Dynamic Evolution Of Major Element Chemistry In Protoplanetary Disks And Its Implications For Chondrite Formation

Modeled profiles of system composition in terms of the relative abundances of (A, C, E) Na (red), Mg (orange), and Al (green) normalized by Si and the composition of solar composition, and (B, D, F) Na (red), Al (green), Si (orange), and S (blue) normalized by H and the solar composition. These plots show the profile after 0.1 Myr of evolution for (A, B) and 40 kyr for (C). The parameters used to create this figure is the same with Figure 1 for (A, B). Cases when (C, D) the normalized pebble size is 10 times larger and when (E, F) the turbulent viscosity is 10 times larger are also shown. The dotted line in (A) indicates the case where the re-condensation of solid-composing elements onto pebbles is neglected. Gray lines in all panels denote unity, corresponding to an unfractionated value. Regions where planetesimals may form by the streaming instability are shaded in (A, E). In (C), the pebble/gas ratio is too low to trigger the streaming instability, so no region is shaded. In (B, D, F), the evaporation fronts of corundum (green), enstatite (orange), albite (red), and troilite (blue) are denoted by arrows. These minerals are the major host phases of Al, Si, Na, and S, respectively, at the time of evaporation.

Chondrites are the likely building blocks of Earth, and identifying the group of chondrite that best represents Earth is a key to resolving the state of the early Earth.

The origin of chondrites, however, remains controversial partly because of their puzzling major element compositions, some exhibiting depletions in Al, Ca, and Mg. Based on a new thermochemical evolution model of protoplanetary disks, we show that planetesimals with depletion patterns similar to ordinary and enstatite chondrites can originate at 1--2~AU just outside where enstatite evaporates.

Around the "evaporation front" of enstatite, the large inward flow of refractory minerals, including forsterite, takes place with a high pebble concentration, and the loss of those minerals result in depletion in Al, Ca, and Mg. When evaporated solid grains re-condense onto pebbles, the concentration of pebbles is further enhanced, potentially triggering the streaming instability. Planetesimals with the composition of ordinary and enstatite chondrites can thus be naturally created in the terrestrial region. T

he preferential loss of forsterite also creates an enhancement of Mg/Si and heavy Si isotopes just inside the potential source region for ordinary and enstatite chondrites. Earth, which shows both features, may originate just inside where ordinary and enstatite chondrites were born.

Yoshinori Miyazaki, Jun Korenaga
Comments: 16 pages, 6 figures, 1 table
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Cite as: arXiv:2004.13911 [astro-ph.EP] (or arXiv:2004.13911v1 [astro-ph.EP] for this version)
Submission history
From: Yoshinori Miyazaki
[v1] Wed, 29 Apr 2020 01:27:34 UTC (1,526 KB)
Astrobiology, Astrochemistry

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