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

Thermochronology of Alkali Feldspar and Muscovite at T > 150 °C Using the 40Ar/39Ar Method: A Review

“…The turbid regions are feldspar that formed via fluid-induced dissolution-reprecipitation and include veins of replacement Na-rich and K-rich sub-grains with incoherent boundaries. Most of these textures are unstable during laboratory heating, as was first hypothesised by Parsons et al (1999) [12] and then experimentally documented by Fitz Gerald et al (2006) [29], Parsons et al (2010) [30] and Popov et al (2020b) [17]. During step-heating, exsolution lamellae in Shap feldspar start to homogenise at ≥500 • C, while complete homogenisation can be achieved at ≥700 • C and the process is complete in less than 1 hr at 1000 • C. Partial homogenisation also occurs between incoherent Na-rich and K-rich sub-grains in replacive veins.…”

“…However, it remains unclear as to which features separate these domains from each other and provide fast diffusion pathways for the effective removal of Ar from their surfaces. On the contrary, several experimental observations show that non-linearity in Arrhenius plots is either due to fracturing or opening of fast diffusion pathways during sample preparation [91], fast pathways for Ar loss that modify during progressive step-heating (e.g., [17,30]), or modifications of intrinsic diffusion parameters by heating-induced structural transitions (e.g., [14,58,92]). Regardless, it is critical to understand how the microtextural evolution of minerals during laboratory step-heating affects 40 Ar/ 39 Ar data, and whether or not laboratory data can be extrapolated to geological temperatures and timescales.…”

“…However, this explanation is not unique, and many studies relate the non-linearity to modifications of texture and structure during laboratory heating. Previous suggestions include homogenisation of compositionally different domains within the analysed fragments along with annealing or formation of crystal defects [12,30,93], fracturing of the analysed fragments and the formation of Ar traps within them [17,91,94], structural transitions [71,92,93] and changes in vacancy-specific diffusion mechanisms [34,55].…”

“…Heating to >700 • C leads to the formation of abundant subspherical nanopores [30]. Finally, heating results in the development of interconnected cracks, which form due to spatially uneven thermal expansion because of regions with varied chemical composition and state of Si-Al order (Figure 7; [17]). Cracks were also reported in heated grains of alkali feldspar from the Chain of Ponds Pluton [73], the Klokken intrusion [82], and the Benson Mines pegmatite [94].…”

“…Here we review published research on whether these assumptions hold for the alkali feldspar and muscovite 40 Ar/ 39 Ar thermochronological methods, where some studies have suggested these can recover thermal histories at~150-350 • C (e.g., [4][5][6]) and 300-600 • C [7][8][9][10][11], respectively. Many studies have questioned the applicability of these methods and argued that the migration of Ar isotopes in alkali feldspar and muscovite is governed by different mechanisms in the laboratory and during the geological past, and thus cannot be described using a single set of diffusion parameters [2,3,[12][13][14][15][16][17]. Only a small percentage of thermochronological studies have developed and applied thermochronometers that can generate continuous t-T paths at T > 150 • C, while the most frequently used (fission track and (U-Th/He)) are sensitive to T < 150 • C. Therefore, we aim to provide ideas to stimulate further research into how to test the aforementioned key assumptions, and perhaps identify intra-grain regions where natural argon diffusion profiles may have been preserved and can be recovered intact.…”