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An experimentally-determined general formalism for evaporation and isotope fractionation of Cu and Zn from silicate melts between 1300 and 1500 °C and 1 bar

Abstract : Vaporisation fractionates elements and their isotopes between condensed phase(s) and the gas phase according to two end-member regimes; kinetic (Langmuir) or equilibrium (Knudsen). The fractionation factor of isotopes i and j of element E between gas and liquid, i/jE(αvap-liq), in the former case depends only on the molar masses of the evaporating species, (Mj/Mi)0.5, whereas, at equilibrium, the fractionation factor is ∼1 at high temperatures. The prevailing regime depends on the production rate of vapour species relative to their rate of transport away from the evaporating surface, for which an accurate theory is lacking. Here, we present results of Cu and Zn isotope fractionation during evaporation of ferrobasaltic melts in the FeO(T)-CaO-MgO-Al2O3-SiO2 (FCMAS) system in a 1 atm gas-mixing furnace at oxygen fugacities (fO2) from 10−8 bar to 10−0.68 bar (air) at temperatures between 1300 and 1500 °C. In air, the fractionation factors are 65/63Cu(αvap-liq) = 0.9972 ± 0.0001 and 66/64Zn(αvap-liq) = 0.9959 ± 0.0002, but increase under reducing conditions, at logfO2 = −8 for 65/63Cu(αvap-liq) (0.9979) and at −3, −5.5 and −8 for Zn, at which 66/64Zn(αvap-liq) = 0.9967, 0.9974 and 0.9978, respectively. This behaviour is caused by slow elemental diffusion through the silicate melt relative to the rate of evaporative loss, thereby depressing i/jE(αvap-liq). When evaporation rates far exceed those for diffusion in the melt, no isotopic fractionation develops in the condensed phase. To account for the 65/63Cu(αvap-liq) and 66/64Zn(αvap-liq) values observed in air, we develop a formalism for mass transfer applicable to any evaporating species over a range of pressures, gas compositions and flow regimes. For natural convection at 1 bar, diffusive transport through the gas is the rate-limiting step, where D(i,j)k is the diffusion rate of isotope i or j through a gas with mean molar mass Mk. Under these conditions i/jE(αvap-liq) is proportional to (Dik/Djk)2/3, and not (Dik/Djk) as previously proposed. Our model predicts i/jE(αvap-liq) in other experiments, and in nature, to a precision of ±0.0005 assuming ideality. The fractionation factors inferred for moderately volatile elements in lunar mare basalts (>0.9994) are inconsistent with a mass transport process, and instead reflect near-equilibrium conditions of vapour loss from the Moon.
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https://hal-enpc.archives-ouvertes.fr/hal-03021529
Contributeur : Robin Treilles <>
Soumis le : mardi 24 novembre 2020 - 12:56:21
Dernière modification le : mardi 15 décembre 2020 - 10:43:43

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Paolo Sossi, Frederic Moynier, Robin Treilles, Marwane Mokhtari, Xiang Wang, et al.. An experimentally-determined general formalism for evaporation and isotope fractionation of Cu and Zn from silicate melts between 1300 and 1500 °C and 1 bar. Geochimica et Cosmochimica Acta, Elsevier, 2020, ⟨10.1016/j.gca.2020.08.011⟩. ⟨hal-03021529⟩

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