Near-equilibrium isotope fractionation during planetesimal evaporation

Abstract

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations.Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26 Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥ 0.1% ) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10 -8 bar and a vapor pressure at the surface of 10 -4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas.We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H 2 on timescales of 10 4 to 10 5 years. .

Isotope Fractionation during Condensation and Evaporation during Planet Formation Processes

“…Mg isotope fractionation factor for Type B CAI-like material, determined experimentally plotted as a function of temperature showing a convergence towards the theoretical kinetic isotope fractionation factor √24/26 at high temperature, as predicted with equation(16).Data source compiled by Richter et al21 .…”

“…Some of the proposed mechanisms that could explain this elemental depletion include (a) an inheritance from interstellar dust that would be volatile poor 5,6 ; (b) early thermal processing when the protoplanetary disk was still hot 7,8 , (c) transient events responsible for the formation of chondrules 9,10 or their precursors, although this is highly debated (e.g., ref 11 ), (d) parent-body processes such as impact events or thermal metamorphism (e.g. ref 12 ) (e) outgassing during magma ocean stage [13][14][15][16] . A common point in all these processes is that there is a reaction between a vapor and a condensed phase (liquid or solid), and this reaction may be either kinetically driven due to strong chemical disequilibrium or it could reach equilibrium when conditions are favorable.…”

“…Notably it has become clear that volatile depletion took place under conditions rather distinct from vacuum evaporation. Similarly, volatile element isotope signatures have provided new insights into the conditions of lunar formation (e.g., ref 48,49 ) or into the volatile loss during the existence of magma oceans [14][15][16] . These studies have highlighted that the gas speciation can be inferred from the pattern of isotope fractionation and the degree of fractionation can be intrinsically linked to the rate of transport away from an evaporating surface.…”

Bulk chondrite variability in mass independent magnesium isotope compositions – Implications for initial solar system 26Al/27Al and the timing of terrestrial accretion

“…The variability of our enstatite chondrite analyses complicates the choice of a terrestrial protolith composition. We use a mean 27 Al ∕ 24 Mg of 0.085 for enstatite chondrites from Lodders and Fegley (1998) is unlikely to be driven by heating from the decay of 26 Al alone (Young et al, 2019) but requires an additional energy source, as might occur from impacts of larger planetesimals (Hin et al, 2017).…”

Potassium isotope compositions of carbonaceous and ordinary chondrites: Implications on the origin of volatile depletion in the early solar system

“…Processes depleting MVEs during planetesimal/planet formation include: 4) accretional volatile loss (Ringwood, 1966;Albarède, 2009;Hin et al, 2017); 5) giant impacts (Paniello et al, 2012;Wang and Jacobsen, 2016b); 6) magma ocean degassing (Day and Moynier, 2014;Kato et al, 2015); 7) extraction into steam atmospheres and atmospheric loss (Fegley et al, 2016;Young et al, 2019).…”