Saddle-shaped 40 Ar / 39 Ar age spectra from young, microstructurally complex potassium feldspars

Details Saddle-shaped 40 Ar / 39 Ar age spectra from young, microstructurally complex potassium feldspars Saddle-shaped 40 Ar / 39 Ar age spectra from young, microstructurally complex potassium feldspars

Jun 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1185z&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 50, Issue 6, pp.1185-1199

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Scientific paper

A suite of young potassium feldspars show markedly saddle-shaped 40 Ar / 39 Ar age spectra as a result of incorporating 10 -10 to 10 -9 mol/g of excess 40 Ar. The minima of these age spectra record reasonable cooling ages, based on the known thermal history and geology of the samples. Acid etching of one sample indicates that excess 40 Ar is concentrated near grain margins. The release of a substantial portion of this excess Ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40 Ar. Anion vacancies have been proposed to act in this role in plagioclase, and we speculate that this is so in K-feldspar as well. Such a mechanism would explain the observation that relative to radiogenic 40 Ar, excess 40 Ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory. Oxygen diffusion provides an appropriate analogy for this phenomenon, being relatively fast under natural, hydrothermal conditions, but extremely slow in anhydrous environments such as an Ar-extraction system. TEM observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns, due to the presence of such microstructures as incoherent exsolution lamellae, dislocations, and stepped twins. TEM observations also reveal the presence in one sample of orthoclase enclaves in a microcline host.

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