Solubilities of noble gases in magnetite: implications for planetary gases in meteorites |
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| Authors: | Michael S Lancet Edward Anders |
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| Institution: | Enrico Fermi Institute and Department of Chemistry, University of Chicago, 5630 Ellis Avenue, Chicago, Illinois 60637, U.S.A. |
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| Abstract: | Solubilities of noble gases in magnetite were determined by growing magnetite in a noble-gas atmosphere between 450 and 700°K. Henry's law is obeyed at pressures up to 10?2 atm for He, Ne, Ar and up to 10?5 atm for Kr, Xe, with the following distribution coefficients at 500° (cc STP g?1 atm?5): He 0.042, Ne 0.016, Ar 3.6, Kr 1.3, Xe 0.88, some 102–105 times higher than previous determinations on silicate and fluoride melts. Apparent heats of solution in kcal/mole are: He ?2.42 ±0.12, Ne ?2.20 ±0.10, Ar ?15.25 ±0.25, Kr ?13.0 ±0.3, Xe ?12-5 ± 0.5. These values, too, stand in sharp contrast with earlier determinations on melts which were small and positive, but are comparable to the values for clathrates. Presumably the gases are held in anion vacancies.Extrapolation of the magnetite data to the formation temperature of C1 chondrites, 360°K, shows that the Arp36 content of Orgueil magnetite could be acquired by equilibrium solubility at a total nebular pressure of 4 × 10?6 atm. In the absence of data for silicates (the principal host phase of planetary gas), an attempt is made to estimate the solubilities required to account for planetary gases in meteorites. These values do not appear grossly unreasonable in the light of the magnetite data, when structural differences between the two minerals are taken into account. It seems that equilibrium solubility may be able to account for four features of planetary gas: elemental ratios, amounts, correlations with other volatiles and retentive siting. It cannot account for the isotopic fractionation of planetary gas, however. |
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