FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2
New Noble Gas Isotope Analysis System: Design and Implementation
Hajimu Tamura1, Keiko Sato1, Takeshi Hanyu2 and Hidenori Kumagai3
1 Research
Program for Data and Sample Analyses, Institute for Frontier Research on Earth Evolution (IFREE)
Program for Geochemical Evolution, Institute for Frontier Research on Earth Evolution (IFREE)
3 Program for Deep Sea Research, Institute for Frontier Research on Earth Evolution (IFREE)
2 Research
1. Specification of Chronos
ure temperature of the crucible but emissivity of the crucible
changes during operation due to the accumulating glasses of melted samples. Temperature is usually controlled with electric power
and temperature at the outside surface of the core tube. The temperature at the outside surface of the core tube is measured by a
HT-THERMIC high temperature sheath thermocouple of Yamari
Industries, Ltd. A W5%Re-W26%Re thermocouple wears BeO
solid insulator and Mo sheath.
The crushing extraction subsystem contains two crushing features (Fig. 1c). One is an electromagnetic crusher consisting of a
38 mm internal diameter stainless tube, a 600 g magnetic stainless
hammer, a driving coil as an electromagnet by Nihon Denjisokki
Co., Ltd., and a Takasago, Ltd. EX-750L2 regulated power supply
as the power source of the electromagnet and another is a valve
modified crusher. A Stanford Research Systems, Inc. RGA200
residual gas analyzer is mounted on the crushing extraction subsystem to measure the amount of carbon dioxide and nitrogen. A
10 cm3 volume of the extracted gas is divided for measuring carbon dioxide and nitrogen and residual gas is exposed to a titanium-zirconium getter.
The noble gas isotope analysis system for Chronological Study
- Chronos - has been launched for K-Ar dating and isotope analyses for helium, neon and argon.
Chronos consists of following subsystems; extraction, standard
gas supply, purification, and mass spectrometry. Two extraction
subsystems with one initial purification stage, six standard gas
tanks, one two-stages purification subsystem, and one NOBLE
GAS 5400He static single focus sector type mass spectrometer are
included in the system for all-purpose on noble gas isotope analysis for rock samples. Figure 1 is the schematic drawing of Chronos.
The ultra high vacuum atmosphere is required for all of the system because of low abundance of noble gas elements and relatively
high abundance in air. Stainless steel tubes and copper gasket conflat flanges are employed overall system to realize low leak rate of
at most 10-11 ccSTP/min and baking up to 250 ºC. Some components should stand higher temperature and gold ring sealing is
employed. 12.7 mm outer diameter stainless tubes and Thermo
Vacuum Generators ZCRD20R all metal angle valves are
employed to reduce the total volume of the system.
The mass spectrometer, NOBLE GAS 5400He of GV
Instruments, is the only commercial mass spectrometer for noble
gas isotope analysis. It has a 27 cm radius magnetic sector for
mass analysis, an electron ion source, and two ion collectors to
mount various detectors [Haines et al., 2001]. One Faraday cup
detector on the high mass side collector (the High Faraday) and a
Daly detector (the Daly) and an ion counting detector on the axial
collector were chosen. Mass resolution of 600 for the 10% valley
definition is realized in type 5400He to eliminate the effect of HD
and H3 from 3He on helium measurement.
The ion source works on the condition of trap 400 µA and
approximate 4.5 kV accelerating voltage. Other variables are
sometimes tuned.
The purification subsystem has two purification stages (Fig. 1a,
1b: V2, V3). As described in figure 1b, each stage has one titanium-zirconium getter (Ti-2, Ti-3) and one activated charcoal trap
(CH1 and CH2). V3 also has one cold SORB-AC getter of SAES
NP-10 and one sintered stainless sieve trap with Advanst
Research Systems CSW-204N refrigerator (Cryo Trap).
Two extraction subsystems are included in Chronos; a heating
extraction subsystem and a crushing extraction subsystem. Each
extraction subsystem has one titanium-zirconium getter for initial
purification.
The heating extraction subsystem employing TH-250T resistance furnace of Horiguchi Iron Works realizes over 2000 ºC in the
crucible. This is called as the tantalum furnace because the heater
and the core tube are made of tantalum. A glass sample holder is
mounted at the top of the tantalum furnace. A molybdenum crucible is sited in the core tube. A pyrometer is employed to meas-
2. Standard gases
Chronos is to use both for the K-Ar dating and the noble gas
isotope analysis. Hence, different standard gases are required for
the isotope analysis of helium, of neon and heavier noble gases,
and for the argon analysis for K-Ar dating.
The basic standard gas for the noble gas isotope analysis is a
diluted air. The most abundant noble gas element in air is argon
which is one million times abundant than xenon of the least abundant noble gas element. A diluted air which contains enough
xenon for calibration of discrimination involves too much argon
for measurement. Three air derived standards, AS, NS, XS were
preserved for Chronos with different densities. AS is for K-Ar
dating, NS is for calibration for neon isotope analysis and XS is
for argon, krypton, and xenon isotope analysis. NS does not contain enough xenon and XS contains too much neon.
Making air derived standard gases, a 500 cm3 tank of air (1 atm)
was sampled in Akiruno, Tokyo and was mounted to the standard
gas supply subsystem. One pipette of the Akiruno air was expanded
to Vstd, V2 and V1 then was purified by Ti-1 and Ti-2. One pipette
of the purified gas was expanded into AS tank. The residual was
expanded into XS tank first and NS tank second. The gas in NS
tank was expanded to Vstd, V2, and V1 to adjust density.
Helium isotope ratios of usual samples are higher than that of
air and non-air derived standard gases are used for calibration.
Two standard gases were preserved for Chronos. One is Helium
Standard of Japan, HESJ [Matsuda et al., 2002] and another is
Kaminoyama Helium Standard [Kumagai, 1999]. HESJ was pro1
FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2
duced from the original HESJ tank of ERI, Tokyo University and
stored into HS tank. Kaminoyama Helium Standard was produced
from a tank from RINS, Okayama University of Science and
stored into KS tank.
AS is used for both calibration of mass discrimination and sensitivity determination. The argon content of original one pipette
and the volume of the pipette were determined. 12 measurements
of standard rock samples gave the original content of
4.628±0.511×10-8 cm3STP for 40Ar. Over 50 measurements of AS
gas gave the pipette volume of 1.3578cm3. The pipetting trend is
shown in Figure 2 and the data points relatively largely scatter
around the regression line. The argon background of the system is
negligibly small to the content of AS, thus this scattering is not
the result of unstable background. There is 5% of standard deviation for the sensitivity of argon, and this balances out to the scattering of AS data points.
The stored non-air derived helium standard gases were calibrated by an air derived helium gas. An diluted air was prepared with
an activated charcoal trap and the cryo trap to remove heavier
noble gases including neon, then was stored into a 3 L stainless
tank as the air derived helium standard gas. It is indicated that
apparent 3He/4He ratio changes logarithmic to 4He intensity in
Chronos and apparent 3He/4He ratios were corrected depending on
this relationship [Tamura et al., 2005]. 9 measurements of HS gas
gave 3He/4He ratio of 2.832±0.020×10-5 and 23 measurements of
KS gas gave that of 7.780±0.198×10-6 (Table 1).
4. Basic Procedure of Noble Gas Analysis
A suite of experiments must be planned including analyses of
standard gases and known rock samples. As described above, AS
is supplied to evaluate the stability of argon sensitivity, XS and
NS are supplied to determine the discrimination for heavier noble
gases, as HS and KS for helium. It is recommended that the suite
includes helium dilution experiments to determine the variation of
discrimination factor for 3He/4He ratio as pointed by Tamura et
al. [2005] Analytical blanks also must be determined in the same
condition as sample analyses.
Neon must be removed from helium-neon mixture by a sintered stainless sieve trap at 20K for 10 minutes after removing of
heavier gases by a charcoal trap chilled by liquid nitrogen. Neon
isotope ratios are varied in helium rich condition as pointed by
Hiyagon [Hiyagon, 1989]. Longer trap period causes adsorption
of helium to the trap and increase uncertainty of abundance and
isotope ratio of helium. If there is too much helium such as over
2×10-7 Torr in V2 + V3, helium in the trap volume should be
released until it reduces to 2×10-7 Torr in V2 + V3 or less before
release of neon. Trapped neon is released from the trap at 50K.
40Ar and 12C16O must be monitored in neon analysis for later cor2
rection. A charcoal trap cooled to liquid nitrogen temperature is
left opened to reduce these peaks interfering neon measurement.
Argon and heavier two noble gases also must be separated to prevent rich 40Ar disturbing xenon (and krypton) analysis. Xenon is
trapped to a charcoal trap at 250K and almost argon is released. A
volume proportional argon remains in the trap and it is recommended to lade out argon until the residual 40Ar is reduced to 2 ×
10-8 Torr as partial pressure in the V2 + V3 volume.
3. Volumetry
The absolute and the relative volumes of each part of the system are important information. A sample gas is lost on a measurement by separating a part of the system to prevent contamination
and unexpected sample loss. The residual sample material in the
extraction subsystem sometimes absorbs sample gases or discharges contaminant gases. Hence, the valve between the extraction subsystem and the purification subsystem is closed after
extraction and a part of helium and neon is lost. Helium is also
lost in the helium-neon separation. On the other hand, a sample is
sometimes diluted because it has so high density that the detectors
cannot measure. The volume information is necessary to account
these losses.
The basic volumetry method is as follows. A known volume is
mounted to the system and gas with known pressure is filled in the
volume. Then the gas is expanded to each part of the system and
the pressure is measured. The capacitance manometer is used to
measure the pressure. This method has serious problems. The air
is used as the gas which has known pressure (1 atm), but the air is
absorbed by titanium-zirconium getters, activated charcoals, and a
SORB-AC getter pump. There are various volumes in the system,
such as some cubic centimeters for the minimum and some liters
for the maximum. The pressure changes a little when the gas is
expanded from a large volume to a small volume, thus poor precision can only get in this situation. Two times volumetries were
performed in this method and the result are Table 2.
Alternative relative volumes of frequently used pairs are determined using standard gases. The result is shown in Table 3. A systematic sensitivity change depending on intensity is exhibited for
Chronos [Tamura et al., 2004] and these alternative relative volumes include the effect of sensitivity change.
5. Basic Correction Methods
The intensity of a peak declines in exponential low usually.
Hence, the intercept of an exponential curve or a line is employed
as the representative value of peak intensity. The average of peak
intensities is also employed in extremely low intensity of which
actual values show so wide distribution that impossible to determine a trend.
The ratio of peaks is usually represented by the average but
varies linearly in a few cases. The intercept of the line is
employed in the latter cases.
At first the variation of helium isotope ratio should be determined. As Tamura et al. [Tamura et al., 2005] says, a log-linear
equation is derived from the helium dilution experiment of standard helium gases:
Rv = 1 + alog10V
where Rv is the ratio of an actual 3He/4He value at given 4He
intensity and the real 3He/4He value. This equation gives correction factors for 3He/4He ratio corresponding given 4He intensities.
All helium isotope ratios are arranged to the value at 1 V.
Corrections for peak overlapping are also corrected in this
stage. As described above, 40 Ar 2+ interference to 20 Ne and
12C16O 2+ to 22Ne are monitored. The ratio of 40Ar2+ / 40Ar+ is 0.3
2
as 12C16O22+ / 12C16O2+ is 0.01. The effect from 12C16O2 is almost
negligible.
Mass discrimination correction factors are determined for
every isotope ratios measured as averages of the results of stan2
FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2
dard gas analyses. In noble gas analysis, mass discrimination is
constant and corrected with discrimination factors derived from
the average in the suite of analyses [Ozima and Podosek, 2002].
Current discrimination factors for helium, neon and argon are
shown in Table 4.
Analytical blanks are also checked in this stage. As the result
of the online, sequential preparation, the blanks easily vary by history of measurements or treatments at sample loading. Typical
sets of analytical blanks are shown in table 5 for each of heating
and crushing (‘cold’ is essential blank for purification subsystem),
but over 1.5×10-9 cm3 STP of 40Ar residual was observed for a
fresh crucible, or 8Ra helium was observed with 5×10-9 cm3 STP
4He after crushing of a MORB glass sample. Low blanks for heating are realized by over two hour baking at 1800 - 2000°C after
sample loading and one hour baking at 1800 - 2000°C after every
analyses. For crushing, several sets of crushing tubes and hammers are applied depending on the sample characteristics.
Contents of noble gases are estimated from intensities and sensitivity derived from analyses of standard rock samples and AS
standard air after corrections above. Sensitivity of argon in
February 2004 is displayed in Sato et al. [Sato et al., 2005] A typical set of relative sensitivities to argon for each noble gas elements are displayed on table 6. Sensitivities should be determined
for each suite of experiments. Recommended amounts of sample
gas for each noble gas elements are shown in table 7.
Ozima, M. and F. A. Podosek, Noble Gas Geochemistry, 286 pp.,
Cambridge University Press, Cambridge, United Kingdom, 2002.
Sato, K., H. Tamura, H. Kumagai and T. Hanyu, Application of K-Ar
dating system to be performed by new noble gas mass spectrometry and its calibration from standard air analysis, In Frontier
Research on Earth Evolution, IFREE Report 2003-2004, 2, 2005.
Tamura, H., H. Kumagai, K. Sato and T. Hanyu, Systematic
Discrimination for 3He/4He depending on 4He Intensity, J. Mass
Spectrom. Soc. Jpn., submitted on January 20th 2005.
Tamura, H., T. Hanyu, K. Sato and H. Kumagai, Evaluation for
Sensitivity Linearity of 5400He Noblegas Mass Spectrometer in
Yokosuka HQ, JAMSTEC (No.SV042), 2004 Annual Meeting of
Isotope Ratio Division of Mass Spectrometry Society Japan
Annual Meeting of Isotope Ratio Division of Mass Spectrometry
Society Japan, Mass Spectrometry Society Japan, Yamagata,
Japan, 24-26 November, 2004.
6. Conclusion
Chronos, the noble gas isotope analysis system of IFREE, has
been designed and implemented for the noble gas isotope analysis
and K-Ar dating. The basic procedure of the analysis for helium,
neon and argon isotopes and the argon analysis for K-Ar dating
are minimally established.
Acknowledgments. We thank Prof. Ichiro Kaneoka of ERI, Tokyo
University and Prof. Hironobu Hyodo of RINS, Okayama University
of Science for their supply of helium standards. Prof. Ichiro Kaneoka
also supplied his mineral standard EB-1 for us. We also thank
Professors Kazuo Saito and Naoyoshi Iwata of Yamagata University
for their supply of various rock and mineral standards for K-Ar dating,
Professors Kyoichi Ishizaka and Takahiro Tagami of Kyoto
University for their supply of Bern4B biotite standard for K-Ar dating,
Dr. Kozo Uto, Dr. Tomoaki Sumii, and Dr. Masafumi Sudo of GSJ for
their supply of SORI93 biotite standard for K-Ar and Ar-Ar dating.
References
Kumagai, H., Variation of noble gas signatures controlled by tectonic
conditions and magmatic processes: a case study for an area
around the Rodriguez Triple Junction in the Indian Ocean, Ph.D
thesis, 129 pp, Tokyo University, December 1999.
Matsuda, J., T. Matsumoto, H. Sumino, K. Nagao, J. Yamamoto, Y.
Miura, I. Kaneoka, N. Takahata, and Y. Sano, The 3He/4He ratio
of the new internal He Standard of Japan (HESJ), Geochemical
Journal, 36, 191-195, 2002.
Haines, R. C., A. N. Eaton and S. N. Dudd, SPECIFICATION NOTE
701 Micromass 5400 Static Vacuum Mass Spectrometers,
Micromass UK Ltd., Manchester, UK, Sept. 2001.
Hiyagon, H., Neon Isotope Measurement in the Presense of Helium,
Mass Spectroscopy, 37, 325-330, 1989.
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2
Figure 1. Schematic Drawing of Chronos
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FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 2
Table 2. Volume
Table 3. Alternative relative volumes
Figure 2. Trend of AS
Table 1. Helium standard gases
Table 4. Degree of discrimination for helium, neon and argon
Table 5. Analytical blanks for most abundant isotopes for noble gas elements (cm3 STP)
Table 6. Relative sensitivities to argon
Table 7. Recommended amount for analysis (cm3 STP)
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