Thermal History of CB_b Chondrules and Cooling Rate Distributions of Ejecta Plumes

Abstract

It has been proposed that some meteorites, CB and CH chondrites, contain material formed as a result of a protoplanetary collision during accretion. Their melt droplets (chondrules) and FeNi metal are proposed to have formed by evaporation and condensation in the resulting impact plume. We observe that the skeletal olivine (SO) chondrules in CB b chondrites have a blebby texture and an enrichment in refractory elements not found in normal chondrules. Because the texture requires complete melting, their maximum liquidus temperature of 1928 K represents a minimum temperature for the putative plume. Dynamic crystallization experiments show that the SO texture can be created only by brief reheating episodes during crystallization, giving a partial dissolution of olivine. The ejecta plume formed in a smoothed particle hydrodynamics simulation served as the basis for 3D modeling with the adaptive mesh refinement code FLASH4.3. Tracer particles that move with the fluid cells are used to measure the in situ cooling rates. Their cooling rates are ~10,000 K hr −1 briefly at peak temperature and, in the densest regions of the plume, ~100 K hr −1 for 1400-1600 K. A small fraction of cells is seen to be heating at any one time, with heating spikes explained by the compression of parcels of gas in a heterogeneous patchy plume. These temperature fluctuations are comparable to those required in crystallization experiments. For the first time, we find an agreement between experiments and models that supports the plume model specifically for the formation of CB b chondrules.

Impact plume‐formed and protoplanetary disk high‐temperature components in CB and CH metal‐rich carbonaceous chondrites

“…The abundance of "normal" chondritic material increases from CB to CH chondrites with decrease of chondrule sizes, possibly reflecting size sorting in the plume and during accretion of the CB/CH parent body(ies). Some of the "normal" chondritic components may have been heated conductively by the silicate vapor of the expanding plume and bow shocks (Hewins et al 2018;Stewart et al 2019aStewart et al , 2019bChoksi et al 2020). Although with a vaporizing impact we expect substantial mixing of target and impactor material, we also expect some large-scale variation in plume composition.…”

“…We infer that some 16 O-depleted igneous CAIs were melted together with precursors of some CH porphyritic chondrules. These observations can be reconciled with a complex thermal history of solid material in the turbulent impact plume that may have experienced multiple pulses of heating (e.g., Hewins et al 2018;Stewart et al 2019a).…”

“…2). The reduction could be due to interaction with the reduced gas of the plume, which is expected to be highly turbulent (e.g., Hewins et al 2018;Stewart et al 2019aStewart et al , 2019b.…”

“…Here, we focus on the origin of Ca, Al-poor ferroan and magnesian cryptocrystalline chondrules, Ca, Al-rich skeletal olivine chondrules, and 16 O-depleted igneous calcium-aluminumrich inclusions (CAIs) surrounded by igneous rims in the metal-rich CB and CH carbonaceous chondrites. All these objects appear to have formed in the impact-generated gas-melt plume (Kallemeyn et al 1978;Wasson and Kallemeyn 1990;Campbell et al 2002;Rubin et al 2003;Krot et al 2005;Fedkin et al 2015;Oulton et al 2016;Hewins et al 2018).…”

Slow cooling during crystallisation of barred olivine chondrules

“…No consensus exists on the process(es) of chondrules formation, and numerous models ranging from nebular to planetary processes have been proposed to explain why chondrules display such a wide variety of textures and chemical compositions (Desch et al, 2012;Connolly and Jones, 2016;Hewins et al, 2018). However, in all cases, it is largely accepted that chondrules resulted from the crystallisation of small beads of liquid that had already more or less crystallised within a gas.…”