Ar data extrapolation from the lab to rocks: systematics of noble gas release in vacuo from micas, feldspars, etc.

Understanding rare-gas release requires interdisciplinarity. Hydrous minerals degas ³⁹Ar and ⁴⁰Ar in vacuo during structural collapse due to dehydroxylation [1,2], never by Fick's Law in an inert, homogeneous matrix. Therefore monomineralic micas always give "plateaus" [3,4] despite age-zoning. Discordant age-spectra instead proceed from polymineralic, heterochemical, diachronous mixtures, unambiguously revealed by common-denominator three-isotope correlations [5,2]. The "Bruderheim staircase archetype" [6] wasn't a "lognormal distribution of monomineralic whole-rock crystals" but a more mundane polyphase maskelynite-pyroxene-anorthite assemblage [7,8]. For micas and Bruderheim, downslope extrapolations are arbitrary.

The inertness of the Itrongay sanidine crystal structure during laboratory heating was assessed by Raman microspectroscopy from 300 to 1000 °C [9]. Raman modes define robust trends at increasing temperature and over time at constant temperature, mirroring progressive excitation of phonon modes associated with structure modifications: interatomic bond stretching/deformation; Si,Al disordering; deformation/rotation of SiO₂ tetrahedra. Differential activation of phonon modes is reversible, but disordering is are irreversible. The K-feldspar structure is not inert during laboratory heating, explaining the observed break in Arrhenian slope of Ar release rate [10]. This forbids downslope extrapolation of laboratory data to geological conditions.

The in vacuo releases between 500-1460 °C of ⁴He*, ²⁰Ne_F, ³⁷Ar_Ca, ³⁸Ar_Cl, ⁸⁰Kr_Br, ¹²⁸Xe_I, ¹³¹Xe_Ba, ¹³⁴Xe_U from irradiated fluorapatite are linear, parallel Arrhenius trajectories. E=62±5 kJ/mol is independent of atomic radius, D₀ values decrease from He to Xe by ~4 orders of magnitude [11]. Differential release diagrams show bimodal degassing patterns for Ne and Ar and a single burst above 1360 °C for Kr and Xe. The crystallographic site of Ba and I had no control on Xe release. All degassing rates steepen by 500 times at 1360 °C, and merge. The extreme Arrhenian break-in-slope and the merger of trajectories widely separated below 1360 °C reflect sudden, major, energetically very costly structural reordering at 1360 °C, which was documented by Raman spectroscopy, XRD, TEM and microchemical analysis by LIBS. Complete outgassing of Ne, Ar, Kr and Xe requires complete defluorination reaction modifying the apatite structure. Discrete phase transitions at high T make downslope extrapolations to low T incorrect.

 

[1] Zimmermann 1970, https://doi.org/10.1016/0016-7037(70)90045-1

[2] Villa 2021, https://doi.org/10.1016/j.chemgeo.2021.1.120076

[3] Foland 1983, https://doi.org/10.1016/S0009-2541(83)80002-3

[4] Hodges &al 1994, https://doi.org/10.1130/0091-7613(1994)022<0055:AAAGIM>2.3.CO;2

[5] Villa & Hanchar 2017, https://doi.org/10.1016/0012-821X(66)90061-6

[6] Turner &al 1966, https://doi.org/10.1016/0012-821X(71)90051-3

[7] Duke &al 1961, https://doi.org/10.1029/JZ066i010p03557

[8] Baadsgaard &al 1961, https://doi.org/10.1029/JZ066i010p03574

[9] Kung & Villa 2021, https://doi.org/10.1016/j.chemgeo.2021.120382

[10] Wartho &al 1999, https://doi.org/10.1016/S0012-821X(99)00088-6

[11] Villa &al 2024, https://doi.org/10.1016/j.chemgeo.2023.121860