Barium silicates of the Berisal Complex, Switzerland: A study in geochronology and rare-gas release systematics

Molecular modeling of the effects of 40Ar recoil in illite particles on their K–Ar isotope dating

“…Similar questions were hypothesized earlier by Goncharov et al (2007). Hetherington and Villa (2007) argued that a minor portion of 40 Ar in coarse muscovite can occur in a position different than the interlayer: within the T-O-T framework. Due to the recent developments of molecular dynamics methodology it is now possible to address these questions quantitatively and to draw specific conclusions relevant for K-Ar geological age estimates.…”

Disequilibrium textures versus equilibrium modelling: geochronology at the crossroads

“…Lovera et al 2002), it has been known for a long time that it does not (Villa 1994). Further evidence that the process governing Ar loss from K-feldspar is not Fick's law diffusion was provided by Hetherington & Villa (2007). These authors compared the kinetics of Ar and Xe release from a variety of barian minerals and observed that a hydrous mineral, ganterite, known with certainty to undergo in vacuo destabilization by dehydroxylation showed the same Arrhenian trajectories (quantifying apparent Ar diffusivity) as anhydrous feldspars.…”

Diffusion of Ar in K-feldspar: Present and absent

“…The ANL parameter, which quantifies the departure of the measured Arrhenius trajectory from this ideal line, is plotted as a function of released Ar. This diagram can be interpreted in two ways: (1) the upward-concave deviations from Arrhenian linearity are an intrinsic phenomenon that affects all silicates during in vacuo degassing (Hetherington & Villa 2007; see above); or (2) the deviations reflect a mixture of discrete 'domains' with a variety of constant grain sizes, each of which would produce a linear Arrhenius trajectory if it were not mixed with others (Lovera et al 1993). In this latter interpretation, a large ANL span means that the analysed mineral consists of a collection of discrete 'domains' having very different grain radii r i and the ANL parameter quantifies the ratio of the average grain size being degassed in one step to that of the lowest temperature step, r/r 0 .…”

“…That noble gas atoms trapped in a crystal do not behave like freely flowing noble gases is made even clearer by the Xe diffusivity measured by Hetherington & Villa (2007). Xe is released from Ba-K-feldspar much later than Ar, and has an activation energy for diffusion indistinguishable from that of Ba.…”

“…The studies quoted in this paragraph imply that k SR ≫ k F , or at least that k F is not the dominant term. It is remarkable that the Arrhenius trajectory of the Xe release (Hetherington & Villa 2007, fig. 6b) is linear, contrary to that of Ar.…”