Total stellar irradiance and lithophile element devolatilization trends for the terrestrial planets

Abundance patterns ranging from (ultra-)refractory (e.g. W, Zr, Al, Ca and Rare Earth Elements), to moderately volatile (e.g. Li, K and Na) and highly volatile (e.g. Zn, Cl, Br, I and In) lithophile elements show a broadly ‘hockey stick’ element depletion trend for Earth and Mars [1,2]. This is because refractory element abundances relative to solar composition and normalized to Mg or Al [3] are weakly affected by devolatilization processes but the moderately volatile elements describe a devolatilization depletion factor with a particular slope (α). Volatile elements with 50% condensation temperatures (T_C,50%) 750-500 K (Zn) are unfractionated with respect to one another [4]. This pattern is recapitulated in some carbonaceous chondrite meteorite groups (CM, CV and CR). Such secular trends in lithophile element abundances potentially yield useful clues regarding the parameter space of planetary accretion such as localization of feeding zones, supply of volatile species to the planets and the degree to which orbital architecture may have changed due, for example, to migration. Nucleosynthetic isotopic tracers (e.g. ⁵⁰Ti, ⁵⁴Cr, ⁶²Ni), as well as mass-independent oxygen isotopes (expressed in the conventional notation as: 'D¹⁷O_VSMOW) of the sampled terrestrial planets, deviate from the volatile-rich carbonaceous chondrites (CC). The terrestrial planets instead show affinities with the non-carbonaceous feedstocks (NC) that are poorer in volatiles than CC. Here, we show that Earth and Mars, along with best estimates of the bulk compositions of Mercury and Venus inferred from geochemical models coupled with remote and in situ analysis, display both a hockey stick element depletion pattern and systematically different devolatilization depletion factors of the moderately volatile lithophiles (slope, α). We further find that the degree of devolatilization in T_C,50% as a function of depletion trend (α) plotted against total stellar irradiance (S₀ in in Wm^-2) correlates with heliocentric distance (au) and thus, stellar luminosity, following a power law . This relationship accounts for the bulk compositions and volatile inventories of the inner planets, and strongly implies that they formed locally, in agreement with recent studies [7-9]. We also find that some NC achondrite meteorite groups (e.g. angrites, ureilites) record extreme lunar-like depletion trends (α) that we interpret to be from a special origin (i.e. colossal impacts with re-condensation). To lowest order, these observations reveal information about complex processes in planet formation scenarios while considering the already-assembled planet. Finally, imminent data (e.g. JWST spectra) for ionized lithophile elements in the atmospheres of ultra-short period exoplanets will allow us to quantitatively assay formation models that are too simple and/or seem to indicate an unwitnessed process in the history of a planet such as migration.

 

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