Shock-darkening in ordinary chondrites: Determination of the pressure-temperature conditions by shock physics mesoscale modeling

1J. Moreau,1,2T. Kohout,3K. Wünnemann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12935]
1Department of Physics, University of Helsinki, Helsinki, Finland
2Institute of Geology, The Czech Academy of Sciences, Prague, Czech Republic
3Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
Published by arrangement with John Wiley & Sons

We determined the shock-darkening pressure range in ordinary chondrites using the iSALE shock physics code. We simulated planar shock waves on a mesoscale in a sample layer at different nominal pressures. Iron and troilite grains were resolved in a porous olivine matrix in the sample layer. We used equations of state (Tillotson EoS and ANEOS) and basic strength and thermal properties to describe the material phases. We used Lagrangian tracers to record the peak shock pressures in each material unit. The post-shock temperatures (and the fractions of the tracers experiencing temperatures above the melting point) for each material were estimated after the passage of the shock wave and after the reflections of the shock at grain boundaries in the heterogeneous materials. The results showed that shock-darkening, associated with troilite melt and the onset of olivine melt, happened between 40 and 50 GPa with 52 GPa being the pressure at which all tracers in the troilite material reach the melting point. We demonstrate the difficulties of shock heating in iron and also the importance of porosity. Material impedances, grain shapes, and the porosity models available in the iSALE code are discussed. We also discuss possible not-shock-related triggers for iron melt.


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