40Ar diffusion in phlogopite: PVT atomistic calibration and implications for Ar-melt-solid kinetic interactions and ascent dynamics of mantle xenoliths and kimberlites

J Nteme and S Scaillet and P Brault and L Tassan-Got and F Duval, GEOCHIMICA ET COSMOCHIMICA ACTA, 382, 103-117 (2024).

DOI: 10.1016/j.gca.2024.07.008

Ar-40/Ar-39 ages of phlogopite from kimberlite-hosted mantle xenoliths are commonly older than the kimberlite eruption, despite the fact that argon is supposed not to be retained by phlogopite at temperatures above hydrothermally-derived Ar closure temperatures (<500 degrees C). Combining Molecular Dynamics (MD) with Nudged Elastic Band (NEB) and Transition State Theory (TST), we investigate Ar-40-PVT (Pressure- Volume-Temperature) relationships in pristine, defect-free, phlogopite and show that Ar-40 diffusivity is several orders of magnitude slower than existing estimates with a strong effect of pressure on diffusion rates and retention of Ar-40 at mantle conditions. These results imply to fundamentally revise residence- and transit-time estimates based on Ar kinetics in phlogopite assuming simple diffusive relaxation during upwelling. When accounting for pressure, Ar-40 retention trends in phlogopite predict substantially slower kimberlite ascent rates than documented by independent chronometers, indicating that Ar-40 resetting during ascent in phlogopite does not result from simple decompression- driven diffusive relaxation. We argue that Ar-40 remobilization probably involves secondary structural-textural modifications induced by reaction-driven recrystallization or rim overgrowth. These findings have far-reaching consequences in terms of argon isotopic mobility at mantle depths as well as for dating and tracing metasomatic events during crust-mantle interactions in the evolution of the subcontinental mantle.

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