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

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

“…The isotopic fractionation factors in our experiments are much higher (closer to unity), indicating less isotopic fractionation for an otherwise constant degree of elemental evaporation (Table 2). Our data can also be compared to those expected in a 1-atm tube furnace [Sossi et al, 2020] in which diffusion through the gas phase is the limiting variable, and again, our isotopic fractionation factors are also closer to unity. This important difference in isotopic fractionation factors between previous experiments (vacuum or 1 bar) and ours must be due to the fundamental difference in the mass transport mechanism involved.…”

“…Table 2. Isotopic fractionation factors from this study in the levitation furnace, compared with those obtained on the same composition and at the same temperature in a vacuum furnace [Richter et al, 2007], and to those expected at 1 bar for a diffusive transport mechanism recalculated from Sossi et al [2020] Advective (this study) 2SD Vacuum [Richter et al, 2007] 1-atm [Sossi et al, 2020] The fact that the saturation factor is the same for Si, Mg, and Cu despite their differing evaporation rates from the melt and differing diffusion rates through the gas implies that this factor is controlled by a property common to all systems. We argue that this property is the effective pressure reduction due to increased mass transport in a flow with a high Sherwood number.…”

“…where D is the diffusion coefficient of the evaporating species in the surrounding gas (on the order of 10 −4 m 2 /s). In the levitation furnace, Pe is on the order of 10, whereas it drops to 0.1 in the tube furnace (gas flows Sossi et al, 2019Sossi et al, , 2020 on the order of 0.1 m/s) and essentially to 0 in a vacuum furnace, highlighting advection as the principal mechanism for mass transport in our experiments compared to those performed in traditional furnaces where the only mode of mass transport away from the sample is due to chemical diffusion of the evaporate in a selfgenerated compositional gradient. These terms are combined in the Sherwood number (Sh), which is the ratio of mass transfer by convection to diffusion (the mass transfer equivalent of the Nusselt number for heat), and is defined as:…”

“…The residual silicon and magnesium content of the melt can be obtained by integrating the evaporation flux (Equation ( 1)) over time [Sossi et al, 2020], and is given by:…”

“…In a realistic system, a gaseous boundary layer builds up around the sample, limiting the evaporating flux by chemical diffusion. This introduces a scaling factor to (Equations ( 12)-( 14)), a function of the pressure saturation factor [Sossi et al, 2020, Young et al, 2019, and ( 14) becomes:…”