Ar/Ar Geochronologie Labor

Leiter

Dr. Masafumi Sudo

E-Mail:
msudo@geo.uni-potsdam.de
Telefon:
+49 331 977 5852
Fax:
+49 331 977 5700
Arbeitsgruppenbild Allgemeine Geologie

Beschreibung

The 40Ar/39Ar dating method is a modification/improvement of the 40K-40Ar method.  The utilized decay system is the electron capture by a 40K parent nuclide and the production of a 40Ar daughter nuclide with a decay constant (Λe) of 0.581x10-10. (In addition, beta-decay of 40K to 40Ca also occurs with a decay constant (Λb) of 4.96x10-10, but this branch is not used.)  For the K-Ar method, direct determination of potassium concentrations on sample aliquots is necessary. In contrast, for the 40Ar/39Ar dating method potassium concentrations are determined indirectly by irradiating the rock or mineral samples in a flux of fast neutrons in a nuclear reactor, resulting in the 39K(n,p)39Ar reaction which produces 39Ar nuclides.  With the J value and decay constants for electron capture and beta-decay, the 40Ar/39Ar age is calculated as follows:

Formel

 

The J value is a combined parameter incorporating the conditions of the neutron irradiation and which is obtained from simultaneously irradiated standard mineral samples of known age.

Due to the long half life of 1.25 x 109 yrs of 40K, the 40Ar/39Ar method can generally be used to determine ages ranging from the age of the Earth to a few ten thousand years before present. 

Foto Aluminium sample holder used for irradiation.

Our present 40Ar/39Ar dating system (see photo at the top of this page) includes: (1) a Micromass 5400 high sensitivity-low background sector-type noble gas mass spectrometer, (2) a New Wave Research DualWave laser system comprising a CO2 continuous laser and an UV pulsed laser, and (3) a ultra-high vacuum metal purification line which includes Zr-Al SAES alloy getters and a cold trap.  The purification line is mainly designed by Dr. Masafumi Sudo, leader of the 40Ar/39Ar laboratory, and is optimized for the analysis of extremely small amounts of argon gas released by laser application.  A single online analysis from laser heating of a sample to the analysis of the argon isotopes takes ca. 25 minutes.  Using the CO2 laser, stepwise heating analyses of grain samples from 0.1 mg to a few tens of mg have been performed in the Potsdam laboratory.  In section samples, the UV pulsed laser is able to ablate spots with minimum diameters of a few tens of micrometers. 

Sample irradiation is presently performed at the Geesthacht Neutron Facility (GeNF) at the GKSS research centre.  Due to the smaller fast neutron flux of 1x1012 n/cm2/s compared to other research reactors, the samples are irradiated for ca. four days in order to produce enough 39Ar atoms.  Grain samples (e.g. mineral or groundmass separates) or polished thick section samples are wrapped in Al foil and subsequently loaded into a sample-container made of 99.999% pure Al (see photo below).  Finally, the sample container is wrapped in Cd foil and irradiated.  Following irradiation, samples are left to cool down at GeNF.  Finally, argon isotope analyses are performed at the 40Ar/39Ar laboratory in Potsdam.

The 40Ar/39Ar ages are obtained as a relative age against a neutron flux monitoring mineral standard, Fish Canyon tuff sanidine, which is irradiated together with samples of unknown ages.  The accuracy of the system has been examined and confirmed by the analyses of some reference materials, such as SORI93 biotite (K-Ar age: 92.6 ± 0.6 Ma; Sudo et al., 1998) or HD-B1 biotite (K-Ar age: 24.21 ± 0.32 Ma; Hess & Lippolt, 1994), shown in the figures below.

Figures
Figures: Results for SORI93 biotite and HD-B1 biotite by stepwise heating.

If you have interest in our 40Ar/39Ar laboratory, please contact Dr. Masafumi Sudo (laboratory leader), Professor Roland Oberhänsli, Professor Patrick O’Brien or Dr. Martin Timmerman (radiation officer) in our institute.

References

  • Hess, J. C. and H. J. Lippolt (1994): Compilation of K-Ar measurements on HD-B1 standard biotite, 1994 status report.  Phanerozoic time Scale, Bull. Liais. Inform. IUGS Subcomm. Geochronol., 122, Paris, 1993
  • Sudo, M, K. Uto, K. Anno, O. Ishizuka and S. Uchiumi (1998): SORI93 biotite: A new mineral standard for K-Ar dating.  Geochem. J, 32, 49-58