  Shock metamorphism in ordinary chondrites allows for reconstructing impact
events between asteroids in the main asteroid belt. Shock-darkening of ordinary
chondrites occurs at the onset of complete shock melting of the rock (>70 GPa)
or injection of sulfide and metal melt into the cracks within solid silicates
(\sim 50 GPa). Darkening of ordinary chondrites masks diagnostic silicate
features observed in the reflectance spectrum of S-complex asteroids so they
appear similar to C/X-complex asteroids. In this work, we investigate the shock
pressure and associated metamorphism pattern in rubble-pile asteroids at impact
velocities of 4-10 km/s. We use the iSALE shock physics code and implement
two-dimensional models with simplified properties in order to quantify the
influence of the following parameters on shock-darkening efficiency: impact
velocity, porosity within the asteroid, impactor size, and ejection efficiency.
We observe that, in rubble-pile asteroids, the velocity and size of the
impactor are the constraining parameters in recording high-grade shock
metamorphism. Yet, the recorded fraction of higher shock stages remains low
(<0.2). Varying the porosity of the boulders from 10% to 30% does not
significantly affect the distribution of pressure and fraction of
shock-darkened material. The pressure distribution in rubble-pile asteroids is
very similar to that of monolithic asteroids with the same porosity. Thus,
producing significant volumes of high-degree shocked ordinary chondrites
requires strong collision events (impact velocities above 8 km/s and/or large
sizes of impactors). A large amount of asteroid material escapes during an
impact event (up to 90%); however, only a small portion of the escaping
material is shock-darkened (6%).
