Physics
Scientific paper
Jun 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.1699f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 6, pp.1699-1708
Physics
11
Scientific paper
Unusual shapes of 40 Ar/ 39 Ar age spectra have long been attributed to the presence of excess argon. We recognize a distinctive age spectrum type brought about by extraneous argon trapped in large diffusion domains. Spectra of this type are characterized by little or no excess argon in low temperature steps, sensible apparent ages over the first ~40 to 50% 39 Ar released, an abrupt increase to ages exceeding the permissible age of the sample, followed by either a gradual decrease in age until fusion or an anomalously old plateau segment. Detailed study of one such orthoclase reveals that the excess argon within these samples is likely located in radiogenic argon sites within large diffusion domains. Argon contained within these features is trapped when the domains close to argon loss at ~400°C. Thus, there exists a genetic difference between large-domain-trapped-argon-effect spectra and those saddle-shaped age spectra which result from excess argon trapped in anion vacancies at temperatures below ~350°C.
Copeland Peter
Foster David A.
Harrison Mark T.
Heizler Matthew T.
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