taken up using a double valve system (pipette) from a calibration canister, and the
40 Ar and
36 Ar contents are
measured.
The results of the analysis by mass spectrometry are shown
in the following table.
40
Ar in volts
36
Ar in millivolts
Sample (LP-6)
8.921
3.110
Atmospheric reference
7.637
28.252
Calibrated dose (3 doses)
7.637
Fig. 5.6 Principle of the K-Ar
method with no tracer. Two
isotopes of argon (
40
Ar and
36
Ar)
are measured by mass
spectrometer. The sample gas is
composed of atmospheric argon
40
Ar at and
36
Ar at , as well as
radiogenic argon
40
Ar
*
, resulting
from the decay of
40
K.
s = sample; at = atmospheric,
cd = calibrated dose
Fig. 5.7 Calibration curve of the
mass-spectrometer on the 19th of
November 2001
Table 5.2 Values of mineral
standards
Standard
K (%)
40
Ar
* (10
−9 mol/g)
Age (Ma)
References
HD-B1
7.987
0.335
24.03 ± 0.41
Fuhrmann et al. (1987)
GL-O
6.56
1.093
93.60 ± 0.90
Charbit et al. (1998)
LP-6
8.37
1.923
127.8 ± 1.4
Odin (1982)
SB-3
7.483
2.213
162.9 ± 0.9
Lanphere and Dalrymple (2000)
5 The
40
K/
40
Ar and
40 Ar/
39
Ar Methods
79