Experimental constraints on the fractionation of 13C/ 12C and 18O/ 16O ratios due to adsorption of CO 2 on mineral substrates at conditions relevant to the surface of Mars
We report here measurements of fractionations of stable carbon and oxygen isotopes between CO 2 vapor and CO 2 adsorbed on kaolinite, basalt, or fluorite. Carbon-isotope fractionations between adsorbate and vapor (∆ 13C a-v = 1000 · ln (α a-v), where α = 13C/ 12C adsorbate)/( 13C/ 12C vapor); ∼δ 13C adsorbate-δ 13C vapor) exhibit a ∼1‰ enrichment of 13C in the vapor phase for all substrates and temperatures over the range 190 to 230 K. These fractionations 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 vapor are complex: Comparison of experiments using vacuum-baked substrates to those in which substrates were not vacuum-baked suggests that exchange between one or both phases of CO 2 and water adsorbed on the sample surfaces influences the 18O/ 16O fractionation between vapor and adsorbate and changes the 18O/ 16O composition of the total CO 2 in the experiment. In experiments on vacuum-baked substrates, oxygen results exhibited 'normal' fractionation of ∼1.8‰ (that is, 18O preferentially partitioned into the adsorbed CO 2). No evidence was found for measurable exchange of oxygen isotopes between structural oxygen in mineral substrates and CO 2 adsorbed on those substrates. The observation that 13C is enriched in the vapor phase may help account for subtle (per mil to tens of per mil) enrichments of 13C estimated for the Martian atmosphere as compared to bulk terrestrial carbon.