  We use new experiments and a theoretical analysis of the results to show that
the isotopic fractionation associated with laser-heating aerodynamic levitation
experiments is consistent with the velocity of flowing gas as the primary
control on the fractionation. The new Fe and Mg isotope data are well explained
where the gas is treated as a low-viscosity fluid that flows around the molten
spheres with high Reynolds numbers and minimal drag. A relationship between the
ratio of headwind velocity to thermal velocity and saturation is obtained on
the basis of this analysis. The recognition that it is the ratio of flow
velocity to thermal velocity that controls fractionation allows for
extrapolation to other environments in which molten rock encounters gas with
appreciable headwinds. In this way, in some circumstances, the degree of
isotope fractionation attending evaporation is as much a velocimeter as it is a
barometer.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics;Fluid Dynamics;Geophysics