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Mass spectrometric analysis of gas inclusions in Muong Nong glass and Libyan Desert glass
EARTH AND PLANETARY SCIENCE LETTERS 14 (1972) 221-225. NORTH-HOLLAND PUBLISHING COMPANY
[]
MASS SPECTROMETRIC ANALYSIS OF GAS INCLUSIONS IN MUONG NONG GLASS AND LIBYAN DESERT GLASS E. JESSBERGER and W. GENTNER Max-Planck-Institu t fiir Kernphysik, Heidelberg, Germany
Received 13 December 1971 Noble and non-noble gases in bubbles of Muong Nong- and Libyan Desert glass were released by vacuum crushing at room temperature and measured.by high sensitivity mass spectrometry. The N2 :Ar:Kr:Xe ratio as well as the rare gas isotope ratios were found to be atmospheric, indicating the terrestrial origin of these glasses. The concentrations of the active gases 02 , CO2 , CO and SO2 vary highly between adjacent bubbles. Total gas pressure in the bubbles of the glasses is in the 100 mm range, much higher than that found for other types of tektites.
1. Introduction
2. Experimental m e t h o d
During the last few years strong evidence for the terrestrial origin of tektites has been accumulated [ 1 - 1 0 ] . One such piece of evidence is provided by the composition of gases included in bubbles of tektites which should reflect the atmosphere in which they were formed. Data obtained by ZS_hringer [1] and Miiller and Gentner [5, 11] indicated that gas bubbles in Moldavites, Phillipinites and Indochinites contain residues of the terrestrial atmosphere. Another important argument for the terrestrial origin of tektites arises from a cogenetic origin of Muong Nong glasses and tektites. Such a genetic relationship is indicated by the common localities of occurrence and by concordant ages [12]. The composition o f gas inclusions in Muong Nong glass could serve to further substantiate this point. The elemental and isotopic abundances of rare gases in two Muong Nong glasses are similar to those in the atmosphere [ 1]. Nitrogen, oxygen and carbon dioxide in Muong Nong glass bubbles were detected by Miiller and Gentner [ 11 ]. The intention of this work was to perform with improved sensitivity complete and simultaneous analyses of both noble and non-noble gases in a larger number of Muong Nong glass bubbles. In addition, bubbles in Libyan Desert glass have also been measured.
The samples, 5 0 - 1 0 0 mg specimens of bubble rich zones were ultrasonically cleaned in acetone and water. Gases adsorbed at the surface were removed by heating in vacuum for 12 hr at 120°C. The stainless steel sample breaker constructed to extract the gases is shown in fig. 1. The force to crush the samples can be increased continuously, thereby allowing a stepwise gas release. The liberated gas expands immediately into the ion source of the gas analyser (fig. 2). Because of the small
Samples Analyzer ~
~ ~\
~Ram Base Plate
Fig. 1. Sample breaker.
E. Jessberger, W. Gentner, Gas inclusions in glasses
222
-8
P
-9
~-10 3.1CC3
He,Ar, Kr, Xe
6 0 ; Nier- S p e c t r o m e t e r I
L
u
g-5,
~
co N2 n
_7 I •
t~
o
,,,,,,,,L,,L,,,,
[ ] = Valve
'
ME N 2 , O 2 , A r , CO2
C
volume of the sample breaker, gas losses due to adsorption are minimized. Metal metal friction is avoided in order to minimize gas release from the walls. The lower plate of the ram is made out of a tempered W-Ti-Co-alloy to prevent penetration of glass splinters. Samples already crushed can be removed from the base plate without venting (fig. 1). A major difficulty in the static analysis of minute amounts of non-noble gases is gas consumption due to reactions with the filament of the ion source. To solve this problem, a very fast operating system is required. Here an ATLAS AMP3 quadrupole mass filter was chosen. Multiplier currents between 5.10 -1~ and 10 -6 Amp are logarithmically amplified and registered on a BRUSH-recorder. With a resolution of 40, only 0.8 sec ar~ required to record a mass spectrum between mass numbers 12 and 50. Exactly known amounts of N2, CO, CO2, Ar and air were used for calibration. The smallest measurable amount of each gas in a sample is determined by the background in the crusher and the mass filter. Test runs using window glass caused no considerable increase in the background. Typical detection limits in units of 10 -9 cm 3 STP for N 2 , 0 2 , Ar and CO2 were 500,40,5 and 500, respectively. After scanning all non-noble gases and 4°At in the quadrupole, the noble gases were treated in a gas purification and separation line and measured in a 60°-magnetic-mass-spectro meter (fig. 2). Detection limits for the latter instrument are 5 . 1 0 ,9 cmaSTP 4°Ar and 0.2 10 -9 cm3STP 4He.
'
I
.
12
I
Fig. 2. Apparatus: B, Sample breaker; C, Calibration gas; CF, Charcoal finger; P, Pump; PG, Pirani gauge; S, Samples; Ti, Titanium furnace.
A r CO2
J _Ak 2 : d . _
IE_ 9
C
lJ
.
.
.
I .....
20
...... I .....
30
0.8 ~ c
I ..... 40
50
__1
Fig. 3. Typical mass filter spectrum.
While the mass filter and the recorder were operating continuously the sample was crushed step by step. After each step, all gases except helium were adsorbed onto a charcoal filled cold trap in the purification section (fig. 2) until the background in the mass filter was restored. Then the valve V~ was closed and the procedure repeated. |n this way, Ar, Kr and Xe from all steps were collected in the purification line. After completion of the mass filter analyses, the rare gases were measured in the 60 °spectrometer. Helium was measured at first. Afterwards, Ar, Kr and Xe were desorbed from the charcoal finger, gettered over hot titanium and then analyzed. The analytical errors of the mass filter data calculated from many repeated calibration measurements are less than -+ 5% for N2, CO and Ar, and less than + 10% for 02. CO and SO2 data are estimates only. The accuracy of the 60°-spectrometer data is + 3% for Ar and -+ 15% for He, Kr and Xe, due to the small amounts involved. Argon data obtained with both spectrometers agreed very well. This cross check increases the confidence in the results.
3. Results
To evaluate peaks of the mass filter spectra with amplitudes as different as five orders of magnitude, spectra were recorded on two parallel channels with
223
E. Jessberger, W. Gentner, Gas inclusions in glasses
N2/Ar
N2/O2
104
t
CO2/Ar" singlesteps without 02
103 .
104
,
N2/Ar"
N2/O2 CO2/AF t single steps i without02
103 265
236
L
102 4
~8~93~836
54
102 7~t11~~ - E a 8 3 6
e, lid
.; 10
10 ~ 37
AtmosphericRatio
a
~
0(33 ~
AtmosphericRatio
b
Fig. 4. N2/Ar-, N2/02 -, and CO2/Ar-ratios. Each dot represents one single analysis. The area of each dot is proportional to the respective N2-amount. (a) Muong Nong glass. (b) Libyan Desert glass. Table 1 Partial and total pressures in bubbles of Muong Nong- and Libyan Desert glass. Specimen no.
4He (10 -8cm3 STP)
Bubble volume (mm3)
pN 2
po 2
PAr
PCO2
PSO 2
P t ot
(ram Hg)
Muong Nong glass 572-1a 572-1i 572-2a 572-2i Kemaraj Kemaraj
0.03 0.14 0.10 0.14 0.39 0.08
0.23 1.06 0.76 1.06 2.96 0.61
27 42 70 65 32 12
0.8 0.6 0.3 1.2 1.5 2.2
0.3 0.3 0.7 0.7 0.4 0.2
135 171 60 125 160 160
0.04 0.1 0.1 -
165 215 130 180 190 175
16.4 1.76 1.38
26 50 40
0.02 0.5 -
0.02 0.7 -
4 15 6
0.1 0.2 -
30 65 45
Libyan Desert glass 589-1 589-2 589-3
2.17 0.23 0.18
different amplifications (fig. 3). Not only the base peaks, but also the cracking patterns, obtained from pure gases, were considered to identify the gases. N2, e.g., is dissociated in the ion source to form a 2+ fixed percentage of N + and N 2 . By this, noting the mass 28/14 ratio, the amounts of N2 and CO can be determined. It might have been doubtful to relate peaks at mass 48 to SO2 ; however, from the mass ratio 64/48 this assignment became obvious. The results of 87 Muong Nong glass (MNG) and 34 Libyan Desert glass (LDG) single step analyses are given as N2/Ar, N2/02 and CO2/Ar ratios in
figs. 4a and 4b. From the cracking patterns discussed above, an average C 0 2 / C 0 ratio of 25+_~° for all MNG specimens is obtained. For LDG this ratio exceeds 200. In samples of both glasses, SO2 was found with an average N2/SO2 = 930 in MNG and N2/SO2 = 330 in LDG. In one MNG sample a trace of NO was detected. The total bubble volume of each sample was determined from the amount of helium (table 1) assuming equilibrium of the He partial pressure between bubbles and atmosphere [12]. A correction is required for the He-diffusion losses
224
E. Jessberger, W. Gentner, Gas inclusions in glasses
Table 2 Isotopic and elemental abundances of the heavy rare gases in Muong Nong glass and Libyan Desert glass bubbles compared with atmospheric ratios. (Av): weighted average ratio from 6 Muong Nong glass and 3 Libyan Desert glass specimens, respectively.
40Ar 36Ar
36At 38Ar
82Kr 84Kr
83Kr 84Kr
86Kr 84Kr
129Xe 131Xe 134Xe 136Xe 36At 132Xe 132Xe 132Xe 132Xe 84Kr
84Kr 132Xe
Muong Nong Glass (av.)
293
5.5
0.198
0.204
0.294
0.99
0.86
0.39
0.34
55.6
28.1
Libyan Desert Glass (av.)
297
5.3
0.198
0.198
0.286
0.98
0.77
0.40
0.32
55.4
25.9
Atmosphere
296
5.35
0.204
0.203
0.305
0.99
0.79
0.39
0.33
48.5
27.7
caused by preheating. It was estimated to be ~ 60% by comparing the He amounts released from single optically measured bubbles in heated and non-heated samples. Then 10 -8 cm 3 STP 4He correspond to (7.6 -+ 2) mm a bubble volume. With this calibration, the He content and the released gas amounts added for all steps yield average partial pressures as well as the total pressure for all bubbles in one sample (table 1). Table 2 contains the isotopic and elemental abundances of rare gases together with the corresponding atmospheric ratios. All deviations from atmospheric ratios are within the experimental errors.
4. Discussion The most striking result of this work is the obvious connection between atmospheric and gas inclusion abundance pattern~ Though the elemental and molecular abundances are not simply atmospheric for the active components, the more inert gases N2, At, Kr and Xe occur roughly in atmospheric ratios (table 2, fig. 4). Isotopic ratios of rare gases are also atmospheric within the limits of error (table 2). This suggests atmospheric gases as the primary source for the gas inclusions. Deviations, especially for active gases, are due to secondary alterations or to an admixture from an additional source. The N2/Ar ratio of about 90 found in all experiments is unique for the earth's atmosphere. It is not changed by secondary reactions since both gases are chemically inactive. For most of the bubbles, the N2/O: ratio exceeds the atmospheric value. This is
easily explained by oxidation processes. On the other hand, it is hardly by chance that the lowest N2/O2 ratios correspond to the atmospheric ratio. The influence of chemical reactions is further illustrated by the variable abundances of CO2, CO, SO2 and NO, which are at least in part the final products o f these reactions. The high total abundance of CO2, especially in MNG, requires an additional source, most probably the decomposition of bedrock carbonates caused by the impact. MNG and LDG are similar with regard to the more inert gases (table 2); however, the total CO2 content is about 15 times higher for MNG than for LDG (fig. 4). This further supports the presence of atmospheric gases as a primary gas component during the formation of both types o f glasses, while the CO2 is an admixture depending on the chemical composition of the bedrock. The varying gas composition for adjacent bubbles of one specimen, determined by stepwise crushing, indicates a rather complex mechanism of gas inclusion. We suggest that inclusion occurred in more than one stage. At first, gases evolved from bedrock may have predominated, while the atmosphere became important in a later stage. Since the differences between adjacent bubbles are preserved over long time periods, it is clear that"the bubbles were in general vacuumtight, thus preventing gas exchange. This was already noted by Z~ihringer [1] for rare gases. Among other reasons, this fact excludes gas penetration into bubbles after deposition. (Assumption o f J.O'Keefe, private communication.) From the total pressures involved and the high
E. Jessberger, I¢. Gentner, Gas inclusions in glasses
C O 2 - c o n t e n t it seems m o s t likely that the f o r m a t i o n o f MNG and LDG t o o k place within a gas phase which was in itself a result o f an impact and which was n o t in equilibrium w i t h the earth's atmosphere. The atmospheric abundance ratios o f inert gases found in Muong N o n g glasses provide a further similarity b e t w e e n MNG and genuine Indochinites. Again it seems reasonable to c o n c l u d e that M u o n g Nong glass and Indochinites are p r o d u c e d by the same impact event taking place on earth 700 000 yr ago.
Acknowledgement The samples # 572 were kindly d o n a t e d b y Professor P. Pellas, Paris.
References [ 1 ] J. Z~/hringer, K-Ar measurements of tektites, IAEA, Vienna, Radioactive Dating (1963) 289. [2] J. Ziihringer and W. Gentner, Radiogenic and atmospheric At-content of tektites, Nature 199 (1963) 583. [3] V.E. Barnes, Variation of petrographic and chemical characteristics of indochinite tektites within their strewn-field, Geochim. Cosmochim. Acta 28, 1 (1964) 893.
225
[4] V.E. Barnes, Petrology of moldavites, Geochim. Cosmoclaim. Acta 33, 9 (1969) 1121. [5] O. Miiller and W. Gentner, Gas contents in bubbles of tektites and other natural glasses, Third Internat. Tektite Symposium, Coming, New York (1969) 47. [6] I. Rybach and I.A.S. Adams, The radioactivity of the Ivory Coast tektites and the formation of the Bosumtwi Crater (Ghana), Geochim. Cosmochim. Acta 33, 9 (1969) 1101. [7] S.R. Taylor and M. Kaye, Genetic significance of the chemical composition of tektites: A review, Geochim. Cosmochim. Acta 33, 9 (1969) 1083. [8] I.M. Wampler, D.H. Smith and A.E. Cameron, Isotopic comparison of lead in tektites with lead in earth materials, Geochim. Cosmochim. Acta 33, 9 (1969) 1045. [9] C.C. Schnetzler, The lunar origin of tektites: R.I.P., 33rd Annual Meeting of the Meteoritical Society, Skyline Drive, Virginia (1970). [10] S. Epstein and H.P. Taylor, Jr., Ol8/Oa6, Si3°/Si28, D/H, and C~3/Ca~ ratios in lunar samples, Proc. Apollo 12 Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 2, 2 (1971) 1421. [ 11] O. Miiller and W. Gentner, Gas content in bubbles of tektites and other natural glasses, Earth Planet. Sci. Letters 4 (1968) 406. [12] W. Gentner, D. Storzer and G. Wagner, New fission track ages of tektites and related glasses, Geochim. Cosmochim. Acta 33, 9 (1969) 1075. [13] J.H. Reynolds, Rare gases in tektites, Geochim. Cosmochim. Acta 20 (1960) 101.
[]
MASS SPECTROMETRIC ANALYSIS OF GAS INCLUSIONS IN MUONG NONG GLASS AND LIBYAN DESERT GLASS E. JESSBERGER and W. GENTNER Max-Planck-Institu t fiir Kernphysik, Heidelberg, Germany
Received 13 December 1971 Noble and non-noble gases in bubbles of Muong Nong- and Libyan Desert glass were released by vacuum crushing at room temperature and measured.by high sensitivity mass spectrometry. The N2 :Ar:Kr:Xe ratio as well as the rare gas isotope ratios were found to be atmospheric, indicating the terrestrial origin of these glasses. The concentrations of the active gases 02 , CO2 , CO and SO2 vary highly between adjacent bubbles. Total gas pressure in the bubbles of the glasses is in the 100 mm range, much higher than that found for other types of tektites.
1. Introduction
2. Experimental m e t h o d
During the last few years strong evidence for the terrestrial origin of tektites has been accumulated [ 1 - 1 0 ] . One such piece of evidence is provided by the composition of gases included in bubbles of tektites which should reflect the atmosphere in which they were formed. Data obtained by ZS_hringer [1] and Miiller and Gentner [5, 11] indicated that gas bubbles in Moldavites, Phillipinites and Indochinites contain residues of the terrestrial atmosphere. Another important argument for the terrestrial origin of tektites arises from a cogenetic origin of Muong Nong glasses and tektites. Such a genetic relationship is indicated by the common localities of occurrence and by concordant ages [12]. The composition o f gas inclusions in Muong Nong glass could serve to further substantiate this point. The elemental and isotopic abundances of rare gases in two Muong Nong glasses are similar to those in the atmosphere [ 1]. Nitrogen, oxygen and carbon dioxide in Muong Nong glass bubbles were detected by Miiller and Gentner [ 11 ]. The intention of this work was to perform with improved sensitivity complete and simultaneous analyses of both noble and non-noble gases in a larger number of Muong Nong glass bubbles. In addition, bubbles in Libyan Desert glass have also been measured.
The samples, 5 0 - 1 0 0 mg specimens of bubble rich zones were ultrasonically cleaned in acetone and water. Gases adsorbed at the surface were removed by heating in vacuum for 12 hr at 120°C. The stainless steel sample breaker constructed to extract the gases is shown in fig. 1. The force to crush the samples can be increased continuously, thereby allowing a stepwise gas release. The liberated gas expands immediately into the ion source of the gas analyser (fig. 2). Because of the small
Samples Analyzer ~
~ ~\
~Ram Base Plate
Fig. 1. Sample breaker.
E. Jessberger, W. Gentner, Gas inclusions in glasses
222
-8
P
-9
~-10 3.1CC3
He,Ar, Kr, Xe
6 0 ; Nier- S p e c t r o m e t e r I
L
u
g-5,
~
co N2 n
_7 I •
t~
o
,,,,,,,,L,,L,,,,
[ ] = Valve
'
ME N 2 , O 2 , A r , CO2
C
volume of the sample breaker, gas losses due to adsorption are minimized. Metal metal friction is avoided in order to minimize gas release from the walls. The lower plate of the ram is made out of a tempered W-Ti-Co-alloy to prevent penetration of glass splinters. Samples already crushed can be removed from the base plate without venting (fig. 1). A major difficulty in the static analysis of minute amounts of non-noble gases is gas consumption due to reactions with the filament of the ion source. To solve this problem, a very fast operating system is required. Here an ATLAS AMP3 quadrupole mass filter was chosen. Multiplier currents between 5.10 -1~ and 10 -6 Amp are logarithmically amplified and registered on a BRUSH-recorder. With a resolution of 40, only 0.8 sec ar~ required to record a mass spectrum between mass numbers 12 and 50. Exactly known amounts of N2, CO, CO2, Ar and air were used for calibration. The smallest measurable amount of each gas in a sample is determined by the background in the crusher and the mass filter. Test runs using window glass caused no considerable increase in the background. Typical detection limits in units of 10 -9 cm 3 STP for N 2 , 0 2 , Ar and CO2 were 500,40,5 and 500, respectively. After scanning all non-noble gases and 4°At in the quadrupole, the noble gases were treated in a gas purification and separation line and measured in a 60°-magnetic-mass-spectro meter (fig. 2). Detection limits for the latter instrument are 5 . 1 0 ,9 cmaSTP 4°Ar and 0.2 10 -9 cm3STP 4He.
'
I
.
12
I
Fig. 2. Apparatus: B, Sample breaker; C, Calibration gas; CF, Charcoal finger; P, Pump; PG, Pirani gauge; S, Samples; Ti, Titanium furnace.
A r CO2
J _Ak 2 : d . _
IE_ 9
C
lJ
.
.
.
I .....
20
...... I .....
30
0.8 ~ c
I ..... 40
50
__1
Fig. 3. Typical mass filter spectrum.
While the mass filter and the recorder were operating continuously the sample was crushed step by step. After each step, all gases except helium were adsorbed onto a charcoal filled cold trap in the purification section (fig. 2) until the background in the mass filter was restored. Then the valve V~ was closed and the procedure repeated. |n this way, Ar, Kr and Xe from all steps were collected in the purification line. After completion of the mass filter analyses, the rare gases were measured in the 60 °spectrometer. Helium was measured at first. Afterwards, Ar, Kr and Xe were desorbed from the charcoal finger, gettered over hot titanium and then analyzed. The analytical errors of the mass filter data calculated from many repeated calibration measurements are less than -+ 5% for N2, CO and Ar, and less than + 10% for 02. CO and SO2 data are estimates only. The accuracy of the 60°-spectrometer data is + 3% for Ar and -+ 15% for He, Kr and Xe, due to the small amounts involved. Argon data obtained with both spectrometers agreed very well. This cross check increases the confidence in the results.
3. Results
To evaluate peaks of the mass filter spectra with amplitudes as different as five orders of magnitude, spectra were recorded on two parallel channels with
223
E. Jessberger, W. Gentner, Gas inclusions in glasses
N2/Ar
N2/O2
104
t
CO2/Ar" singlesteps without 02
103 .
104
,
N2/Ar"
N2/O2 CO2/AF t single steps i without02
103 265
236
L
102 4
~8~93~836
54
102 7~t11~~ - E a 8 3 6
e, lid
.; 10
10 ~ 37
AtmosphericRatio
a
~
0(33 ~
AtmosphericRatio
b
Fig. 4. N2/Ar-, N2/02 -, and CO2/Ar-ratios. Each dot represents one single analysis. The area of each dot is proportional to the respective N2-amount. (a) Muong Nong glass. (b) Libyan Desert glass. Table 1 Partial and total pressures in bubbles of Muong Nong- and Libyan Desert glass. Specimen no.
4He (10 -8cm3 STP)
Bubble volume (mm3)
pN 2
po 2
PAr
PCO2
PSO 2
P t ot
(ram Hg)
Muong Nong glass 572-1a 572-1i 572-2a 572-2i Kemaraj Kemaraj
0.03 0.14 0.10 0.14 0.39 0.08
0.23 1.06 0.76 1.06 2.96 0.61
27 42 70 65 32 12
0.8 0.6 0.3 1.2 1.5 2.2
0.3 0.3 0.7 0.7 0.4 0.2
135 171 60 125 160 160
0.04 0.1 0.1 -
165 215 130 180 190 175
16.4 1.76 1.38
26 50 40
0.02 0.5 -
0.02 0.7 -
4 15 6
0.1 0.2 -
30 65 45
Libyan Desert glass 589-1 589-2 589-3
2.17 0.23 0.18
different amplifications (fig. 3). Not only the base peaks, but also the cracking patterns, obtained from pure gases, were considered to identify the gases. N2, e.g., is dissociated in the ion source to form a 2+ fixed percentage of N + and N 2 . By this, noting the mass 28/14 ratio, the amounts of N2 and CO can be determined. It might have been doubtful to relate peaks at mass 48 to SO2 ; however, from the mass ratio 64/48 this assignment became obvious. The results of 87 Muong Nong glass (MNG) and 34 Libyan Desert glass (LDG) single step analyses are given as N2/Ar, N2/02 and CO2/Ar ratios in
figs. 4a and 4b. From the cracking patterns discussed above, an average C 0 2 / C 0 ratio of 25+_~° for all MNG specimens is obtained. For LDG this ratio exceeds 200. In samples of both glasses, SO2 was found with an average N2/SO2 = 930 in MNG and N2/SO2 = 330 in LDG. In one MNG sample a trace of NO was detected. The total bubble volume of each sample was determined from the amount of helium (table 1) assuming equilibrium of the He partial pressure between bubbles and atmosphere [12]. A correction is required for the He-diffusion losses
224
E. Jessberger, W. Gentner, Gas inclusions in glasses
Table 2 Isotopic and elemental abundances of the heavy rare gases in Muong Nong glass and Libyan Desert glass bubbles compared with atmospheric ratios. (Av): weighted average ratio from 6 Muong Nong glass and 3 Libyan Desert glass specimens, respectively.
40Ar 36Ar
36At 38Ar
82Kr 84Kr
83Kr 84Kr
86Kr 84Kr
129Xe 131Xe 134Xe 136Xe 36At 132Xe 132Xe 132Xe 132Xe 84Kr
84Kr 132Xe
Muong Nong Glass (av.)
293
5.5
0.198
0.204
0.294
0.99
0.86
0.39
0.34
55.6
28.1
Libyan Desert Glass (av.)
297
5.3
0.198
0.198
0.286
0.98
0.77
0.40
0.32
55.4
25.9
Atmosphere
296
5.35
0.204
0.203
0.305
0.99
0.79
0.39
0.33
48.5
27.7
caused by preheating. It was estimated to be ~ 60% by comparing the He amounts released from single optically measured bubbles in heated and non-heated samples. Then 10 -8 cm 3 STP 4He correspond to (7.6 -+ 2) mm a bubble volume. With this calibration, the He content and the released gas amounts added for all steps yield average partial pressures as well as the total pressure for all bubbles in one sample (table 1). Table 2 contains the isotopic and elemental abundances of rare gases together with the corresponding atmospheric ratios. All deviations from atmospheric ratios are within the experimental errors.
4. Discussion The most striking result of this work is the obvious connection between atmospheric and gas inclusion abundance pattern~ Though the elemental and molecular abundances are not simply atmospheric for the active components, the more inert gases N2, At, Kr and Xe occur roughly in atmospheric ratios (table 2, fig. 4). Isotopic ratios of rare gases are also atmospheric within the limits of error (table 2). This suggests atmospheric gases as the primary source for the gas inclusions. Deviations, especially for active gases, are due to secondary alterations or to an admixture from an additional source. The N2/Ar ratio of about 90 found in all experiments is unique for the earth's atmosphere. It is not changed by secondary reactions since both gases are chemically inactive. For most of the bubbles, the N2/O: ratio exceeds the atmospheric value. This is
easily explained by oxidation processes. On the other hand, it is hardly by chance that the lowest N2/O2 ratios correspond to the atmospheric ratio. The influence of chemical reactions is further illustrated by the variable abundances of CO2, CO, SO2 and NO, which are at least in part the final products o f these reactions. The high total abundance of CO2, especially in MNG, requires an additional source, most probably the decomposition of bedrock carbonates caused by the impact. MNG and LDG are similar with regard to the more inert gases (table 2); however, the total CO2 content is about 15 times higher for MNG than for LDG (fig. 4). This further supports the presence of atmospheric gases as a primary gas component during the formation of both types o f glasses, while the CO2 is an admixture depending on the chemical composition of the bedrock. The varying gas composition for adjacent bubbles of one specimen, determined by stepwise crushing, indicates a rather complex mechanism of gas inclusion. We suggest that inclusion occurred in more than one stage. At first, gases evolved from bedrock may have predominated, while the atmosphere became important in a later stage. Since the differences between adjacent bubbles are preserved over long time periods, it is clear that"the bubbles were in general vacuumtight, thus preventing gas exchange. This was already noted by Z~ihringer [1] for rare gases. Among other reasons, this fact excludes gas penetration into bubbles after deposition. (Assumption o f J.O'Keefe, private communication.) From the total pressures involved and the high
E. Jessberger, I¢. Gentner, Gas inclusions in glasses
C O 2 - c o n t e n t it seems m o s t likely that the f o r m a t i o n o f MNG and LDG t o o k place within a gas phase which was in itself a result o f an impact and which was n o t in equilibrium w i t h the earth's atmosphere. The atmospheric abundance ratios o f inert gases found in Muong N o n g glasses provide a further similarity b e t w e e n MNG and genuine Indochinites. Again it seems reasonable to c o n c l u d e that M u o n g Nong glass and Indochinites are p r o d u c e d by the same impact event taking place on earth 700 000 yr ago.
Acknowledgement The samples # 572 were kindly d o n a t e d b y Professor P. Pellas, Paris.
References [ 1 ] J. Z~/hringer, K-Ar measurements of tektites, IAEA, Vienna, Radioactive Dating (1963) 289. [2] J. Ziihringer and W. Gentner, Radiogenic and atmospheric At-content of tektites, Nature 199 (1963) 583. [3] V.E. Barnes, Variation of petrographic and chemical characteristics of indochinite tektites within their strewn-field, Geochim. Cosmochim. Acta 28, 1 (1964) 893.
225
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