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Dating fine-grained sericite and illite: new ideas in interpretation of 40Ar/39Ar age spectra using sample encapsulation - Abstract
The potassium-argon (K-Ar) and argon-argon (40Ar/39Ar) methods are commonly used to determine the ages of sericite and other potassium bearing mica/clay minerals. A problem in dating of fine-grained (<20 microns) clay-like material is that the K-Ar age is often younger than that from other geochronometers (e.g., Rb-Sr, biostratigraphy). This may be due to loss of 40Ar in nature either by ejection of the 40Ar atom by “recoil” during the decay of 40K, or by diffusion from the “leaky” structure of these minerals resulting in an closure temperature lower (<150°C) than that determined from laboratory diffusion experiments.
As part of the 40Ar/39Ar dating method, samples are irradiated in a nuclear reactor to produce 39Ar from 39K. This allows both the parent and daughter to be measured simultaneously in the mass spectrometer and allows for step-heating and other experiments to assess the loss of 40Ar during geologic time. For fine-grained minerals, the energy of the 39K to 39Ar transmutation during irradiation may cause 39Ar to recoil from the mineral. For most minerals, the grain size is sufficiently large enough that recoil is insignificant. However for clays and sericites, up to 40% of all 39Ar can be lost by recoil. Because age is proportional to 40Ar/39Ar, this can significantly bias the age by the 40Ar/39Ar method (compared to the K-Ar age) if this recoiled 39Ar is not measured. We present a process of encapsulating a sample in a quartz ampoule prior to sample irradiation that allows for the capture of recoil 39Ar and the ability to measure this gas. The ratio of the radiogenic 40Ar in a sample to the total 39Ar (including the “captured” argon) would be proportional to the K-Ar age of the sample.
The “retention age” model for interpreting argon data (Hall et al., 2000, Econ. Geol., v. 95, p 1739–1752) is based on the idea that 40Ar lost in nature is proportional to the 39Ar lost in sample irradiation. Thus, the ratio of the total amount of 40Ar in a sample to the 39Ar in the sample following irradiation excluding the encapsulated 39Ar (the integrated age of an unencapsulated sample) would yield the most geologically meaningful information and is more consistent with other geochronometers.
We present results from encapsulated and unencapsulated sericite, illite, biotite and muscovite samples, illustrating the effect of recoil on an age spectrum and the application of the retention age model for determining “true” ages of fine-grained material. In general, large (>100 micron), well crystallized, biotite and muscovite have negligible (<0.3%) 39Ar recoil loss, while sericites and illites range from 5% to more than 40% 39Ar recoil loss. With minor loss, classic “plateau analysis” and “isochron analysis” of 40Ar/39Ar spectra yield geologically “valid” results and can identify “reset” events. For the finer grained minerals with complex geologic histories, these approaches need to be modified to reflect the redistribution of 39Ar within the mineral, however cooling (retention) and reset ages can still be determined.
Acknowledgments and Associated Footnotes
2 Jeff Drake: Department of Geology & Geophysics, and Geophysical Institute, University of Alaska, Fairbanks, AK
3 Walt Munly: Department of Geology & Geophysics, and Geophysical Institute, University of Alaska, Fairbanks, AK
Copyright © 2014 by the Alaska Geological Society