Variations in the isotopic compositions of oxygen, hydrogen, and carbon in the near-surface environment of Mars are likely influenced by condensation, evaporation, and sublimation of major volatile species (H2O, CO2). We present here an experimental study of the fractionations of O-18/O-16 and C-13/C-12 ratios between CO2 ice and vapor at conditions relevant to the present near-surface of Mars; these experiments constrain isotopic variations generated by the current Martian CO2 condensation/sublimation cycle. Oxygen-isotope fractionation between ice and vapor (Delta(ice-vapor) = 1000 . In ([O-18(ice)/O-16(ice)] / [O-18(vapor)/O-16(vapor)]) varies approximately linearly vs. 1/T between temperatures of 150 and 130 K (from 4.2 and 7.5 parts per thousand, respectively). Carbon isotopes are unfractionated (Delta(13)C(ice-vapor) = 0.2 parts per thousand) at temperatures greater than or equal to 135 K and only modestly fractionated (Delta(13)C(ice-vapor) less than or equal to 0.4 parts per thousand) at temperatures between 135 and 130 K. Martian atmospheric volumes that are residual to high extents of condensation (i.e., at high latitudes during the winter) may vary in delta(18)O by up to tens of per mil, depending on thr scales and mechanisms of ice/vapor interaction and atmospheric mixing. Precise (i.e., per mil level) examination of the Martian atmosphere or ices could be used as a tool for examining the Martian climate; at present such precision is only likely to be had from laboratory study of returned samples or substantial advances in the performance of mass spectrometers on landers and/or orbital spacecraft. Oxygen-isotope fractionations accompanying the CO2 condensation/sublimation cycle may play a significant role in the oxygen-isotope geochemistry of secondary phases formed in SNC meteorites, in particular as a means of generating O-18-depleted volatile reservoirs. Copyright (C) 1999 Elsevier Science Ltd.
