Diffusion of sup 40 Ar and sup 39 Ar in irradiated orthoclase

Journal Article · Thu Nov 01 04:00:00 EST 1990 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(90)90130-D· OSTI ID:6106111

Foland, K A; Xu, Yuping ^[1]

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1. Ohio State Univ., Columbus (USA)

The important concerns of whether neutron irradiation affects Ar diffusion behavior in minerals and whether the diffusivities of natural radiogenic {sup 40}Ar and induced {sup 39}Ar are identical are considered. Both issues are addressed with isothermal and incremental-heating experiments on natural, homogeneous orthoclase from Benson Mines which was subjected to irradiation similar to typical {sup 40}Ar/{sup 39}Ar measurements. Previous study of this feldspar shows that laboratory {sup 40}Ar loss occurs by volume diffusion with physical grain sizes as the effective transport dimensions following a single Arrhenius relation. Isothermal-heating experiments on irradiated feldspar show the same loss and apparent {sup 40}Ar diffusion coefficients, within uncertainty, as unirradiated sample. For these heating times and temperatures, the experiments indicatet that the defects accompanying irradiation have only very minor, if any, effects on Ar behavior with regard to both diffusion kinetics and effective transport domains and that {sup 39}Ar and {sup 40}Ar diffusivities do not differ radically. Step-heating experiments yield essential flat spectra but with minor yet significant apparent discordance which is the same for different grain sizes. The spectra show high ages for small, initial low-temperature fractions and a slight increase in age with progressive Ar release. While these variations are potentially explicable by sample heterogeneities, the preferred explanation is that they result from {sup 39}Ar recoil loss and a slightly higher diffusivity of {sup 39}Ar relative to {sup 40}Ar. The results have important implications for diffusion processes in feldspar and the application of the {sup 40}Ar/{sup 39}Ar method.

• OSTI ID: 6106111
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 54:11; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALKALI METAL COMPOUNDS
ALUMINIUM COMPOUNDS
ALUMINIUM SILICATES
ARGON 39
ARGON 40
ARGON COMPOUNDS
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
CHEMISTRY
CRYSTAL STRUCTURE
DIFFUSION
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FELDSPARS
GEOCHEMISTRY
IRRADIATION
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MINERALS
NUCLEI
ORTHOCLASE
OXYGEN COMPOUNDS
POTASSIUM COMPOUNDS
POTASSIUM SILICATES
RADIATION EFFECTS
RADIOISOTOPES
RARE GAS COMPOUNDS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
STABLE ISOTOPES
YEARS LIVING RADIOISOTOPES