TY - JOUR

T1 - Experimental investigation of elemental and isotopic evaporation processes by laser heating in an aerodynamic levitation furnace

AU - Badro, James

AU - Sossi, Paolo A.

AU - Deng, Zhengbin

AU - Borensztajn, Stephan

AU - Wehr, Nicolas

AU - Ryerson, Frederick J.

N1 - Funding Information: James Badro expresses his infinite gratitude to the Bourcart–Gentil Foundation of the French Academy of Science for their award, and to the editors of the proceedings of that noble society (Comptes Rendus Géosciences) for the invitation to contribute this work in recognition of this honour. Paolo A. Sossi thanks the SNSF through Ambizione Fellowship grant 180025. We thank Maylis Landeau, Sébastien Charnoz, Marc Chaussidon, and Frank Richter for invigorating and refreshing discussions. We acknowledge the financial support of the Uni-vEarthS labex program at the University of Paris (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Publisher Copyright: © 2021 Elsevier Masson SAS. All rights reserved.

PY - 2021

Y1 - 2021

N2 - We carried out evaporation experiments on a B-type calcium-aluminium-rich inclusion (CAI) melt in a gas-mixing aerodynamic levitation laser furnace, at 1873 K and an oxygen partial pressure of 10−9.1 atm, for durations ranging from 60 to 600 s. Evaporation of SiO2 and MgO follow the same trend as those observed in vacuum furnace experiments at the same temperature and starting composition, showing that their evaporation relative to one another from the melt is independent of pressure, oxygen fugacity, and hydrodynamical regime specific to the furnace. Isotopic ratios of Mg and Si in evaporation residues are used to derive fractionation factors of α26/24Mgvap−liq = 0.9906 ± 0.0004 and α30/28Sivap−liq = 0.9943 ±0.0003, which are both significantly closer to unity than those found for evaporation in a vacuum, which translates to less isotope fractionation. The residues are also less isotopically fractionated than expected for cases in which transport of the gas species away from the melt is diffusion-controlled at 1-atm. By analysing the flow regimes in our furnace, we find that advection by the levitating gas is the primary mode of mass transport away from the melt surface, as opposed to diffusion-limited transport in a vacuum or 1-atm tube furnace. A modified Hertz-Knudsen-Langmuir formulation accounts for this process, and shows that isotopic fractionation of both Si and Mg reflect a saturation factor (ratio of the pressure of the evaporating species to vapour saturation pressure) equal to 0.75. This is in perfect accord with recent measurements of Cu isotopic fractionation using a similar furnace. The fact that three elements (Mg, Si, Cu) with varying equilibrium vapour pressures, activity coefficients in the liquid, and diffusion coefficients in the gas have the same scaling behaviour to saturation pressure is a strong indication that the mechanism controlling evaporation is driven by the hydrodynamical regime imposed in the furnace. Therefore, this class of experiments can be used to constrain processes in which advection dominates over diffusion, such as (but not limited to) planetary ejecta, tektites, giant impacts, nebular condensation in a turbulent flow, or nuclear fallout material. Finally, the possibility to reach high temperatures (in excess of 3500 K) in this furnace allows it to be used to evaluate the activity coefficients of melt components in extreme conditions relevant to molten planetary interiors (i.e., magma oceans), with a specific focus on refractory elements.

AB - We carried out evaporation experiments on a B-type calcium-aluminium-rich inclusion (CAI) melt in a gas-mixing aerodynamic levitation laser furnace, at 1873 K and an oxygen partial pressure of 10−9.1 atm, for durations ranging from 60 to 600 s. Evaporation of SiO2 and MgO follow the same trend as those observed in vacuum furnace experiments at the same temperature and starting composition, showing that their evaporation relative to one another from the melt is independent of pressure, oxygen fugacity, and hydrodynamical regime specific to the furnace. Isotopic ratios of Mg and Si in evaporation residues are used to derive fractionation factors of α26/24Mgvap−liq = 0.9906 ± 0.0004 and α30/28Sivap−liq = 0.9943 ±0.0003, which are both significantly closer to unity than those found for evaporation in a vacuum, which translates to less isotope fractionation. The residues are also less isotopically fractionated than expected for cases in which transport of the gas species away from the melt is diffusion-controlled at 1-atm. By analysing the flow regimes in our furnace, we find that advection by the levitating gas is the primary mode of mass transport away from the melt surface, as opposed to diffusion-limited transport in a vacuum or 1-atm tube furnace. A modified Hertz-Knudsen-Langmuir formulation accounts for this process, and shows that isotopic fractionation of both Si and Mg reflect a saturation factor (ratio of the pressure of the evaporating species to vapour saturation pressure) equal to 0.75. This is in perfect accord with recent measurements of Cu isotopic fractionation using a similar furnace. The fact that three elements (Mg, Si, Cu) with varying equilibrium vapour pressures, activity coefficients in the liquid, and diffusion coefficients in the gas have the same scaling behaviour to saturation pressure is a strong indication that the mechanism controlling evaporation is driven by the hydrodynamical regime imposed in the furnace. Therefore, this class of experiments can be used to constrain processes in which advection dominates over diffusion, such as (but not limited to) planetary ejecta, tektites, giant impacts, nebular condensation in a turbulent flow, or nuclear fallout material. Finally, the possibility to reach high temperatures (in excess of 3500 K) in this furnace allows it to be used to evaluate the activity coefficients of melt components in extreme conditions relevant to molten planetary interiors (i.e., magma oceans), with a specific focus on refractory elements.

KW - Aerodynamic levitation

KW - Elemental fractionation

KW - Extreme conditions

KW - Isotopic fractionation

KW - Laser furnace

KW - Planetary evaporation

U2 - 10.5802/CRGEOS.56

DO - 10.5802/CRGEOS.56

M3 - Journal article

AN - SCOPUS:85106858134

VL - 353

SP - 101

EP - 114

JO - Comptes Rendus Geoscience

JF - Comptes Rendus Geoscience

SN - 1631-0713

IS - 1

ER -