Pathways for 39Ar loss during step-heating of alkali feldspar megacrysts from the Shap granite (UK): Combined evidence from diffusion experiments and characterisation of heating-induced texture modifications

• Published in: Chemical geology Vol. 547; p. 119677
• Main Authors: Popov, Daniil V., Spikings, Richard A., Kouzmanov, Kalin
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.08.2020
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2020.119677

This study aims to identify the dominant fast pathways for 39Ar loss during step-heating 40Ar/39Ar analysis of alkali feldspar from the Shap granite (UK). In our analysis we combined step-heating 40Ar/39Ar data acquired from variably sized feldspar fragments (0.4 to 2.2 mm in the shortest dimension) and optical and electron microscopy observations of microtextural modifications caused by heating in a muffle furnace in air. Our optical and electron microscopy results suggest that laboratory heating of the Shap feldspar causes it to fracture. The resulting cracks form an interconnected network, which may link with some of previously documented nanoscale defects such as nanotunnels, slots, pull aparts and bubbles. Our step-heating 40Ar/39Ar analyses yielded non-linear Arrhenius trajectories of 39Ar release. The non-linearity is best explained by heating-induced reduction of the effective diffusion length due to fracturing, likely with some contribution from nanotunnels, pull-aparts, slots and bubbles. Incoherent grain boundaries apparently played a minor role in providing fast pathways for 39Ar loss, if any. Importantly, unambiguous evidence for a reduction of the diffusion length during step-heating was only observed in experiments with large fragments (≥1 mm in the shortest dimension). The Arrhenius trajectory obtained from the smaller fragments (≤0.5 mm in the shortest dimension) appears to be consistent with the presence of variably-sized non-interacting intra-grain diffusion domains that existed prior to mineral separation. Heating-induced fracturing must be taken into account when interpreting 40Ar/39Ar dates of alkali feldspar, with particular focus on the following two points. First, this process is clearly inconsistent with the underlying assumptions of commonly utilised multi-diffusion domain (MDD) theory, although it can result in 39Ar release patterns that appear consistent with them. Therefore, it is important to identify on a case by case basis the dominant fast pathways for 39Ar loss from alkali feldspar during step-heating before applying MDD theory. Second, if heating-induced fracturing is ubiquitous in alkali feldspar and affects even gem-quality samples, then presently available diffusion parameters of Ar in alkali feldspar structure may be inaccurate to some degree, since all of them were obtained without considering the presence of cracks. •Laboratory heating of the Shap feldspar creates interconnected cracks within it•The interconnected cracks dominate among fast pathways for Ar loss during step-heating 40Ar/39Ar analysis•Step-heating 40Ar/39Ar data indicative of fracturing during the analysis were only obtained from fragments with ≥1 mm width•Step-heating 40Ar/39Ar data derived from fragments with 0.4 mm width erroneously appear consistent with MDD theory•Heating-induced fracturing should be carefully considered when constraining Ar diffusion parameters of alkali feldspar