| Popis: |
For six decades geoscientists have been trying to quantitatively understand the nature of radiogenic Ar loss from alkali feldspar. Some researchers suggest that volume diffusion is the dominant mechanism, and they use conventional step-heating ⁴⁰Ar/³⁹Ar data from alkali feldspar to recover the thermal histories of rocks. They argue that a high degree of correlation between log(r/r₀) and ⁴⁰Ar/³⁹Ar age spectra, which is observed in a number of natural examples, justifies this hypothesis. In contrast, other investigators suggest that fluid-mediated recrystallisation and alteration control the radiogenic Ar redistribution, hence rendering alkali feldspar useless as a thermochronometer. By means of numerical modelling, we found that the latter mechanism as well is able to produce samples with highly correlated log(r/r₀) and ⁴⁰Ar/³⁹Ar age spectra. In addition, we show that apparent thermal histories recovered for altered alkali feldspar crystals by interpreting step-heating ⁴⁰Ar/³⁹Ar data may be grossly inaccurate, and yet seemingly fit the prevailing understanding of regional geology. Such inaccurate apparent thermal histories can be obtained even from alkali feldspar crystals that underwent volumetrically low degrees of alteration. Therefore, we conclude that conventional step-heating ⁴⁰Ar/³⁹Ar data are insufficient to support the assumption that radiogenic Ar loss from alkali feldspar occurred solely by volume diffusion and validate the constrained thermal histories, even if upheld by a priori knowledge of regional tectonics. We further suggest that all thermochronological constraints obtained using such data should be supported by detailed petrological characterisation of alkali feldspar. |
Full Text from ScienceDirect