Sorption of noble gases by solids,with reference to meteorites. III. Sulfides,spinels, and other substances; on the origin of planetary gases

To simulate trapping of noble gases by meteorites, we reacted 15 FeCr or FeCrNi alloy samples with CO, H₂O or H₂S at 350–720 K, in the presence of noble gases. The reaction products, including (Fe,Cr)₂O₃, FeCr₂S₄, FeS, C, and Fe₃C, were analyzed by mass spectrometry, usually after chemical separation by selective solvents. Three carbon samples were prepared by catalytic decomposition of CO or by dehydration of carbohydrates with H₂SO₄.The spinel and carbon samples were similar to those of earlier studies (Yang et al., 1982 and Yang and Anders, 1982), with only minor effects attributable to the presence of Ni. All samples sorted substantial amounts of noble gases, with distribution coefficients of 10^?1–10^?2 cm³ STP/g atm for Xe. On the basis of release temperature three gas components were distinguished: a generally dominant physisorbed component (20–80% of total), and two more strongly bound, chemisorbed and trapped components. Judging from the elemental pattern, the adsorbed components were acquired at the highest noble gas partial pressure encountered by the sample—atmosphere or synthesis vessel.Sulfides, particularly daubréelite, showed three distinctive trends relative to chromite or magnetite: the high-T component was larger, 30–70% of the total; $\text{Ne}\text{Xe}$ ratios were higher, by up to 10², possibly due to preferential diffusion of Ne during synthesis. In one synthesis, at relatively high P, the gases were sorbed with only minimal elemental fractionation, presumably by occlusion.Most of the features of primordial noble gases can be explained in terms of the data and concepts presented in the three papers of this series. The elemental fractionation pattern of Ar, Kr, Xe in meteorites, terrestrial rocks, and planets resembles the adsorption pattern on the solids studied: carbon, spinels, Sulfides, etc. The variation in $\text{Ne}\text{Ar}$ ratio may be explained by preferential diffusion of Ne. The high release temperature of meteoritic noble gases may be explained by transformation of physisorbed to chemisorbed gas, as observed in some experiments. The ready loss of meteoritic heavy gases on surficial oxidation (“Phase Q”) is consistent with adsorption, as is the high abundance. Extrapolation of the limited laboratory data suggests that the observed amounts of noble gases could have been adsorbed from a solar gas at 160–170 K and 10^?6–10^?5 atm, i.e. in the early contraction stages of the solar nebula. The principal unsolved problem is the origin of isotopically anomalous, apparently mass-fractionated noble gases in the Earth's atmosphere and in meteoritic carbon and chromite.