11:30 AM - 11:45 AM
[PPS07-27] Viscous heating in shock-comminuted rocks: A reappraisal of the shock melting threshold by using a shock physics code
Keywords:Hypervelocity collisions, Impact melts, Numerical modeling of impact phenomena
A vertical impact of a sphere onto a flat target are numerical modeled in a two-dimensional cylindrical coordinate. The analytical equation of state (ANEOS) for dunite were used for both projectile and target. Impact velocity was fixed at 6 km/s, which is slightly lower than the bulk sound velocity of dunite. The projectile radius was divided into 50 cells, which is thought to be large enough to investigate the shock pressure distribution with a high accuracy. We assumed that the projectile and the target have any temperature gradients at initial. The initial temperature was set to 220 K, which is close to a radiative-equilibrium temperature at the main belt region. The constitutive model for dunite parameterized in Johnson et al. (2015) was also used with the same input parameters except for the coefficient of internal friction. Lagrangian tracer particles were inserted into each computational cell. We stored the time variation of pressure and entropy into the tracers.
We found that the entropy gradually increases during pressure release in the case of a highly-frictional target contrary to the assumption of isentropic release. A larger value of the internal friction leads to a larger increase of entropy. We also found that the shock melting occurs after ~40 GPa shock compression under our experimental conditions if we used a typical value for the coefficient of the internal friction. This value is lower than a widely-used threshold for shock-induced melting. Our results suggest that (1) the shock melting occurs at a lower impact velocity than previously thought and that (2) the input parameters of the constitutive model in numerical models largely affect the thermodynamic response of geologic materials.
Acknowledgement: We appreciate the developers of iSALE, including G. Collins, K. Wünnemann, B. Ivanov, J. Melosh, and D. Elbeshausen.