Apparent partial loss ~(40)Ar/~(39)Ar age spectra of hornblende from the Palaeoproterozoic Lapland-Kola orogen (arctic European Russia): insights from numerical modelling and multi-method in-situ micro-sampling geochronology

摘要

~(40)Ar/~(39)Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic ~(40)Ar by solid-state volume diffusion suggests that staircaseshaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of ~(40)–50% during reheating to 450 ± 25 oC in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. ~(40)Ar/~(39)Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic ~(40)Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra - a proxy for ~(37)ArCa/~(39)ArK - of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.