Cr stable isotope fractionation by evaporation from silicate melts

We present new experimental results of Cr isotope fractionation during degassing of silicate melts. Our experiments at oxidizing conditions (in air) at 1 bar total pressure show that evaporative loss of Cr is substantial and depends on run duration and on temperature (between 1350 and 1500 degrees C). The stable Cr (delta Cr-53) isotope compositions of the evaporation residues were analyzed by thermal ionization mass spectrometry using the double spike method. Our results indicate that during degassing the lighter isotope Cr-52 is enriched in the gas phase and consequently the residual melt becomes enriched in the heavier Cr-53 isotope in a mass-dependent fashion. The kinetic isotopic fractionation factor, alpha, derived from our experimental data is 0.9995 +/- 0.0001, which is significantly different from the fractionation factor predicted for an ideal gas composed of CrO3, alpha(kin), = 0.995. Our experimentally-determined value of alpha is consistent with diffusive transport of CrO3 (g) away from the melt surface, proportional to the inverse cube-root of the reduced masses of the two isotopologues. We hypothesize that Cr loss by evaporation may be recorded by Cr isotopes in terrestrial magmas with sufficiently high oxygen fugacities erupting near the surface, which are plausibly found in lava lakes or erupting arc magmas.