Heat Diffusion in Numerically Shocked Chondrites: Towards a Better Understanding of Shock Melting Features in Meteorites

Abstract:

Shock metamorphism in ordinary chondrites (OC) is characterized by shock heating, melting and mechanical deformation of minerals. In the range of high pressure shock metamorphism (40–60 GPa), shock heating and melting of iron sulfide, metal, plagioclase and olivine have been investigating in earlier studies using 2D numerical models (Moreau et al. 2018, 2019). These models could reproduce the partial to complete melting of iron sulfide and plagioclase observed in shocked OC, but could not explain other common features like the melting of metal-sulfide mixtures or the intermixed melting of metals and silicates.

One shortcoming of these earlier models is that the used shock physics code could not account for the relaxation of local temperature contrasts by heat diffusion. Post-shock temperatures can differ strongly (by > 400K) between adjacent phases with different elastic properties, which leads to strong post-shock temperature contrasts on the grain scale. Diffusion of heat between adjacent grains alters the temperature distribution in the rock and hence needs to be considered to understand the local post-shock heating and melting processes.

We modeled the heat diffusion among adjacent grains in simplified 2D textures, using the post-shock temperature output of the shock physics code iSALE as initial condition. Model textures were designed to represent various grain configurations and associated melting features observed in OC.

We found that diffusion of heat from strongly shock heated phases plays a key role in melting moderately shock heated phases like iron metal. Considering this diffusive heating, we could reproduce the eutectic melting of metals and the intermixed melting of metals and silicates observed in OC. In addition, our results shed light on the effects of grain sizes, geometries, textural relations and the direction of the shock wave on the in initial post-shock heat distribution and their key role in facilitating melting or solidification of different phases.

As heat diffusion is time dependent, the heat distribution in shocked targets might differ depending on the duration of the shock pulse. Hence considering post-shock heat diffusion might be important for understanding possible differences in the melting behavior of targets in small (experimental) and large (natural) impacts.