Experimental constraints on the fractionation of C-13/C-12 and O-18/O-16 ratios due to adsorption of CO2 on mineral substrates at conditions relevantto the surface of Mars

We report here measurements of fractionations of stable carbon and oxygen i sotopes between CO2 vapor and CO2 adsorbed on kaolinite, basalt, or fluorit e. Carbon-isotope fractionations between adsorbate and vapor (Delta C-13(a- v) = 1000 . ln (alpha (a-v)), where alpha = C-13/C-12(adsorbate))/(C-13/C-1 2(vapor)); similar to delta C-13(adsorbate)-delta C-13(vapor)) exhibit a si milar to1 parts per thousand enrichment of C-13 in the vapor phase for all substrates and temperatures over the range 190 to 230 K. These fractionatio ns are 'reversed' relative to the common expectation that heavy isotopomers should be concentrated into condensed phases relative to coexisting vapor (Lindemann, 1919). Oxygen-isotope fractionations between adsorbate and vapo r are complex: Comparison of experiments using vacuum-baked substrates to t hose in which substrates were not vacuum-baked suggests that exchange betwe en one or both phases of CO2 and water adsorbed on the sample surfaces infl uences the O-18/O-16 fractionation between vapor and adsorbate and changes the O-18/O-16 composition of the total CO2 in the experiment. In experiment s on vacuum-baked substrates, oxygen results exhibited 'normal' fractionati on of similar to1.8 parts per thousand (that is, O-18 preferentially partit ioned into the adsorbed CO2). No evidence was found for measurable exchange of oxygen isotopes between structural oxygen in mineral substrates and CO2 adsorbed on those substrates. The observation that C-13 is enriched in the vapor phase may help account for subtle (per mil to tens of per mil) enric hments of C-13 estimated for the Martian atmosphere as compared to bulk ter restrial carbon. Copyright (C) 2001 Elsevier Science Ltd.