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Redox Conditions in the Solar Nebula: Observational, Experimental, and Theoretical Constraints

Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago Chicago, Illinois 60637, U.S.A., yosi{at}midway.uchicago.edu

John R. Beckett

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.

Alexei V. Fedkin and Steven B. Simon

Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois 60637, U.S.A.

Fred J. Ciesla

Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015-1305, U.S.A.

Crystallization experiments on liquids with compositions similar to those of compact Type A, Type B1 and Type B2 refractory inclusions were conducted under controlled temperature and f_O2 conditions. Application of the results to the compositions of coexisting Ti³⁺ -bearing fassaitic clinopyroxene + melilite pairs in natural inclusions shows that, if they crystallized at ~1509 K, they did so at log f_O2 = –19.8 ± 0.9, only slightly below the equilibrium log f_O2 of a partially condensed system of solar composition at the same temperature, –18.1_–0.3^+0.2, or IW-6.8. Fassaite is the only f_O2 indicator that shows that anything in chondrites formed in a system that was close to solar in composition. Solar composition is so reducing that equilibrium calculations predict vanishingly small FeO/(FeO + MgO) ratios in the condensate until temperatures fall below 800 K, where significant oxidation of metallic iron and formation of fayalite in solid solution with previously condensed forsterite begin. The mechanism for the latter process is diffusion of Fe²⁺ through forsterite, but the diffusion rate is nearly zero at these temperatures. By comparison to what is achievable in a system of solar composition, the mean FeO/(FeO + MgO) ratio of the olivine in chondrules in unequilibrated ordinary chondrites (UOCs) is very high, ~0.15. Making such ratios in chondrule precursors by solar nebular processes requires sufficiently high f_O2 for iron to become oxidized above temperatures where diffusion of Fe²⁺ becomes very slow. Two dynamic models for enrichment of oxygen relative to carbon and hydrogen were investigated quantitatively: radial transport of water ice-rich migrators across the snow line into the inner part of the solar nebula where the ice evaporates; and coagulation, vertical settling and evaporation of anhydrous dust in the median plane of the inner nebula. In both cases, the maximum achievable f_O2, ~IW-4.5, produces a maximum X_Fa before diffusion ceases that is a factor of >7 less than would be required for UOC chondrule precursors, even for grains only 0.1 µm in radius and nebular cooling times as high as 10⁶ yr. The same dynamic models are also incapable of creating environments sufficiently oxidizing to produce olivine with X_Fa = 0.15 during formation of chondrules by melting of FeO-poor precursors. If, instead, chondrule precursors were made of very FeO-rich, non-equilibrium condensates, reduction of chondrule melts by nebular gas may have been arrested before the mean X_Fa of chondrule olivine could fall below 0.15 because chondrules were hot for such a short time. A nebular origin for the mineral assemblage of unequilibrated enstatite chondrites (UECs) requires f_O2 significantly below that of a system of solar composition. In particular, after fractionation of specific amounts of predicted high-temperature condensates, equilibrium condensation in a system whose P^tot = 10^–4 atm and whose initial composition is solar except for a C/O ratio of 0.83 yields an assemblage characterized by a very large enstatite/forsterite ratio, the presence of oldhamite and niningerite, metallic nickel-iron containing several wt% Si, and small amounts of pure silica and albitic plagioclase, very similar to the mineral assemblage of EH3 chondrites. Log f_O2 in this system varies from IW-8.9 at 1500 K to IW-13 at 900 K. The mechanisms proposed to date for fractionation of C, O and H from one another are quantitatively insufficient to produce the magnitude of nebular f_O2 variations needed to account for primitive features of UOCs and UECs.