Argon Diffusion in Hypogene and Supergene Alunites: Implications to Geochronology and Thermochronometry on Earth and Mars

1Z.Ren,1P.M.Vasconcelos
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.014]
1The University of Queensland, Brisbane, Qld 4072 Australia
Copyright Elsevier

Argon release mechanisms and diffusivity were quantified for coarsely crystalline hypogene alunite [KAl3(SO4)2(OH)6] from Marysvale, Utah, and microcrystalline supergene alunite aggregates from Coober Pedy, South Australia. Prior to diffusivity studies, all alunite samples, sorted in the 500-200, 200-100, 100-50, 50-10, and < 10 µm sieve-size ranges, were vacuum encapsulated to quantify 39Ar recoil losses. Incremental-heating of single capsules inserted into Ta-crucibles and heated by a projector-lamp in a specially designed diffusion cell permits quantifying 39Ar released at precisely measured temperature steps (± 1-3 °C). Incremental-heating 40Ar/39Ar analyses of the various sieve-size ranges yield reproducible ages that are indistinguishable from single grain (1-2mm) laser-heating 40Ar/39Ar dating of the same samples. The results, cast in Arrhenius plots, yield activation energies (Ea) ranging from 248.0 ± 18.5 to 281.7 ± 15.2 kJ/mol and ln(Do/a2) from 26.2 ± 2.0 to 27.8 ± 3.2 ln(s-1) for hypogene alunite; supergene alunite yield Ea ranging between 233.3 ± 5.4 and 293.8 ± 13.7 kJ/mol and ln(Do/a2) between 27.3 ±1.0 and 36.8 ± 2.6 ln(s-1). These diffusion parameters correspond to closure temperatures of 264 ± 22 °C and 246 ± 19 °C for hypogene and supergene alunite, respectively, assuming a cooling rate 100 °C·Ma-1. In-situ TEM experiments on aliquots of alunite crystals from the same samples indicate that alunite single crystals undergo transformation to nanocrystalline aggregates at 430-460 °C, showing that alunite releases Ar by volume diffusion below ∼ 430 °C, retains a significant amount of Ar during phase transformation, and proceeds to release Ar by volume or multipath diffusion from a modified polycrystalline structure at T > 460 °C. Isothermal holding time and AGESME modelling using our calculated diffusion parameters indicate that alunite should preserve Ar quantitatively for long periods (4.0 Ga) at Earth and Mars surface conditions, and both hypogene and supergene alunite should preserve original formation ages, independently of precipitation mechanism.

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