Incremental heating of hornblende in vacuo: Implications for 40Ar/39Ar geochronology and the interpretation of thermal histories

Diffusion of 40Ar in muscovite

“…This contrasts with hydrous phases such as biotite which yield flat, convex upward, or irregular release patterns from samples that have experienced 40 Ar * loss (Berger, 1975;Dallmeyer, 1975;Harrison et al, 1985;Gaber et al, 1988). Although hornblende has been found to degas Ar in a fashion that mimics diffusion (Harrison and McDougall, 1980;Harrison, 1981;Copeland et al, 1991), this reflects its decomposition by an inwardly propagating reaction front during vacuum heating (Lee et al, 1991;Wartho, 1995).…”

High-strain deformation and fluid infiltration diachronism of the middle crust: New Devonian–Permian Alice Springs ages (365–290 Ma) of shear zones in the Strangways Metamorphic Complex, Central Australia

“…Further work would unambiguously confirm whether the younger dates in the age spectra represent a truly heterogeneous 40 Ar distribution (cf. Lee et al, 1991). Nevertheless, assuming that the low-T dates represent diffusive loss, this could have been achieved by: i) the infiltration of a hot and 40 Ar-poor fluid present at the grain boundary after crystallization, or ii) slow cooling.…”

Thermal history of the Ecstall pluton from 40Ar/39Ar geochronology and thermal modeling

“…Gaber et al, 1988;Harrison et al, 1985;Grove and Harrison, 1996;Lee et al, 1991). In particular, Lee et al, 1991 suggest that breakdown of hornblende, and formation of new phases during in vacuo laser heating homogenizes the Ar released, and therefore it is unlikely that original spatial gradients in Ar over any one hornblende grain will be preserved. Thus, a flat age spectrum given by in vacuo laser heating of hornblende and biotite does not in itself preclude diffusive loss of 40 Ar.…”

“…Biotite and hornblende are both hydrous minerals that are unstable during in vacuo laser heating, and for this reason may not produce meaningful release patterns that reflect spatial Ar isotopic gradients (e.g. Gaber et al, 1988;Harrison et al, 1985;Grove and Harrison, 1996;Lee et al, 1991). In particular, Lee et al, 1991 suggest that breakdown of hornblende, and formation of new phases during in vacuo laser heating homogenizes the Ar released, and therefore it is unlikely that original spatial gradients in Ar over any one hornblende grain will be preserved.…”