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In reality, the ‘closure temperature’ of a mineral to Ar diffusion is more likely a temperature range, however, this range depends upon factors such as the rate of cooling, the grain size and also upon the degree of crystallinity of the mineral (e.g. Ar mineral ages can be variously interpreted as the age of mineral crystallisation, the age of cooling through a nominal closure temperature, or the age of mineral alteration.
Choosing between these alternative interpretations, particularly for results from fluid alteration systems, can be assisted by comparison of results with Ar dating of samples from outside the alteration system.
Details of sample preparation, analytical procedures and full analytical data are given in Nicolson et al. Each sample was heated incrementally with a continuous-wave Ar-ion laser with a 2 mm wide defocused beam.
Evidence in support of the model is significant, however, as confirmation of both the rare preservation of an ancient epithermal system and the regional prospectivity for this style of mineralisation.
1585 Ma, based on the dating of alunite within the advanced argillic alteration assemblage. These age spectra suggest that the alunite has not remained completely closed to Ar diffusion since initial crystallisation.
However, the alunite samples do not provide flat age spectra (Fig. Rather, in three-step heating experiments on three alunite grains from two different samples, the age spectra obtained show a progressive increase in age from minimum ages of c. In general, alunite has been shown to retain Ar over geological time where the crystals have remained unaffected by secondary alteration processes or thermal events exceeding temperatures of ~200–220 °C (Landis, Snee and Juliani 2005). However, in Cenozoic examples, alunite Ar age spectra typically yield relatively simple age profiles consistent with minimal post-crystallisation disturbance.
Consequently, muscovite from samples of high-grade metamorphic rocks of Warrow Quartzite both close to and distal to the alteration system were also analysed.
Muscovite has a nominal closure temperature of ~350 °C (Mc Dougall and Harrison 1999).