Recovery of muscovite age gradients by 40Ar/39Ar vacuum furnace step-heating analysis

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos, Greece: a record of continuous (re)crystallization during exhumation?

“…However, greenschist facies overprinting did not completely reset argon isotopes in phengite grown during HP metamorphism and it is unknown whether the form of age spectra results from the outgassing of intra‐ or intergrain argon isotopic variations. Step heat experiments on white mica have been shown to document core‐rim 40 Ar/ 39 Ar age variations obtained using in situ laser analysis (Kula & Spell, ). However, in case of Syros, it is not possible to compare the new step heat results with previously published in situ analyses on HP mica because only a subset of the laser analyses were presented (Putlitz et al ., ).…”

40 Ar/ 39 Ar geochronology and the diffusion of 39 Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo

“…There has thus been increased interest in the diffusion of argon in white mica as more data emerge that support the hypothesis that solid-state diffusion is responsible for the release of argon during step-heating experiments in vacuo during 40 Ar/ 39 Ar geochronology (Sletten & Onstott 1998;Forster & Lister 2004;Harrison et al 2009;Cosca et al 2011;Forster et al 2011;Kula & Spell 2012). It would be interesting if this is true, for then geochronologists could thereby more readily extract information as to the heating and cooling path followed by rocks in nature (e.g.…”

“…Fortunately, it now appears that this is not always the case (Harrison et al 2009;Forster et al 2011;Kula & Spell 2012) and solid-state diffusion during step-heating experiments may be pertinent to a wider range of minerals than has so far been assumed. There is, perhaps, some justification for the same careful step-heating experiments with white micas as are conducted with K-feldspar in order to perform multi-diffusion domain (MDD) analysis (Lovera et al 1997(Lovera et al , 2002.…”

“…It should be emphasized that there is considerable variation in the parameters that can be estimated in this way and, therefore, claims that results from step-heating experiments in vacuo which mimic experimental data (e.g. Harrison et al 2009;Kula & Spell 2012) should be treated with caution. The range of diffusion parameters that apply in nature may be far greater than presently published experimental data would allow.…”

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

“…The age patterns reflected by Ar release spectra are often interpreted as corresponding to Ar distribution in the crystal(s) such that the thermal history of the grain(s) can be recovered (e.g. Harrison et al, 2009;Kula and Spell, 2012). However, metamorphosed and deformed rocks often record complex geological and thermal histories.…”