Other
Scientific paper
Nov 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.3147f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 11, pp.3147-3158
Other
6
Scientific paper
The important concerns of whether neutron irradiation affects Ar diffusional behavior in minerals and whether the diffusivities of natural radiogenic 40 Ar and induced 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 40 Ar / 39 Ar measurements. Previous study of this feldspar shows that laboratory 40 Ar loss occurs by volume diffusion with physical grain sizes as the effective transport dimensions following a single Arrhenius relation. Isothermal-heating experiments (600 to 800°C, 4 to 21 days) on irradiated feldspar show the same loss and apparent 40 Ar diffusion coefficients, within uncertainty, as unirradiated sample. For these heating times and temperatures, the experiments indicate 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 39 Ar and 40 Ar diffusivities do not differ radically. The results suggest intrinsic control of Ar diffusion by a single mechanism for all isotopes, regardless of origin, in this specimen. Step-heating experiments yield essentially 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 39 Ar recoil loss and a slightly higher diffusivity of 39 Ar relative to 40 Ar. The relative behavior is consistent with a kinetic mass effect in which the diffusion rates of 39 Ar and 40 Ar are inversely proportional to the square root of mass according to rate theory. The results have important implications for diffusion processes in feldspar and the application of the 40 Ar / 39 Ar method.
Foland Kenneth A.
Xu Yuping
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