Talk by Megan Duncan at AGU 2019

San Francisco.

“Variations in Moderately Volatile Elements in Planetary Bodies from Impact Vaporization”

Abstract:

Planet-building processes have a strong effect on the distribution of the elements in the resulting bodies. In particular, moderately volatile elements (MVEs) have distinct differences in depletion between the meteorite groups and the planets. Understanding the chemical effects of impacts and post-impact processes are necessary in order to place constraints on the bulk composition of the planet (pre-differentiation). Vaporizing collisions, common in energetic periods of accretion such giant planet migration and the terrestrial giant impact stage, can separate refractory and volatile components as the system re-equilibrates during vapor plume expansion and cooling. In order to understand the chemical changes caused by high velocity collisions, we must know the liquid-vapor phase boundary of the dominant mineral(s) of the original bodies, likely a silicate mineral such as olivine or pyroxene. To constrain the equations of state of these minerals, we conducted shock-and-release experiments with the Sandia Z machine on single crystal forsterite, olivine, and bronzite to determine the pressure (P), temperature (T), entropy, and density of the shocked and post-shocked states.

Upon decompression from the shocked state, the high pressure fluid intersects the phase boundary and separates into a two-phase mixture at high P and T (e.g., ~1–1000 bar and ~4000–6500 K), driving melt-vapor partitioning under near-equilibrium conditions with limited isotopic fractionation. As the system continues to cool, the MVE concentrations evolve until the system reaches the triple point and the melt freezes. The remaining condensable vapor adsorbs onto the larger solid droplets or condenses as new dust. Thus, the MVE concentrations in the post-impact system may have large variations that are correlated with particle size. We explore the hypothesis that impact vaporization leads to a net enrichment of MVEs in dust-sized particles. Nebular size-sorting processes may separate the larger, MVE-poor solids and the MVE-rich dust, creating zones of variable depletion. During accretion, the MVE-enriched dust may be variably incorporated into planetesimals and contribute to the chemical diversity observed in meteorites and planets.

SNL is managed by NTESS under DOE-NSSA contract DE-NA0003525.