Observation of centimetre-scale argon diffusion in alkali feldspars: implications for ⁴⁰ Ar/ ³⁹ Ar thermochronology

Abstract

New data from a gem-quality feldspar from Itrongay, Madagascar, record naturally occurring 40 Ar/ 39 Ar age profiles which can be numerically modelled by invoking a single diffusion mechanism and show that microtexturally simple crystals are capable of recording complex thermal histories. We present the longest directly measured, naturally produced 40 Ar*-closure profiles from a single, homogeneous orthoclase feldspar. These data appear to confirm the assumption that laboratory derived diffusion parameters are valid in nature and over geological timescales. Diffusion domains are defined by crystal faces and ancient cracks, thus in gem-quality feldspars the diffusion domain size equates to the physical grain size. The data also illustrate the potential of large, gem-quality feldspars to record detailed thermal histories over tens of millions of years and such samples should be considered for future studies on the slow cooling of continental crust.

Interpreting and reporting 40Ar/39Ar geochronologic data

“…It is especially important to recognize that the most appropriate value of a to use for a particular dated mineral can be ambiguous. Many He and Ar diffusion studies for minerals and empirical observations suggest that half of the physical grain size is commonly the effective diffusion dimension for samples that are devoid of alteration, microfractures, and other potential fast-diffusion pathways (Anderson et al, 2019;Cassata and Renne, 2013;Flude et al, 2014;Hodges and Bowring, 1995;Kula and Spell, 2012;Skipton et al, 2017;Wartho et al, 1999). However, for deformed crystals that contain internal subgrain boundaries that may define fast-diffusion pathways (Lee, 1995) or structurally complex feldspars, a may be substantially smaller than the physical grain size would suggest (Lovera et al, 1989(Lovera et al, , 1991Cassata and Renne, 2013).…”

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

“…This process is referred to as Multi-Diffusion Domain (MDD) modelling, and the inversion theory was developed by Lovera et al (1989) [4] and Lovera and Richter (1991) [5], while several authors have written code to perform this task (e.g., [21][22][23]). The second approach is to use in situ 40 Ar/ 39 Ar analysis, where Ar isotopic compositions are measured in polished grains by ablation using a UV laser to characterise intra-grain 40 Ar/ 39 Ar date variations (e.g., [6,16,24,25]). Potential t-T paths can be generated by forward modelling using software such as Diffarg(P) and choosing an appropriate set of diffusion parameters (Figure 2A; [8,26,27]).…”

“…The second key assumption is that laboratory diffusion experiments at high temperatures replicate the physical mechanisms that occur with natural rates of cooling and heating over geological timescales. 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. 40 Ar/ 39 Ar Method Some studies of Ar in K-feldspars and muscovite suggest that their Ar isotopic compositions can be exploited to recover accurate, continuous t-T paths at T~150-350 • C (e.g., [4][5][6]) and~300-600 • C [7][8][9][10][11], respectively (Figure 1), in some cases. Following Fick's law of diffusion, thermochronological methods utilise a mathematical description that relates properties of the host structure (activation energy of diffusion, E a , and absolute diffusivity, D 0 ), cooling rate, the geometry and size of the diffusion domain(s) (i.e., length scale(s) for diffusion) and t-T history with degree of daughter isotope loss (e.g., [4]).…”

Comment on “Systematic variations of argon diffusion in feldspars and implications for thermochronometry” by Cassata and Renne

“…Because both the Benson Mines and Madagascar samples are homogeneous K-feldspars, it is unsurprising that the largest diffusion domain (domain 2) coincides approximately with the size of the crystal while the much smaller domain (domain 1), documented in both materials by Lovera et al (1997) and comprising only a small fraction of the sample, are surface related or distributed within the grain. Further evidence is provided by Flude et al (2013) who used laser depth profiles to infer the existence of small, surface related domains. Thus the http://dx.doi.org/10.1016/j.gca.2014.11.016 0016-7037/Ó 2014 Elsevier Ltd All rights reserved.…”