Mass-Spectrometric Analysis
• Stage 1: pumping—pre-degassing. The sample (crystal
or microcrystalline groundmass) is placed in a vacuum
chamber including a crucible in the case of fusion in a
furnace, and a viewport in the case of fusion by Laser
CO 2 ), then placed under secondary high vacuum by
means of turbo-molecular pumps. The sample is then
heated to about 500 °C. This first gas extracted is eliminated by pumping.
• Stage 2: fusion. The sample is melted either by a CO 2
laser or in a double vacuum resistance furnace. In the case
of fusion by laser, it applies essentially to the analysis of
single crystals and to small groups of crystals (about 5–15
grains). The furnace is used for the analysis of microcrystalline groundmass. As seen before, this allows a step
by step fusion, and hence a gradual degassing of the
sample required for the ‘step-heating’ method.
• Stage 3: purification. The gas extracted from the sample
is purified by the combined effect of Getters pumps and a
titanium sublimation pump.
• Stage 4: measurement by mass spectrometer. After
purification, the gas is introduced into the mass spectrometer. The purified gas is measured using a
high-sensitivity noble gas GV5400 instrument operated in
ion-counting mode. One analytical run consists of 20
peak scans of each argon isotope with integration times of
1 s (
40 Ar,
39 Ar) or 10 s (
36 Ar,
37
Ar,
38 Ar, baseline), first
preceded by a peak centering routine on the five Ar isotopes, upon admission of the sample into the
mass-spectrometer. The precision and accuracy of the
mass discrimination correction is monitored by periodical
measurements of air argon. This monitoring is performed
using a dedicated air-calibration system featuring a 6 L
tank filled with purified atmospheric argon. This tank is
connected to the mass spectrometer vacuum line via two
pneumatically- actuated air pipettes of approximately 0.1
and 1.0 cc. This system allows for a 1 cc (e.g. 600,000
counts s
−1 (cps on
40 Ar)) and a 0.1 cc (e.g. 70 000 cps on
40 Ar) atmospheric aliquots to be delivered into the mass
spectrometer and permits a careful monitoring of the mass
discrimination over a wide dynamic range. The mineral
standards used to calculate the flux are analyzed in the
same way as the ordinary samples.
Calculation of Age
Determination of the J factor (Neutron flux received
during irradiation)
J is determined for each sample. See the following analysis
of a ACR-2 standard grain (1.194 Ma) subjected to a fast
neutron flux for 30 min (Osiris reactor, CEA Saclay):
40
Ar
39
Ar
38
Ar
37
Ar
36 Ar
Measured
(mV)
9.246 Â 10
−3
2.321 Â 10
−3
3.576 Â 10
−5
1.188 Â 10
−6
1.375 Â 10
−6
Blank (mV)
2.351 Â 10
−5
2.586 Â 10
−7
2.238 Â 10
−8
6.672 Â 10
−7
2.383 Â 10
−7
Corrected
measurement
2.312 Â 10
−3
7.967 Â 10
−4
1.234 Â 10
−5
3.161 Â 10
−7
2.981 Â 10
−6
Using Eq. (5.9), J can be calculated by setting the
40 Ar
* /
39 Ar K ratio or R e as follows:
Re ¼
40 Ar=
39 Ar
½
m À
40 Ar=
36 Ar
Â
Ã
A
36 Ar=
39 Ar
Â
Ã
m
þ
40 Ar=
36 Ar
Â
Ã
A
36 Ar=
37 Ar
Â
Ã
Ca
37 Ar=
39 Ar
½
m
1 À 39 Ar= 37 Ar
½
Ca
37 Ar= 39 Ar
½
m
À
40 Ar
39 Ar
!
K
with m: measured ratios (see table above).
[
40 Ar/
36 Ar] A = atmospheric reference ratio = 292.8 (for this
sample);
[
36 Ar/
37 Ar] Ca = (given by calcium salt) 5.60 Â 10
−4 ;
[
39 Ar/
37 Ar] Ca = (given by calcium salt) 6.95 10
−4 ;
[
40 Ar/
39 Ar] K = (given by potassium salt) 3.52 10
−3 ;
R e ¼
3:974 À 292:8 Â 4:898 Â 10
À4 þ 292:8 Â 5:60 Â 10
À4 Â 5:09 Â 10
À2
1 À 6:95 Â 10 À4 Â 5:09 Â 10 À2
À 3:52 Â 10
À3 ¼ 3:8355
If t s = 1.194 Ma and k = 5.543 Â 10
−10 ; J is then calculated as follows:
J ¼ e
1:194Â0:0000000005543
À 1
À
Á =3:8355 ¼ 1:726 Â 10
À4
The measurement is repeated on at least three grains to
quantify any possible external errors originating from the
heterogeneity of the age standard (*1%). A weighted
84
H. Guillou et al.
• Stage 1: pumping—pre-degassing. The sample (crystal
or microcrystalline groundmass) is placed in a vacuum
chamber including a crucible in the case of fusion in a
furnace, and a viewport in the case of fusion by Laser
CO 2 ), then placed under secondary high vacuum by
means of turbo-molecular pumps. The sample is then
heated to about 500 °C. This first gas extracted is eliminated by pumping.
• Stage 2: fusion. The sample is melted either by a CO 2
laser or in a double vacuum resistance furnace. In the case
of fusion by laser, it applies essentially to the analysis of
single crystals and to small groups of crystals (about 5–15
grains). The furnace is used for the analysis of microcrystalline groundmass. As seen before, this allows a step
by step fusion, and hence a gradual degassing of the
sample required for the ‘step-heating’ method.
• Stage 3: purification. The gas extracted from the sample
is purified by the combined effect of Getters pumps and a
titanium sublimation pump.
• Stage 4: measurement by mass spectrometer. After
purification, the gas is introduced into the mass spectrometer. The purified gas is measured using a
high-sensitivity noble gas GV5400 instrument operated in
ion-counting mode. One analytical run consists of 20
peak scans of each argon isotope with integration times of
1 s (
40 Ar,
39 Ar) or 10 s (
36 Ar,
37
Ar,
38 Ar, baseline), first
preceded by a peak centering routine on the five Ar isotopes, upon admission of the sample into the
mass-spectrometer. The precision and accuracy of the
mass discrimination correction is monitored by periodical
measurements of air argon. This monitoring is performed
using a dedicated air-calibration system featuring a 6 L
tank filled with purified atmospheric argon. This tank is
connected to the mass spectrometer vacuum line via two
pneumatically- actuated air pipettes of approximately 0.1
and 1.0 cc. This system allows for a 1 cc (e.g. 600,000
counts s
−1 (cps on
40 Ar)) and a 0.1 cc (e.g. 70 000 cps on
40 Ar) atmospheric aliquots to be delivered into the mass
spectrometer and permits a careful monitoring of the mass
discrimination over a wide dynamic range. The mineral
standards used to calculate the flux are analyzed in the
same way as the ordinary samples.
Calculation of Age
Determination of the J factor (Neutron flux received
during irradiation)
J is determined for each sample. See the following analysis
of a ACR-2 standard grain (1.194 Ma) subjected to a fast
neutron flux for 30 min (Osiris reactor, CEA Saclay):
40
Ar
39
Ar
38
Ar
37
Ar
36 Ar
Measured
(mV)
9.246 Â 10
−3
2.321 Â 10
−3
3.576 Â 10
−5
1.188 Â 10
−6
1.375 Â 10
−6
Blank (mV)
2.351 Â 10
−5
2.586 Â 10
−7
2.238 Â 10
−8
6.672 Â 10
−7
2.383 Â 10
−7
Corrected
measurement
2.312 Â 10
−3
7.967 Â 10
−4
1.234 Â 10
−5
3.161 Â 10
−7
2.981 Â 10
−6
Using Eq. (5.9), J can be calculated by setting the
40 Ar
* /
39 Ar K ratio or R e as follows:
Re ¼
40 Ar=
39 Ar
½
m À
40 Ar=
36 Ar
Â
Ã
A
36 Ar=
39 Ar
Â
Ã
m
þ
40 Ar=
36 Ar
Â
Ã
A
36 Ar=
37 Ar
Â
Ã
Ca
37 Ar=
39 Ar
½
m
1 À 39 Ar= 37 Ar
½
Ca
37 Ar= 39 Ar
½
m
À
40 Ar
39 Ar
!
K
with m: measured ratios (see table above).
[
40 Ar/
36 Ar] A = atmospheric reference ratio = 292.8 (for this
sample);
[
36 Ar/
37 Ar] Ca = (given by calcium salt) 5.60 Â 10
−4 ;
[
39 Ar/
37 Ar] Ca = (given by calcium salt) 6.95 10
−4 ;
[
40 Ar/
39 Ar] K = (given by potassium salt) 3.52 10
−3 ;
R e ¼
3:974 À 292:8 Â 4:898 Â 10
À4 þ 292:8 Â 5:60 Â 10
À4 Â 5:09 Â 10
À2
1 À 6:95 Â 10 À4 Â 5:09 Â 10 À2
À 3:52 Â 10
À3 ¼ 3:8355
If t s = 1.194 Ma and k = 5.543 Â 10
−10 ; J is then calculated as follows:
J ¼ e
1:194Â0:0000000005543
À 1
À
Á =3:8355 ¼ 1:726 Â 10
À4
The measurement is repeated on at least three grains to
quantify any possible external errors originating from the
heterogeneity of the age standard (*1%). A weighted
84
H. Guillou et al.