An Earth with affinities to Enstatite Chondrites

Abstract:

The Enstatite chondrite model for the Earth, as envisaged by Marc Javoy and colleagues, has strengths and weaknesses. The overwhelming evidence against layered mantle scenarios makes the existing enstatite Earth models unacceptable. Increasingly, stable and radiogenic isotope data for the Earth and the range of chondrites find that many (but not all) isotopic ratios are shared between the Earth and enstatite chondrites. This significant amount of overlap in isotope space compels one to reconsider the enstatite chondrite model for the Earth.

During early solar system formation (circa +1 Ma) radial inward migration of the Jupiter and Saturn in the disk (e.g., Grand Tack model) would fully disrupted an asteroid belt, resulting in mixing and redistribution of preexisting components, while much later after the disk is gone (e.g., +100 Ma) gravitational scattering by these planets may have transported small bodies from the outer reaches of the solar system inward towards the rocky planets (Nice model).

Astromineralogy reveals variations in the proportion of olivine to pyroxene in accretion disks, some with inner disk regions being richer in olivine relative to the disk wide composition, while other disks show the abundance of olivine is greater in the outer (vs the inner) part of the circumstellar disk, with differences in disk mineralogy being relating to type of star (e.g., T Tauri vs Herbig Ae/Be stars). The inner disk regions (a few AU) show higher abundances of large grains and generally higher crystallinity as compared to outer disk regions, suggesting grain growth occurs more rapidly in the inner disk regions.

Recent results from geoneutrino measurements are most consistent with geochemical models that predict 20 TW of radiogenic power, less so with existing enstatite Earth models predicting less power in the planet.

At 1 AU the Earth accreted a greater proportion of olivine to pyroxene (i.e., Mg/Si of pyrolite) than that available to the known enstatite chondrite parent body. The Earth accreted early in a reduced state, perhaps to the point of differentiating silicides into the core. Later accreted material was increasingly more oxidized. Stirring and mixing in the early solar system created opportunities for the Earth and enstatite chondrites to share some, but not all chemical and isotopic characteristics.