Comparative 40Ar/39Ar and K–Ar dating of illite-type clay minerals: A tentative explanation for age identities and differences

Integrating 40Ar–39Ar, 87Rb–87Sr and 147Sm–143Nd geochronology of authigenic illite to evaluate tectonic reactivation in an intraplate setting, central Australia

“…Respective retention ages range from 107.9 ± 0.6 Ma to 112.8 ± 1.1 Ma. All samples have well developed plateaux that fall close-to (~45%) or within (>50%) the theoretical definition of Fleck et al (50% of heating spectrum;1977), and can be attributed to 39 Ar release from a single, homogenous reservoir (Clauer et al, 2012). However, the height of such plateaux do not necessarily represent geologically significant ages as age spectra can be significantly distorted by recoil artefacts such as the presence of point defects that are created by recoiling 39 Ar nuclei .…”

“…While some earth scientists have attributed these features to discrete hydrothermal Villa and Williams, 2013) or heating (van de Pluijm et al, 2001;Cassata and Renne, 2013;Piacentini et al, 2013) events, others have cast doubt on the geological meaningfulness of plateaux (Hodges et al, 1994;Sletten and Onstott, 1998;Forster and Lister, 2004). These intricacies are further exacerbated when analysing finegrained mineral phases, such as illite, due to the degree of 39 Ar recoil that occurs during irradiation (Clauer et al, 2012). Recoil loss of 39 Ar is arguably dependent on factors such as the surface area of the analyzed aliquot, illite crystallinity (clay packet thickness), presence of structural defects and grain size (Hunziker et al, 1986;Reuter and Dallmeyer, 1987;Dong et al, 1995;Dong et al, 1997;Villa, 1997;Clauer et al, 2012;Clauer, 2013).…”

“…These intricacies are further exacerbated when analysing finegrained mineral phases, such as illite, due to the degree of 39 Ar recoil that occurs during irradiation (Clauer et al, 2012). Recoil loss of 39 Ar is arguably dependent on factors such as the surface area of the analyzed aliquot, illite crystallinity (clay packet thickness), presence of structural defects and grain size (Hunziker et al, 1986;Reuter and Dallmeyer, 1987;Dong et al, 1995;Dong et al, 1997;Villa, 1997;Clauer et al, 2012;Clauer, 2013). To mitigate the complete loss of recoil 39 Ar, clay samples are encapsulated in quartz vials prior to irradiation.…”

“…As samples are quartz vial encapsulated, 40 Ar- 39 Ar illite age data can be in interpreted using total-gas ages or retention ages (Dong et al, 1995;Clauer et al, 2012 Ar was previously lost in nature (Dong et al, 1995;Hall et al, 2000). Total-gas ages are calculated using all the 39 Ar, including that lost due to recoil, and are equivalent to conventional K-Ar age measurements.…”

Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians

“…K-Ar dating is a reliable tool to constrain authigenic formation of fined-grain illite clay minerals in brittle faults to circumvent 39 Ar recoil by 40 Ar/ 39 Ar dating (Clauer et al, 2012, Torgersen et al, 2015.…”