Abstract
High pressure shock metamorphism in ordinary chondrites involves heating and melting of individual phases from shock entropy, pore collapse, frictional heating, and heat transfer. Numerical models using shock physics codes have recently been used to comprehend the mechanism of shock heating and melting in multiphase mesoscale models. Such models suggest that the formation of sulfide and metal melt veins in ordinary chondrites (shock-darkening) can be explained by preferential heating and melting of iron and iron sulfides during shock. However, those models usually dismissed heat transfer between heterogeneously shock heated phases. This leads to an underestimation of the degree of melting in phases that experienced low degrees of shock heating (e.g. iron metal) but are in direct contact with strongly shock heated phases (e.g. iron sulfides). In our study, we implemented a finite difference 2-D heat diffusion code to model heat diffusion among neighboring grains in shock heated multiphase meshes that represent typical textural relations of silicate, sulfide and metal grains in ordinary chondrites. Post-shock temperature maps for each textural model were calculated using the iSALE shock physics code and used as input for the diffusion code. We find that heat diffusion, not initial shock heating, is the principal cause for heating and melting of metals in eutectic contact with iron sulfides at ~50 GPa of pressure. In addition we study the effects of iron and troilite grain sizes, shock pressures and pre-shock porosities of the silicate matrix, and discuss the preservation of melt allowing melt migration in shock-darkened meteorites and the observation of metal-silicate intermixed melting. With our work, we demonstrate that the consideration of heat diffusion during and after shock is crucial for a better understanding of melting features in both experimentally and naturally shocked ordinary chondrites.
Highlights
Shock metamorphism in ordinary chondrites is associated with impact processes between asteroids (Stoffler et al, 1991, 2018; Fritz et al, 2017) inducing fracturing, crystal and lattice deformation, heat ing, melting, and phase changes under shock compression
Our work considers the combined effects of two processes controlling heating and melting during shock metamorphism in ordinary chon drites: increase of internal heat by the shock wave and redistribution of this heat by diffusion
All results for shock heating and diffusion of heat after time t are compiled in Figs. 1 and 2 and in Table A.1 (Supplementary Material) that compiles peak shock pressures, initial and final post-shock temperatures and melt fractions recorded from the iSALE and diffusion models
Summary
Shock metamorphism in ordinary chondrites is associated with impact processes between asteroids (Stoffler et al, 1991, 2018; Fritz et al, 2017) inducing fracturing, crystal and lattice deformation, heat ing, melting, and phase changes under shock compression. Fracturing and crystal/lattice deformation are more characteristic of low shock stages (C–S1 to C–S4, 30–45 GPa, Stoffler et al, 1991, 2018, Moreau et al, 2019a), followed by iron sulfides (>40 GPa, Moreau et al, 2019a). Iron sulfides melt at their grain borders (Bennett and McSween Jr., 1996), or as isolated droplets in silicate shock veins (Bennett and McSween Jr., 1996); Sharp et al, 2015), and display intermixed melting with plagioclase (Moreau, 2019), depending on the shock conditions (Moreau et al, 2018a, 2019a)
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