet al. (1969)
reported that the water content of Darwin glass was 0.046%.
SiOz contents of obsidians are 71 to 76%. There seem to
be no special differences in major element compositions
among obsidian samples used in this study, except that the
obsidians from Turkey have a CaO content systematically
lower than that of the other samples.
TAYLOR and SOLOMON(1964) reported the major and
trace ~om~sitions of Darwin glass samples. The data in this
paper are very similar to theirs. Darwin glass is dominated
by SiOz up to 84.9%, much higher than those in obsidian
samples. MgO and TiOz are present in higher abundances
than those in obsidians. Nat0 and CaO in Darwin glass are
lower than those in the obsidians by a factor of about 10 or
more. K20 in Darwin glass is also low in comparison with
that in obsidians, which is rather constant (4.3-6.2’1’0).High
absolute MgO content, low CaO, and high K20/Na20 have
been observed in several impact glasses (TAYLOR and SOLOMON, 1964).
NOBLE GASES
Experimental and results
Noble gas abundances were determined by mass spectrometry using the stepwise heating technique. The mass
spectrometer installed in Kobe University is a single focusing
inst~ment of 15 cm radius with both incident and exit angles
of 55”. About 0.3-J g of sample was loaded into a sample
line and heated under a vacuum at about 150°C overnight
to remove atmospheric contamination,
and then dropped
into a molybdenum crucible in a tantalum tube surrounded
by a tantalum heater. The sample was kept for 20 min at
each tern~mtu~
step, and then cooied to room temperature.
The temperature of the crucible was determined by an optical
pyrometer, which has an uncertainty of about 100°C. The
gases were purified by exposure to two-stage Ti and Zr getters
at 800°C. After adsorbing the heavy noble gases (Ar. Kr, and
Xe) on activated charcoal at liquid nitrogen temperature, He
and Ne were measured. Argon was separated from Kr and
Xe by keeping the charcoal at -5OOC. Both Kr and Xe were
released by heating the charcoal at 100°C. The mass spectrometer sensitivity was calibrated by measuring pipetted
amounts of air. The reproducibility in the measurements of
elemental abundances is within about 10%. The hot blank
was measured before each sample measurement, and blank
corrections were applied to the data. If blank contribution
was larger than 50%, we listed the measured data as an upper
limit without blank correction. The results are listed in Tables
3 and 4.
The concentrations of noble gases were obtained only as
upper limits in many temperature steps except for the 800°C
fraction of Ne and Ar. Although silica contents in obsidians
are rather restricted and the chemical compositions are quite
uniform, the concentrations of noble gases in obsidians are
very variable. The 36Ar contents in obsidians have a range
of about two orders of magnitude, and “Ne varies by about
an order of magnitude. There seem to be no correlations of
36Ar and “Ne with the chemical composition such as SiO,
content given in Table 2. The extremely high concentration
of 36Ar in NE3 may be due to alteration of the sample (Dr.
UI, personal communication). though its water content was
not high enough to be detected. As to 4He and 13*Xe, the
correlations are not known because they are given only as
upper limits in many cases. The noble gases of Darwin glass
DG#3 (60-100 mesh fraction) are considerably lower than
those of DG#2 (chip), although Ne content of DG#2 agrees
well with that of DG#l (chip). We observed many vesicles
Tabie
2.
sampie
N%W
Si02
T10
Al a
T~&~Fe203
W”0
WJ
cao
Na 0
% li
pz*
H20I -)
H20(+)
Total
Major
element
NE2
data
NE3
for
obsidians
ME1
-
74.66
0.14
13.19
2.41
O.051
0.034
0.33
5.22
4.96
0.008
tr.
tr.
101.00
71.11
0.37
9.25
7.78
0.17
0.045
0.32
6.15
4.60
0.021
tr.
tr.
99.82
and Darwin glass.
USIIS
US1823
_I_--.
14.46
0.062
13.28
1.56
0.069
0.14
0.43
4.47
4.50
0.008
tr.
2.61
101.81
74 37
0.095
13.49
1.28
0.041
0.092
0.87
4.36
3.55
0.030
tr.
tr.
98.16
~-
US33Oa 7473104
-~
73.93
0.28
13.76
2.14
0.032
0.30
1.25
4.30
4.55
0.045
tr
tr
,OO.Sh
Darwin
Glass
72.55
0.23
13.96
2.59
0.060
0.11
0.88
5.05
4.22
0.029
tr.
tr.
75.94
0.089
13.34
1.08
0.097
O.I7
0 67
4.33
3.92
0.036
tr.
tr.
84.90
0.62
7.44
3.44
O.OIS
0.63
a.053
0.12
1.89
0.047
tr.
tr.
99.66
99 67
99.16
3027
Ne content of natural glasses
100,
sample
Weight
Temp.
ZDNe =Ar
4lle
Y
e4Kr 132X,
NE2
NE3
1.1757
0.7682
(OC)
600
CO.169
1600
CO.147
Total
~0.306
800
0.9139
0.3336
0.0251
1O.B
16.1
6.55
0.331
0.667
0.0967
800
1.65
1.15
<0.203
13.3
3.01
CO.154
25.3
0.471
0.155
0.0151
CO.00151
0.366
0.144
0.00659
(0.00132
0.300
CO.160
1400
to.154
CO.00540
0.368
0.632
0.00976
1600
CO.163
to.00075
0.0292
0.182
0.00976
CO.833
3.01
1.53
1.57
0.0473
to.421
0.610
0.0372
CO.124
~0.0290
CO.527
CO.0388
CO.0963
~0.207
CO.0290
CO.948
0.610
CO.160
CO.331
<0.0580
CO.469
0.253
0.401
0.219
CO.0246
CO.496
CO.0338
0.592
1.14
CO.0406
CO.965
0.253
0.993
1.36
CO.0656
to.450
(0.434
0.179
CO.0193
0.0413
CO.0370
to.101
<O.lll
0.0190
CO.0241
to.664
0.179
0.0482
CO.212
CO.0603
SO0
600
1600
800
1600
Total
1.03
CO.497
4.69
0.0422
CO.0216
1.10
4.69
Samples
are
60-100
mesh fraction
which
chipped
sample
was used.
by
size
0.457
0.0645
<0.0310
0.0422
CO.226
CO.0471
NF3
Sample
NO.
4.
Noble
Weight
(g)
gases
Temp.
in Darwin
4He
(“C)
240lk
36Ar
(x10-6cm3STP/g)
01
1600
2.61
29.7
0.805
DGXZ
(chip)
0.9106
600
1600
1.75
0.190
12.2
16.4
0.0297
0.851
1.94
26.6
DGx3
(60-100
mesh)
0.3749
for
‘32X,
~3.76
CO.377
(chip)
Total
600
1000
1200
1400
1600
Total
CO.641
CO.446
(0.362
co.375
CO.376
12.6
0.00653
CO.00534
~0.0136
<0.0176
C2.20
12.6
0.681
0.0552
0.126
0.0713
CO.0618
CO.0566
0.306
1400
1600
0.0490
1.61
1.66
\,
i
800
Temperature
(x10-10cm3STP/g)
0.1046
DGPI
64KT
Ar
,
\,
,/
\
\
-%4
glass.
2ONe
i
-
z
CO.0161
CO.125
except
5
less than 1 mm in diameter in the thin section under the
microscope. It is estimated that considerable amounts of noble
gases were present in such vesicles and were degassed when
we prepared 60-100 mesh fraction by crushing. The 36Ar
content of Darwin glass is in the same range as in obsidians,
but *‘Ne in the former is higher than those in the latter.
Figure 1 shows the release patterns of Ne, Ar, Kr, and Xe
of MEI. Almost all Ne was released by 800°C. Argon was
released at 800 to 1400°C and Kr mainly at 1200 to 1400°C.
Although the 800°C fraction of Xe is highest, a considerable
amount of Xe was released even at 1600°C. It is conceivable
that the 800°C fraction of Xe contains an adsorped component which could not be removed by baking the samples
at 150°C. Thus, lighter noble gases were released at lower
temperatures. This feature was also observed for NE3 (Table
3) where almost all He was also released below 800°C. In
Table
1200
-I
,,
,,
a
V2 60 -
0.00610
CO.0393
seperation
1000
’
1
80-
0.648
1200
600
I
0.421
2.42
Total
7473104
0.169
0.0452
1600
0.3265
0.0605
0.0174
Total
us330a
0.0152
0.00259
Total
0.3343
1.13
0.00986
CO.189
1600
US1623
1.65
0.0866
0.0603
CO.205
Total
0.3263
2.67
0.0320
0.0265
1200
1000
US115
2.67
CO.00322
(x10-10cm3STP/g)
1600
Total
HE1
(x10-6C,3STP/g)
I
/
Nme
(g)
I
Ne ‘;
<0.0102
0.0460
0.0494
t0.100
(0.0637
CO.0669
to.114
to.164
t0.0166
~0.0246
~0.0124
<0.0124
~0.0248
to.551
CO.0930
( “C 1
FIG. 1. Thermal release pattern of noble gases in the obsidian sample
MEI. Helium was not shown because all data were determined as
upper limits. All Ne was released at the lowest temperature of 800°C.
Argon was released at 800-14OO”C, and Kr at 1200-1400°C. Large
portions of Xe were released at higher temperatures except for 8OO”C,
at which temperature an adsorped component may have been included.
all samples except the chipped Darwin glass (Table 4) it was
observed that Ne was released below 800°C. Therefore, we
made the stepwise heating experiments of Ne below 800°C
for NE2, MEl, 7473 104, and Darwin glass samples.
For these experiments, we improved our sample line with
new attachments. Samples weighing about 0.5 g were loaded
into a sample line and dropped in a stainless steel crucible
which was heated by a conventional Kanthal heater. We did
not bake the sample at 150°C under a vacuum in these runs.
The temperature ofthe crucible was monitored with AlumelChrome1 thermocouples. The sample was heated up to the
setting temperature taking lo- 15 min and kept at that temperature for 30 min, and then cooled to the room temperature
after each step. The precision of the temperature setting was
+2”C. The purification and the measurement of noble gases
are the same as that we already stated. The results are listed
in Table 5. The Ne contents of 7473 104 and DG#3 were
lower than those in Tables 3 and 4, but the difference seems
to be within errors of 10% in each measurement. NE2 and
ME1 in Table 5 show higher Ne concentrations than those
in Table 3. The differences are larger than the reproducibility
of our measurement and are perhaps due to the fact that we
did not bake the sample prior to the measurements of Ne in
Table 5. Otherwise, although we used 60- 100 mesh fractions,
there may be inhomogeneity of Ne contents in these samples.
Figure 2 shows the release pattern of Ne below 800°C showing that Ne was released mainly at 400 to 500°C for all samples.
J.-i. Matsuda et al.
3028
Table
5.
Stepwise
Sample
Weight
NE2
0.4762
heating
(“C)
(9)
ME1
of
Ne
0.00776
300
0.482
400
1.51
500
1.55
600
0.368
700
0.0295
26
800
0.00265
76
51
2.1
0.67
0.65
2.7
3.95
200
0.00189
300
0.354
1.7
400
1.37
0.46
500
1.32
0.47
600
0.670
700
0.114
44
0.72
5.2
100
Total
0.4282
4.04
200
0.0629
300
0.723
400
500
1.57
1.62
600
700
600
0.207
Total
0.3125
OCXJ
39
5.2
2.5
2.4
15.9
100
100
4.16
200
0.0409
0.264
300
400
(60-100
mesh)
8OO’C.
(x10-%n%TP/g)
600
7473104
below
Blank(X)
200
Total
0.5004
data
2ONe
Temp.
21
3.6
3.97
4.45
500
600
0.26
0.25
1.23
700
0.151
600
0.0161
Total
0.90
6.6
41
10.1
Elemental abundances and Ne excess
In Figs. 3 and 4, the elemental noble gas abundance
terns were plotted by using fractionation factor F(m)
pat-
where “‘X represents a noble gas isotope of mass “m.” Although the trend is not so clear since most of data are given
by upper limits, the fractionations of obsidians seem to show
the monotonic changes from Ar to Xe; and there are directions of both increasing and decreasing of the fractionation
with increasing mass number (Fig. 3). Noble gases in submarine glasses show fractionations for heavy noble gases of
Kr and Xe and an excess of Ne relative to air, which are
classified as type 2 (OZIMA and ALEXANDER.1976). The absolute concentrations of noble gases in obsidians are compatible with those in submarine glasses. The fractionation
patterns of noble gases in obsidians are similar to type 2,
although Kr/Ar and Xe/Ar ratios in some of the former samples are lower than those in air. The Xe/Ar ratios in the
submarine glass samples (DYMOND and HOGAN, 1973;
OZIMA and ZASHU, 1983) are actually higher than that of
air, of which F( 132) varies up to about 100. F( 132) in obsidians are lower than 15, indicating that the fractionations are
small for heavy noble gases in obsidians. In contrast with
this, the fractionations of Ne to Ar in obsidians are very variable, and are very large in some samples compared to that
expected from the fractionation pattern of heavy noble gases.
Five of seven samples show very high Ne/Ar ratios compared
to that of air, and two other samples show a ratio lower than
the air. F(20) in submarine glasses has a range from 0.8 to
250 (OZIMA and ZASHU, 1983; DYMONDand HOGAN, 1973)
similar to the values obtained from our obsidian samples
(Fig. 3).
As seen in Fig. 4, the fractionation patterns of Darwin
glass samples are very similar to each other and are characterized by its high Ne/Ar ratio compared to the air, of which
F(m) = (m~I’6Ar),,,,,I(mXI’6Ar)ail
50
Ne
0 NE2
0 ME1
0 7473104
n DGtl3
40
i,
,
h
$ 30
Y
.-s
z
2 20
IA
0
LI
0
v
0
0
0
10
0
NE2
NE3
ME1
us115
US1823
US33Oa
7473104
c] Submarineglass
0.001
I
200
300
400
500
I
I
,
I
he
‘ONe
36Ar
uKr
132Xe
FIG. 3. Elemental abundance pattern of noble gases in obsidians,
I
Temperature
,
600
700
800
(“C 1
Fm. 2. Thermal release pattern of Ne below 800°C. Neon was
mainly released at 400-500°C, which was commonly observed in
obsidians and Darwin glass.
displayed as a fractionation factor
F(m) = (mX/MAr),,,,J(mX136Ar),,
where “X represents a noble gas isotope of mass “m”. The data field
of 23 submarine glass samples (DYMOND
and HOGAN, 1973; OZIMA
and ZASHU, 1983) is also shown. The values of F(20) vary widely
compared to fl84) and F( 132).
Ne content of natural glasses
excess which is now observed was not shown in the noble
gas pattern of impact glass immediately after the shock event
on the Earth’s surface. The Ne/Ar ratio of the Darwin glass
should be lower or almost the same as that of air when it was
formed. This suggests that Ne excess occurred by gas-solid
interaction during the time since the impact glass solidified.
As the shock-implanted noble gases in natural samples are
lower than those shock-implanted in the laboratory, it is also
conceivable that the gas implantation did not occur during
terrestrial impacts (BOGARD et al.. 1986). However, solubility
data of noble gases in silicate melt show that the fractionation
patterns of noble gases are monotonic for the atomic number
(OZIMA and PODOSEK, 1983). The Ne/Ar fractionation has
no correlations with the ratio of the other heavy noble gases
such as Kr/Ar and Xe/Ar ratios (Figure 5), also suggesting
0 !xPl
A DGttZ Darwin glass
n DGa3 1
0 Lonar tektite
0 Thatland toktile
I
‘He 20Ne 36Ar
&Kr
3029
I
13’Xe
loo0
m
FIG. 4. Elemental abundance pattern of noble gases in Darwin
glassusing the fractionation factor defined in Fig. 3. Those of Thailand
tektite (HENNECKEet al., 1975) and of Lonar tektite-like glass (BoCARD et al., 1986) are also plotted for comparison. The value of
F(20) in Darwin glass is as high as about 70.
F(20)
is about 70. The Kr/Ar and Xe/Ar ratios of DG#2 are
almost the same as those in air, although those of DG# 1 and
DG#3 are shown as their upper limits. There have been several reports of noble gases in impact glass and similar samples.
HENNECKEet al. (1975) measured Thailand tektites of which
F(20) was as high as 1800. Recently, BOGARDet al. (1986)
measured the noble gas in Lonar tektite-like glasses from
India, of which Ne/Ar was also high and F(20) was about
10. These results are also shown in Fig. 4. Degassing from
vesicles in Darwin glass by crushing is the cause of the difference between the chipped samples (DG# 1 and DG#2) and
the 60-100 mesh fraction (DG#3) in Table 4, which also
appears to have a Ne/Ar ratio (F(20) = 53) as high as that
in the glass. O’KEEFE et al. ( 1962) also observed high Ne and
He in Tektite bubbles although they did not give Ar content.
The occurrence of Ne excess in impact glasses and tektites
indicates that Ne enrichment may have occurred on the
Earth’s surface. The laboratory experiments of shock implanted noble gases in silicates showed that no elemental
fractionations of noble gases were detected (BOGARD et al.,
1986; WIENS and PEPIN, 1988). Shock-implanted Ne was
lightly retained and lost subsequent to shock in many samples,
and, therefore, the Ne/Ar ratios implanted in silicates were
lower than that of the ambient gas in their experiments. Examination of the stepwise heating experiment showed that
Ne is degassed at the very lowest temperatures (200-4OO”C),
suggesting that Ne diffusion from these samples readily occurs
(BOGARDet al., 1986). The Ne implanted in shock-produced
diamonds was trapped very tightly, being degassed at temperatures as high as 2000°C (MATSUDA and NAGAO, 1989)
but the fractionation pattern of noble gases showed a small
depletion in light elements relative to air (F(20) < l), contrary
to the Ne excess. These experiments indicate that the Ne
.:I!,
,,,,,,,,,
or
,Eij
0.01
0.1
1
84Kr I 36Ar
loo0 m
13’Xe I 36Ar
FIG. 5. (a) 20Ne/MArvs. s4Kr/36Arand (b) 20Ne/36Arvs. ‘3*Xe/SbAr
for obsidian, Darwin glass, tektite (HENNECKE et al., 1975; BOGARD
et al., 1986), and submarine glass (DYMOND and HOGAN, 1973;
OZIMAand ZASHU, 1983). There seem to he no correlations among
these ratios of noble gases.
3030
J.-i. Matsuda et al.
that Ne was not trapped into glasses with heavy noble gases
at the same time.
There are two possibilities for trapping noble gas in solidgas interaction: physical adsorption and diffusion. In the
former case, the adsorbed noble gases should show progressive
enrichment in heavy noble gases, and the Ne/Ar ratio in the
adsorbed gases would be lower than that of air. Therefore, it
is estimated that the possible mechanism to produce Ne excess
is a diffusion process in the gas-solid interaction, where light
noble gases enter the material more easily than heavy noble
gases.
woo
500
Temperature (“C)
300 200
100
Diffusion of noble gases in silica-rich glasses
The diffusion coefficients of noble gases can be determined
from the stepwise heating data by assuming that the sample
is composed of spheres of a certain radius and has homogeneous concentrations of noble gases before heating (CRANK,
1956). When we calculated the diffusion coefficient in the
intermediate temperature step, we summed up all the released
gas fractions below that temperature and calculated the diffusion coefficient, and then subtracted the diffusion coefficient
obtained from the last temperature step. In this procedure,
the released gas fraction is compared to the total gas fraction
under the assumption that the gas was homogeneously distributed. The obtained diffusion coefficients were in good
agreement with those calculated from the approximate equations given by FECHTIG and KALBITZER (1966)who took it
into consideration that after the first run the gas distribution
was not homogeneous. The diffusion coefficients of Ne in
NE2, ME 1, 7473 104, and Darwin glass have been calculated
from Table 5, and those of Ar and Kr in ME3 from Table
3. We used the 60-100 mesh fraction in our measurement
and assumed the average radius of the grain is 100 pm. The
results are listed in Table 6. The accuracy of the diffusion
coefficient is thought to be within a factor of two when the
elemental abundances of noble gases have 10% error for reproducibility. After the Ne measurement below 800°C we
checked the sample in the holder and confirmed that all the
samples were not molten at 800°C. However, ME3 should
be molten at some temperature when it was heated above
1000°C. As the geometry of the sample was not valid above
the melting temperature. we only listed the diffusion coefficients of Ar and Kr at 1000°C in parentheses with those at
800°C in Table 6. We did not calculate the diffusion coefficient of Xe in ME3 because it is likely that the 800°C fraction
Table
6.
Diffusion
coeffipients
and Darwin
glass
(cm2/sec)
of
noble
gases
in
obsidians
Temp.
(“C)
200
300
400
500
600
700
800
1000
*
8.oxlo-‘l
1.7x10-9
8.4XlO-9
1.4XN8
4.1x10-1’
1.1x10-9
4.1X1O-9
l.2x1o-8
2.7XlO-12
2.1x10-‘0
2.1x10-9
1.1X1O-8
5.0x10-1*
1.1x10-9
7.2X10-’
1.2x10-@
1.3XN8
8.2x10-10
(2.3x10-9)
1.5x10-‘1
(2.2xlo-‘o)
For the release
fraction
of noble
gas
less
than
1X, the diffusion
coefficients
were
not
calculated.
As for
AT and Kr. we listed
the diffusion
coefficients
at 1OOO’C 1” parentheses,
because
the
sample
should
be molten
at some temperature
above
1000°C.
_
o-5
10
l-5
20
25
1OOOI T(K)
3-o
3-5
FIG. 6. Arrhenius plots of diffusion coefficients of noble gases in
obsidians and Darwin glass. The line represents a linear least squares
fit to the data, of which the slope corresponds to the activation energy.
As the linearity for the Darwin glass was not good, the line was not
drawn. The lines obtained for the quartz, commercial silica glass,
tektite, and natural glasses from other authors are also given for comparison. The numerical values of these lines are the reference numbers
given in Table 7 (I: tektite; 2, 3, 6: fused quartz; 4. 5: Si02 glass: 7:
obsidian; 8: basalt glass).
of Xe in ME3 contains adsorped gas, judging from the release
pattern of Xe in Fig. 1.
The diffusion coefficient D has a temperature dependence
given by an Arrhenius equation D = Do exp(-E/RT)
where
DOis a characteristic constant, E is the activation energy, R
is the gas constant, and T is the absolute temperature. Figure
6 is an Arrhenius plot in which log D is plotted against l/T.
The slope of the Arrhenius plot then gives the activation
energy E. There have been extensive reports on diffusion
coefficients of noble gases in quartz and commercial silica
glass (FRANK et al., 196 1; SWETS et al.. 196 1: FRISCHAT and
OEL, 1967; PERKINS and BEGEAL. 197 1: SHELBY, 1972; JAMBON and SHELBY, 1980). All these data are mainly on He.
The diffusion coefficients of He and Ne in tektites (REYNOLDS, 1960) and He in volcanic glass (KURZ and JENKINS,
I98 1) have been also reported. In Fig. 6, we plotted all these
data. The activation energies and characteristic constants of
noble gases have been summarized in Table 7.
The diffusion coefficients of Ne in three obsidian samples
are very similar, and seem to lie on a straight line (Fig. 6)
corresponding to an activation energy of 18-20 Kcal/mole
(Table 7). The diffusion coefficients of Ne in Darwin glass
are similar to those in obsidians, but the linearity is not as
good as those of obsidians. If we draw a line from all five
points, the activation energy from the slope of the line is
about 22 Kcal/mole, a value similar to those for obsidians
(Table 7). REYNOLDS(1960) measured the diffusion coefficient of Ne in an Australian tektite (Fig. 6, line I) in the
temperature range of 110 to 450°C for which diffusion coefficients agree very well with our data between 300 and 600°C.
303 1
Ne content of natural glasses
Table
7.
The
(Do)
activation
of noble
sample
Noble
gas
NEZ(obs1di.m)
N+Z
Mgl(obsidian)
Ne
?473104(obsidian)
Ne
ffiltJ(Daruin
glass)*Ne
Tektite
He
Ne
Fused quartz
Ne
Fused quartz
He
Ne
Cmmerc~ai
glass
Ne
Commercial giass
Ne
*r
Fused ~l”artz
He
Temp.
and the characteristic
constant
m silica-rich
glasses.
E
(Kcal/mole)(cm
(“C)
300.
600
300-
600
18.6
200-
500
20.1
300-76.
IlO-
700
50
450
(21.7)
6.23
440.
985
11.4
24-
300
300-1034
He
He
948.1196
26- 600
650. 920
103. 165
90. 171
zoo- 300
200. 300
He
125-
He
Iceland
obsidian
Valles
Caldera
obsidian
8asa1t glass
energy (E)
gas diffusion
400
17.5
14.1
5.58
6.61
13.7
9.65
26.7
5.63
5.51
8.01
8.59
19.9
LID
Reference
/WC)
5.5x10-4
This work
7.2x10-4
This work
6.9X10-!
This work
(3.4X10m3) This work
1.4x10-5
(1)
3.5x10-S
(1)
2. 7.x1o-4
(2)
3.0x10-4
(31
7.4x10-4
(3)
4.9x&
(4)
5.1x10-5
(5)
1.2x10-4
(5)
4.6x10-4
(6)
3.8x10-4
(6)
7.7x10-4
(7)
1.6x10-3
(7)
6.7x10-2
(6)
References
(1) Reynolds
(1960);
(2) Prank et al.
(1961);
(3) S”ets et al.
(1961):
(4) Frischat
and Oel (1967);
(5) Perkins and Begeal (1971);
(6) Sheiby (1972);
(7) Jantbon and Shelby (1980); (8) Kurz and
Jenkins
11981)
* Although
the linearity
was “at good,
the activation
energy and
the characteristic
constant
were calculated
from all five data
points.
However,
diffusion coefficients at low temperatures in
(1960)seem to be higher than those of our data,
giving a lower activation energy of 14.1 Kcal/moie. Three
sets of Ne data for quartz and commercial glass (lines 5, 2,
and 4 in Fig. 6) from FUNK et al. (1961) FRISCHAT and
OEL ( 1967) and PERKINSand BEGEAL(197 1) seem to lie on
a single line for a wide range of temperature, giving an activation energy of lo- 14 Kcallmofe. The diffusion coefhcients
of Ne obtained by us and those by REYNOLDS(1960) are
much lower than those from quartz and commercial glass.
The SiOz content of the commercial glass sample used by
FRISCHATand OEK.(1967) is 73.9%, which is very similar to
those of our obsidians. Therefore, it is conceivable that the
silica content is not the only parameter to control the Ne
diffusivity. Other elements such as Fe and Mg in natural glass
probably decrease the Ne diffusivity. The content of such
elements may have an affect on the glass structure, and the
mobility of Ne atoms may be influenced by these elements
(NORTON, 1957).
The He diffusivity (see Fig. 6) in tektites (line 1) seems to
be less than in obsidians (Iine 7), and basalt glass (line 8) has
much lower diffusivity than obsidians. Thus, He shows a large
variation of diffusivity with glass composition. The diffusion
coefficient seems to increase and the activation energy decreases with increasing SiOz content. ALTEMOSE(1961)reported that the permeation rate of He increases with the mole
percent of glass network former (SiOz, B203, and P,Os).
SHELBYand EAGAN(1976) showed that He diffusion in albite
glass depended on the AI/Na ratio of the glass composition.
The effect of elemental compositions on the diffusivity of
noble gases in natural glass may be more complicated. More
data may be necessary to determine these effects.
The diffusion coefficients of Ar in our samples are approximately plotted in the extension of the line obtained by
PERKINSand BEGEAL(197l), but the data point at 800°C in
our obsidians is slightly high (Fig. 6). The difference of diffusivities of Ar and Kr is small compared to that of Ar and
Ne (Fig. 6).
REYNOLDS
We suppose that the fresh glass with negligible amounts
of noble gases is exposed to air. If 0, is the diffusion coefficient
of noble gas of mass m, the penetration depth of noble gas
during time t is given by (&J)‘/*_ Therefore, the fmctionation
factor F(20) of noble gas in the sample is approximately given
which is as far as we take the sample whose
by (&c@#~,
size is sufficiently larger than the (D,t)i”. Except for very
high F(20) in one sample of a Thailand tektite (HENNECKE
et al., 1975), almost all values of F(20) in silica-rich glass are
below 300. This corresponds to a value of D&Dj6 as high as
about 10’. The difference of diffusivity between Ar and Ne
is about three orders of magnitude at 700°C as shown in Fig.
6. It is very likely that the difference of diffusivity of Ne and
Ar increases up to approximately lo5 at low temperatures.
Thus, the Ne excess may have been produced by the difference
in diffusivity of noble gases. The diffusion coefficients of He
are very much higher than those of Ne in obsidians. Helium
diffuses very readily into and out of these samples. It is well
known that He can pass through Pyrex glass very well at
room temperature. It is conceivable that the sample lost its
He under a vacuum in the sample line of the mass spectrometer. Neon can also diffuse readily into these samples, but its
mobility is smaller than that of He. By comparison, heavy
noble gases cannot easily enter the sample because of their
very small diffusivities. This is probably the mechanism that
produces the Ne excess. The large variety of Ne/Ar ratios is
also compatible with this model. The physicai conditions of
the material should be very sensitive to the diffusivity. and
Ne also escapes from the material easily.
NE EXCESS IN SUBMARINE
GLASSES
DYMOND and HOGAN (1973) and KIRSTEN et al. (198 I)
emphasized that the high NefAr ratios in submarine glasses
reflected the solar type noble gas which still exists in the mantle. OZIMA and ALEXANDER(1976) and OZIMA and ZASHU
(1983) explained the Ne excess by the preferential leakage of
Ne into the magma source from the su~ounding mantle.
However, LUX ( 1987) pointed out that such large differences
in diffusivity are based on values determined in materials
only at temperatures below their liquidus. It seems difficult
to explain Ne excess by the difference of diffusivity at the
magma generation. LUX (1987) proposed linear equations of
Henry’s constant as a function of density of silicate liquids
for densities in the range 2.1-2.7 g/cm3. From these equations,
the fractionation pattern of noble gases was calculated if submarine glasses were equilibrated with the planetary reservoir.
According to Lux (1987),
the calculated result covers the Ne
excess only by an extreme increase in density of the basalt
liquid. This model requires that the Ne/Ar ratio should correlate with Kr/Ar and XefAr ratios. However, Ne/Ar ratios
seem to have no correlations with Kr/Ar and Xe/Ar ratios
for submarine glasses, and for obsidians and impact glasses
(Fig. 5). Therefore, it is difficult to explain the Ne excess by
this model. JAMBONet al. (1986) also measured the soIubility
of noble gas in silica liquid and discussed the distribution of
noble gases between vesicles and glass as a function of vesicularity. In their calculation, heavy noble gases enter vesicles
preferentially. If gas is lost because of vesicuiation. both He/
Ne and He/Ar are increased in the residual melt. They pro-
J.-i. Matsuda et al.
3032
posed that this model could be a reasonable explanation of
high Ne/Ar and He/Ar ratios in submarine glasses. However.
this model again predicts the negative correlation between
Ne/Ar and Xe/Ar ratios, which is incompatible with the
present data (Fig. 5).
There are no correlations of Ne/Ar with the ratio of the
other noble gases such as Xe/Ar and Kr/Ar (Fig. 5). The Ne
excess is observed in the case of increasing and/or decreasing
fractionation of heavy noble gases with increasing mass
number, and in all sorts of silica glass of submarine glass,
obsidian, impact glass, and tektite. Therefore, the mechanism
to produce the Ne excess should be different from that which
produced the fractionation pattern of heavy noble gases and
may be a characteristic of the glass structure.
CRAIG and LUPTON ( 1976) first reported Ne isotopic
anomalies in MORB of about 2.5% in the “Ne/“Ne ratio
compared to that in air. POREDAand RADICATIDI BROZOL.O
(1984) also found an excess of 20Ne/22Ne ratio of about 7%
in basalt glasses from Reykjanes Ridge, and SARDA et al.
( 1988) recently reported “Ne/“Ne ratios as high as about 13
in MORB glassy samples. It seems difficult to explain these
2oNe/22Ne anomalies by the diffusion model proposed in this
study. RAMA and HART (1965) examined Ne isotope fractionation during the transient permeation process, reporting
as much as 45% enrichment of “Ne relative to “Ne. This is
a rapid diffusion process. and in a steady state fractionation
can be achieved to about 5% depending on the ratio of the
diffusion coefficients of mass 20 and 22. HENNECKEet al.
(1975) reported Ne isotopic compositions identical to air in
a Thailand tektite, though the Ne/Ar ratio (F(20) = 1800)
was much higher than that in air. The uniform 3He/4He ratios
and the very high 40Ar/36Ar ratios observed in MORB are
difficult to explain by the diffusion model. Therefore. it should
be emphasized that, if submarine glasses have different noble
gas isotopic features from those in air, these samples really
contain noble gases from the upper mantle. The problem is
the very high Ne/Ar ratios and their wide variation which
are especially observed in some submarine glasses. We could
not examine the correlation between the 20Ne/22Ne ratio and
the Ne/Ar ratio in submarine glasses because many authors
gave Ne isotopes without Ar abundances (CRAIG and LUPTON, 1976; POREDA and RADICATI DI BROZOLO. 1984;
SARDA et al., 1988) or gave noble gas abundances without
Ne isotopes (DYMOND and HOGAN, 1978). OZ~MA and ZASHU (1983) gave both abundances of all noble gases and Ne
isotopes, but they could not get meaningful Ne isotopic results
because of low Ne concentration. At present. we may only
suggest the possibility that the high Ne/Ar ratios observed in
some submarine glasses are influenced by sea water or the
atmosphere, rather than representing the original ratio derived
directly from the mantle. If this is the case, there are no correlations between the Ne/Ar ratio and the ratio of the other
noble gases (Fig. 5).
CONCLUSIONS
Neon was enriched in obsidians and impact glass compared
to the relative pattern of noble gases in air, as is also the case
in submarine glass. Ne was released at 400-5OO”C, very low
compared to the degassing temperature for heavy noble gases
such as Ar and Kr. Judging from shock-implanted experiments of noble gases and the occurrence of Ne excess in
natural impact glass, the Ne excess may have been produced
by gas-solid interaction after the sample solidified, and probably the high diffusivity of Ne into solids is the main cause.
The diffusion coefficients and the activation energy of noble
gases have been determined.
It is conceivable that this Ne excess is a common feature
generally seen in glasses. There are no correlations of Ne
excesses with heavy noble gases, suggesting that the mechanism responsible for Ne excess is different from any which
produce heavy noble gas patterns. It is also likely that the
very high Ne/Ar ratios observed in some submarine glasses
are not mantle-derived but are the result of diffusion into the
sample from sea water or air.
Acknowledgments-We
would like to thank Prof. Ui, Kobe University, for giving us some of the obsidian samples and for the petrographic observation of samples. Thanks are also due to Dr. Kaneoka,
Tokyo University, for his constructive criticism on this manuscript.
We gratefully acknowledged to Drs. D. E. Fisher, 0. K. Manuel, and
anonymous reviewers for their comments and suggestions.
Editorial handling: D. E. Fisher
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